AN ABSTRACT OF THE THESIS OF
Ryan L. Gerber for the degree of Master of Science in Radiation Health Physics
presented on December 6, 2010.
Title: A COMPARISON OF COMPACT IONIZATION CHAMBER
PERFORMANCE AND RELATIVE READINGS
Abstract approved:
Kathryn A. Higley
The purpose of this study is to investigate the relative performance of
compact ionization chambers as it changes based on the speed of detector motion
and collection volume. To quantify changes, multiple scans were made with each
of a selection of compact chambers and repeated varying detector speed. Each scan
was then used to compute the necessary statistics for each field sampled. These
results were then compared to analyze differences in relative ionization readings
across entire scan ranges.
The results and conclusions of this study further reinforce existing studies,
in particular those released since 2007 relating to the study of newly available
compact ionization chambers. When choosing a chamber, one should use the
smallest chamber available that has been proven to respond appropriately for the
field sizes to be measured. As for detector speed, generally smaller field sizes are
shown to be more sensitive to detector speed changes. There is not one
recommendation for detector speed, as the optimum speed is determined by the
type of scan being performed, the energy being scanned, the field size being
scanned, and the end use of the data being captured. Finally, optimizing pdd scan
depths and profile penumbra margins is an important step to maximizing efficient
use of time when capturing LINAC beam characteristics.
© Copyright by Ryan L. Gerber
December 6, 2010
All Rights Reserved
A COMPARISON OF COMPACT IONIZATION CHAMBER
PERFORMANCE AND RELATIVE READINGS
by
Ryan L. Gerber
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Presented December 6, 2010
Commencement June 2011
Master of Science thesis of Ryan L. Gerber presented on December 6, 2010.
APPROVED:
________________________________________
Major Professor, representing Radiation Health Physics
________________________________________
Head of the Department of Nuclear Engineering & Radiation Health Physics
________________________________________
Dean of the Graduate School
I understand that my thesis will become part of the permanent collection of Oregon
State University libraries. My signature below authorizes release of my thesis to
any reader upon request.
____________________________________
Ryan L. Gerber, Author
TABLE OF CONTENTS
Page
Introduction ................................................................................................................ 1
Background ................................................................................................................ 4
Radiation Therapy with Linear Accelerators .........................................................4
Medical Linear Accelerators ..................................................................................... 4
Treatment Planning ..................................................................................................... 8
Beam Measurement ..............................................................................................10
Ionization Chambers ................................................................................................. 10
Electrometers ............................................................................................................. 12
3D Beam Scanning Systems .................................................................................... 13
Equipment Used and Data Collection Conditions ................................................... 14
Equipment/Parameters ..........................................................................................14
Data Collection Conditions ..................................................................................14
Data Analysis Legend ..........................................................................................15
Additional Notes on Data Collection ...................................................................15
Data Presentation and Analysis ............................................................................... 16
Detector Speed Change ........................................................................................16
Small fields – Data .................................................................................................... 16
Small fields - Analysis ............................................................................................. 17
Small fields - Recommendation .............................................................................. 21
Medium fields – Data ............................................................................................... 21
Medium fields – Analysis ........................................................................................ 22
Medium fields – Reommendation........................................................................... 27
Large Fields – Data ................................................................................................... 27
Large Fields – Analysis ............................................................................................ 28
Large fields – Recommendation ............................................................................. 32
Physical Detector Change.....................................................................................33
Chamber response to field size change – Data...................................................... 33
TABLE OF CONTENTS (Continued)
Page
Chamber response to field size change – Analysis............................................... 37
Small fields – Data .................................................................................................... 37
Small fields - Analysis ............................................................................................. 37
Small field – Recommendation ............................................................................... 39
Medium field – Data ................................................................................................. 39
Medium field – Analysis .......................................................................................... 39
Medium fields – Recommendation ......................................................................... 43
Large fields – Data .................................................................................................... 44
Large fields – Analysis ............................................................................................. 44
Large fields – Recommendation ............................................................................. 47
Conclusion ............................................................................................................... 48
Bibliography ............................................................................................................ 49
Appendix .................................................................................................................. 51
LIST OF FIGURES
Figure
Page
1 – Varian Trilogy LINAC......................................................................................... 1
2 – IBA Welhoefer Blue Phantom Scanning System ................................................ 2
3 – LINAC Console with Scanning System............................................................... 3
4 – PTW 30013 Farmer Chamber ............................................................................ 11
5 – IBA Welhoefer cc01 Compact Chamber ........................................................... 12
6 – 6 MV Photons, .5x.5cm2, cc01, profile, all speeds ............................................ 20
7 – 6 MV Photons, .5x.5cm2, cc01, pdd, speeds ...................................................... 21
8 – Field Size Dependence – 6 MV photons ............................................................ 35
9 – Field Size Dependence – 18 MV photons .......................................................... 36
10 – 6 MV Photons, .5x.5cm2, cc01(blue)/cc04(green)/cc13(red), pdd .................. 38
11 – 6 MV Photons, .5x.5cm2, cc01(blue)/cc04(green)/cc13(red), profile .............. 39
12 – 6 MeV electrons, 3cm circle, cc01/cc04/cc13, pdd .........................................42
13 – 6 MeV electrons, 10x10cm2, cc01/cc04/cc13, pdd .......................................... 42
14 – 6 MV photons, 10x10cm2, cc01/cc04/cc13, pdd .............................................43
15 – 6 MV photons, 40x40 cm2, cc01/cc04/cc13, pdd ............................................46
16 – 6 MeV, 25x25cm2, cc01, profile ...................................................................... 47
17 – 6 MV photons, .5x.5cm2, cc01, fast pdd ..........................................................52
18 – 6 MV photons, .5x.5cm2, cc01, medium pdd ...................................................53
19 – 6 MV photons, .5x.5cm2, cc01, slow pdd ........................................................54
20 – 6 MV photons, .5x.5cm2, cc01, all speeds, pdd ...............................................55
21 – 6 MV photons, .5x.5cm2, cc01, fast profile .....................................................56
22 – 6 MV photons, .5x.5cm2, cc01, medium profile ..............................................57
23 – 6 MV photons, .5x.5cm2, cc01, slow profile....................................................58
24 – 6 MV photons, .5x.5cm2, cc01, all speeds profile ...........................................59
25 – 6 MV photons, .5x.5cm2, cc04, fast pdd ..........................................................60
26 – 6 MV photons, .5x.5cm2, cc04, medium pdd ...................................................61
27 – 6 MV photons, .5x.5cm2, cc04, slow pdd ........................................................62
LIST OF FIGURES (Continued)
Figure
Page
28 – 6 MV photons, .5x.5cm2, cc04, all speeds pdd ................................................63
29 – 6 MV photons, .5x.5cm2, cc04, fast profile .....................................................64
30 – 6 MV photons, .5x.5cm2, cc04, medium profile ..............................................65
31 – 6 MV photons, .5x.5cm2, cc04, slow profile....................................................66
32 – 6 MV photons, .5x.5cm2, cc04, all speeds profile ...........................................67
33 – 6 MeV electrons, 3cm circle, cc01, fast pdd ....................................................68
34 – 6 MeV electrons, 3cm circle, cc01, medium pdd.............................................69
35 – 6 MeV electrons, 3cm circle, cc01, slow pdd .................................................. 70
36 – 6 MeV electrons, 3cm circle, cc01, all speeds pdd .......................................... 71
37 – 6 MeV electrons, 3cm circle, cc01, fast profile ............................................... 72
38 – 6 MeV electrons, 3cm circle, cc01, medium profile ........................................73
39 – 6 MeV electrons, 3cm circle, cc01, slow profile ............................................. 74
40 – 6 MeV electrons, 3cm circle, cc01, all speeds profile .....................................75
41 – 6 MeV electrons, 3cm circle, cc04, fast pdd ....................................................76
42 – 6 MeV electrons, 3cm circle, cc04, medium pdd.............................................77
43 – 6 MeV electrons, 3cm circle, cc04, slow pdd .................................................. 78
44 – 6 MeV electrons, 3cm circle, cc04, all speeds pdd .......................................... 79
45 – 6 MeV electrons, 3cm circle, cc04, fast profile ............................................... 80
46 – 6 MeV electrons, 3cm circle, cc04, medium profile ........................................81
47 – 6 MeV electrons, 3cm circle, cc04, slow profile ............................................. 82
48 – 6 MeV electrons, 3cm circle, cc04, all speeds profile .....................................83
49 – 6 MeV electrons, 3cm circle, cc13, fast pdd ....................................................84
50 – 6 MeV electrons, 3cm circle, cc13, medium pdd.............................................85
51 – 6 MeV electrons, 3cm circle, cc13, slow pdd .................................................. 86
52 – 6 MeV electrons, 3cm circle, cc13, all speeds pdd .......................................... 87
53 – 6 MeV electrons, 3cm circle, cc13, fast profile ............................................... 88
54 – 6 MeV electrons, 3cm circle, cc13, medium profile ........................................89
LIST OF FIGURES (Continued)
Figure
Page
55 – 6 MeV electrons, 3cm circle, cc13, slow profile ............................................. 90
56 – 6 MeV electrons, 3cm circle, cc13, all speeds profile .....................................91
57 – 6 MeV electrons, 10x10 cm2, all chambers, medium pdd ............................... 92
58 – 6 MeV electrons, 25x25 cm2, cc01, fast pdd ................................................... 93
59 – 6 MeV electrons, 25x25 cm2, cc01, med pdd .................................................. 94
60 – 6 MeV electrons, 25x25 cm2, cc01, slow pdd.................................................. 95
61 – 6 MeV electrons, 25x25 cm2, cc01, all speeds pdd.......................................... 96
62 – 6 MeV electrons, 25x25 cm2, cc01, fast profile ............................................... 97
63 – 6 MeV electrons, 25x25 cm2, cc01, med profile.............................................. 98
64 – 6 MeV electrons, 25x25 cm2, cc01, slow profile ............................................. 99
65 – 6 MeV electrons, 25x25 cm2, cc01, all speeds profile ................................... 100
66 – 6 MeV electrons, 25x25 cm2, cc04, fast pdd ................................................. 101
67 – 6 MeV electrons, 25x25 cm2, cc04, med pdd ................................................ 102
68 – 6 MeV electrons, 25x25 cm2, cc04, slow pdd................................................ 103
69 – 6 MeV electrons, 25x25 cm2, cc04, all speeds pdd........................................ 104
70 – 6 MeV electrons, 25x25 cm2, cc04, fast profile ............................................. 105
71 – 6 MeV electrons, 25x25 cm2, cc04, med profile............................................ 106
72 – 6 MeV electrons, 25x25 cm2, cc04, slow profile ........................................... 107
73 – 6 MeV electrons, 25x25 cm2, cc04, all speeds profile ................................... 108
74 – 6 MeV electrons, 25x25 cm2, cc13, fast pdd ................................................. 109
75 – 6 MeV electrons, 25x25 cm2, cc13, med pdd ................................................ 110
76 – 6 MeV electrons, 25x25 cm2, cc13, slow pdd................................................ 111
77 – 6 MeV electrons, 25x25 cm2, cc13, all speeds pdd........................................ 112
78 – 6 MeV electrons, 25x25 cm2, cc13, fast profile ............................................. 113
79 – 6 MeV electrons, 25x25 cm2, cc13, medium profile ..................................... 114
80 – 6 MeV electrons, 25x25 cm2, cc13, slow profile ........................................... 115
81 – 6 MeV electrons, 25x25 cm2, cc13, all speeds profile ................................... 116
LIST OF FIGURES (Continued)
Figure
Page
82 – 6MV photons, .5x.5cm2, cc01, pdd ................................................................ 117
83 – 6MV photons, .5x.5cm2, cc04, pdd ................................................................ 118
84 – 6MV photons, .5x.5cm2, cc13, pdd ................................................................ 119
85 – 6MV photons, .5x.5cm2, all chambers pdd ....................................................120
86 – 6MV photons, .5x.5cm2, cc01, profile ...........................................................121
87 – 6MV photons, .5x.5cm2, cc04, profile ...........................................................122
88 – 6MV photons, .5x.5cm2, cc13, profile ...........................................................123
89 – 6MV photons, .5x.5cm2, all chambers, profile .............................................. 124
90 – 6 MeV electrons, 3cm circle, cc01, pdd ......................................................... 125
91 – 6 MeV electrons, 3cm circle, cc04, pdd ......................................................... 126
92 – 6 MeV electrons, 3cm circle, cc13, pdd ......................................................... 127
93 – 6 MeV electrons, 3cm circle, all chambers pdd ............................................. 128
94 – 6 MeV electrons, 10x10 cm2, all chambers, pdd ........................................... 129
95 – 6 MV photons, 10x10 cm2, all chambers, pdd ...............................................130
96 – 6 MV photons, 10x10 cm2, all chambers, pdd (near surface zoom) ..............131
97 – 6 MeV electrons, 3cm circle, cc01, profile .................................................... 132
98 – 6 MeV electrons, 3cm circle, cc04, profile .................................................... 133
99 – 6 MeV electrons, 3cm circle, cc13, profile .................................................... 134
100 – 6 MeV electrons, 3cm circle, all chambers, profile .....................................135
101 – 6 MeV electrons, 25x25cm2, cc01, pdd ....................................................... 136
102 – 6 MeV electrons, 25x25cm2, cc04, pdd ....................................................... 137
103 – 6 MeV electrons, 25x25cm2, cc13, pdd ....................................................... 138
104 – 6 MeV electrons, 25x25cm2, all chambers pdd ........................................... 139
105 – 6 MV photons, 40x40cm2, all chambers, pdd .............................................. 140
106 – 6 MeV electrons, 25x25cm2, cc01, profile................................................... 141
107 – 6 MeV electrons, 25x25cm2, cc04, profile................................................... 142
108 – 6 MeV electrons, 25x25cm2, cc13, profile................................................... 143
LIST OF FIGURES (Continued)
Figure
Page
109 – 6 MeV electrons, 25x25cm2, all chambers, profile ...................................... 144
LIST OF TABLES
Table
Page
1 – 6 MV Photons, .5x.5cm2, cc01, profile, fast ...................................................... 17
2 – 6 MV Photons, .5x.5cm2, cc01, profile, medium ............................................... 17
3 – 6 MV Photons, .5x.5cm2, cc01, profile, slow ....................................................17
4 – 6 MV Photons, .5x.5cm2, cc01, profile, fast ...................................................... 18
5 – 6 MV Photons, .5x.5cm2, cc01, profile, medium ............................................... 18
6 – 6 MV Photons, .5x.5cm2, cc01, profile, slow ....................................................18
7 – 6 MV Photons, .5x.5cm2, cc01, pdd, fast ........................................................... 18
8 – 6 MV Photons, .5x.5cm2, cc01, pdd, medium ................................................... 19
9 – 6 MV Photons, .5x.5cm2, cc01, pdd, slow ......................................................... 19
10 – 6 MV Photons, .5x.5cm2, cc01, pdd, fast ......................................................... 19
11 – 6 MV Photons, .5x.5cm2, cc01, pdd, medium ................................................. 19
12 – 6 MV Photons, .5x.5cm2, cc01, pdd, slow ....................................................... 19
13 – 6 MeV electrons, 3cm circle, cc01, profile, fast ..............................................22
14 – 6 MeV electrons, 3cm circle, cc01, profile, medium .......................................22
15 – 6 MeV electrons, 3cm circle, cc01, profile, slow ............................................ 23
16 – 6 MeV electrons, 3cm circle, cc04, profile, fast ..............................................23
17 – 6 MeV electrons, 3cm circle, cc04, profile, medium .......................................23
18 – 6 MeV electrons, 3cm circle, cc04, profile, slow ............................................ 23
19 – 6 MeV electrons, 3cm circle, cc13, profile, fast ..............................................24
20 – 6 MeV electrons, 3cm circle, cc13, profile, medium .......................................24
21 – 6 MeV electrons, 3cm circle, cc13, profile, slow ............................................ 24
22 – 6 MeV electrons, 3cm circle, cc01, pdd, fast ...................................................24
23 – 6 MeV electrons, 3cm circle, cc01, pdd, medium............................................ 25
24 – 6 MeV electrons, 3cm circle, cc01, pdd, slow ................................................. 25
25 – 6 MeV electrons, 3cm circle, cc04, pdd, fast ...................................................25
26 – 6 MeV electrons, 3cm circle, cc04, pdd, medium............................................ 25
27 – 6 MeV electrons, 3cm circle, cc04, pdd, slow ................................................. 25
LIST OF TABLES (Continued)
Table
Page
28 – 6 MeV electrons, 3cm circle, cc13, pdd, fast ...................................................26
29 – 6 MeV electrons, 3cm circle, cc13, pdd, medium............................................ 26
30 – 6 MeV electrons, 3cm circle, cc13, pdd, slow ................................................. 26
31 – 6 MeV electrons, 25x25 cm2, cc01, profile, fast .............................................. 28
32 – 6 MeV electrons, 25x25 cm2, cc01, profile, medium ...................................... 28
33 – 6 MeV electrons, 25x25 cm2, cc01, profile, slow ............................................ 29
34 – 6 MeV electrons, 25x25 cm2, cc04, profile, fast .............................................. 29
35 – 6 MeV electrons, 25x25 cm2, cc04, profile, medium ...................................... 29
36 – 6 MeV electrons, 25x25 cm2, cc04, profile, slow ............................................ 29
37 – 6 MeV electrons, 25x25 cm2, cc13, profile, fast .............................................. 30
38 – 6 MeV electrons, 25x25 cm2, cc13, profile, medium ...................................... 30
39 – 6 MeV electrons, 25x25 cm2, cc13, profile, slow ............................................ 30
40 – 6 MeV electrons, 25x25 cm2, cc01, pdd, fast .................................................. 30
41 – 6 MeV electrons, 25x25 cm2, cc01, pdd, medium ........................................... 31
42 – 6 MeV electrons, 25x25 cm2, cc01, pdd, slow................................................. 31
43 – 6 MeV electrons, 25x25 cm2, cc04, pdd, fast .................................................. 31
44 – 6 MeV electrons, 25x25 cm2, cc04, pdd, medium ........................................... 31
45 – 6 MeV electrons, 25x25 cm2, cc04, pdd, slow................................................. 31
46 – 6 MeV electrons, 25x25 cm2, cc13, pdd, fast .................................................. 32
47 – 6 MeV electrons, 25x25 cm2, cc13, pdd, medium ........................................... 32
48 – 6 MeV electrons, 25x25 cm2, cc13, pdd, slow................................................. 32
49 – 6 MeV electrons, 3cm circle, cc01, profile ...................................................... 40
50 – 6 MeV electrons, 3cm circle, cc04, profile ...................................................... 40
51 – 6 MeV electrons, 3cm circle, cc13, profile ...................................................... 40
52 – 6 MeV electrons, 3cm circle, cc01, pdd ........................................................... 40
53 – 6 MeV electrons, 3cm circle, cc04, pdd ........................................................... 41
54 – 6 MeV electrons, 3cm circle, cc13, pdd ........................................................... 41
LIST OF TABLES (Continued)
Table
Page
55 – 6 MeV electrons, 25x25 cm2, cc01, profile...................................................... 44
56 – 6 MeV electrons, 25x25 cm2, cc04, profile...................................................... 44
57 – 6 MeV electrons, 25x25 cm2, cc13, profile...................................................... 45
58 – 6 MeV electrons, 25x25 cm2, cc01, pdd ..........................................................45
59 – 6 MeV electrons, 25x25 cm2, cc04, pdd ..........................................................45
60 – 6 MeV electrons, 25x25 cm2, cc13, pdd ..........................................................45
61 – Ion Chamber Technical Details........................................................................ 51
62 – Ion Chamber Wall Material Information ......................................................... 51
COMMONLY USED TERMS AND ABBREVIATIONS
LINAC:
Linear Accelerator
RT:
Radiation Therapy/Treatment
RTPS:
Radiation Treatment Planning System
PDD:
Percent Depth Dose
CAX:
Central Axis
MLC:
Multi-Leaf Collimator
Farmer:
PTW Farmer Chamber model 30013
cc01:
IBA Welhoefer Compact Chamber model cc01 (volume .01 cm3)
cc04:
IBA Welhoefer Compact Chamber model cc04 (volume .04 cm3)
cc13:
IBA Welhoefer Compact Chamber model cc13 (volume .13 cm3)
Introduction
Accurate modeling of the multiple poly-energetic beams available for use in
clinical external beam radiotherapy is of utmost importance to patient care. The
first and foremost need for understanding the beam characteristics is from a safety
and efficacy of treatment standpoint. Without this accurate understanding the
potential penalties include the potential for death in the patient population from
either a lack of curative treatment if undertreated or an unintended overexposure to
a beam with a higher output than modeled.
Modern radiation therapy (RT) involves a complex network of
interconnected systems to allow for all steps of the treatment planning process from
diagnostic imaging to actual treatment on a clinical Linear Accelerator (LINAC,
see Figure 1). An important key in the middle of this process is the radiation
therapy treatment planning system (RTPS). Today’s RT treatments are
increasingly complex and are rarely anymore done by hand. However, whether
performed by hand or a RTPS, the calculations rely on data gathered either during
the initial machine commissioning or during an additional RTPS commissioning
session.
Figure 1 – Varian Trilogy LINAC
2
Capturing commissioning data has traditionally been a time intensive
process that utilized large square water phantoms (see Figure 2) connected to
computer control equipment that moves and tracks the location in 3 dimensions of
ionization chambers (see Figure 3). The ion chambers used for scanning have
continued to shrink in size, and improve accuracy over time. However, as noted by
many authors, with the number of treatments where field sizes are being reduced to
well below the traditional small 4x4 cm2 field and into the sub centimeter
dimensions, one of the main dosimetric challenges is the availability of even
smaller, minimally perturbing detectors appropriate for measuring these newer very
small fields. (Sauer & Wilbert, 2007) (Das, Ding, & Ahnesjo, 2007) (Klein, Tailor,
Archambault, Wang, Therriault-Proulx, & Beddar, 2010) These smaller chambers
for use in measuring very small fields hold the potential to provide more accurate
readings of the penumbra and other high dose gradient regions.
Figure 2 – IBA Welhoefer Blue Phantom Scanning System
3
Figure 3 – LINAC Console with Scanning System
In addition to the potential for higher accuracy from smaller detectors, the
newer 3-d water scanning systems also allow for customizable detector motion
speeds for different scan types. Increasing detector speed will save time during the
data acquisition, but there has been no study on how the change in detector speed
affects the data collected. The biggest benefits of shortened scanning time are the
ability to get the LINAC treating patients sooner, and saving the organization the
costs associated with additional physics commissioning time.
This thesis will attempt to address these two questions;
1 - How will changing the speed of the chambers affect their relative
readings?
2 - How will changing the chamber size affect the readings of a range of
fields sizes from .5x.5 cm2 to 40x40 cm2?
4
Background
Many of the separate items here could each be written on at length, and
could easily deserve a complete, independent study. I have attempted to summarize
a relevant history and working functional understanding of each topic as necessary
for the express purpose of having a framework for the reader of this study to place
the results into. None of these backgrounds should be considered all
encompassing.
Radiation Therapy with Linear Accelerators
Medical Linear Accelerators
A brief history
The modern medical LINAC has had a long history that started with the
first rf powered linear accelerator build by Wideroe in 1928. However, it wasn’t
until the advent of the Klystron in the summer of 1937 and introduced in 1939
(Ginzton, 2010) that would allow for the construction of the first generation of
modern medical LINACs. The first LINAC used to treat a patient worldwide was
London in 1953. The first use in the USA was at Stanford in 1956. Since these
first relatively simple single low energy photon (4-8MV) machines were
developed, accelerators have gone through a rapid development in the past ~60
years. Today’s LINACs can produce multiple low and high energy electron beams
along with a selection of typically five or six available electron beam energies.
These machines also possess the ability to highly modulate the radiation beam with
on-board computer controlled shaping equipment known as multi-leaf collimators.
How they work
The LINAC used for final data set capture was a Varian Trilogy unit
installed in the fall of 2008. The following discussion of how LINACs work will
be constrained to this and other similar units available from a few different vendors
worldwide. Each vendor has a slightly different method of creating their desired
beams, but this background will not attempt to tease out all of individual details.
One should consult vendor documentation for machine operations specifics.
5
Podgorsak et al classifies the main beam forming components of the
modern medical LINAC into six classes: (i) Injection system; (ii) Radiofrequency
(RF) power generation system; (iii) Accelerating waveguide; (iv) Source beam
transport system; (v) Clinical beam production, collimation, and monitoring
system; (vi) Auxiliary system. (Podgorsak, Metcalf, & Van Dyk, 1999) In addition
to the beam forming components there are several machine structure and control
components. The machine structure is composed primarily of the gantry stand that
houses the bulk of the equipment necessary, a gantry which will rotate around a
patient 360, a patient support table that may move in up to six different directions;
angle, in/out, left/right, up/down, tilt, and roll. (see Figure 1) Finally, the majority
of control components are housed at the treatment console. (see Figure 2)
The injection system is the source of electrons for the accelerator and
consists of an electrostatic accelerator known as an electron gun and its associated
control systems. The electron gun may be of the diode or triode type, which
consists of a heated cathode, a perforated grounded anode, and in the case of the
triode electron gun an additional grid. In the triode electron gun, used in the Varian
Trilogy, “the cathode is held at a static negative potential (typically -20 kV). The
grid of the triode gun is normally held sufficiently negative with respect to the
cathode to cut off the current to the anode. The injection of electrons into the
accelerating waveguide is then controlled by voltage pulses, which are applies to
the grid and must be synchronized with the pulses applied to the microwave
generator.” (Podgorsak, Metcalf, & Van Dyk, 1999)
The RF power generation system is comprised of three main components.
First is the RF power source, either a magnetron or a klystron and RF driver. The
Varian Trilogy unit uses a klystron, which is a RF power amplifier that amplifies
the RF signal created by the RF driver. Second is a pulsed modulator. The
modulator creates the high voltage (~100 kV), high current (~100 A), short
duration (~1s) synchronized pulses needed by both the RF power source and
electron gun. The third and final component of the RF power system is the RF
6
power transmission waveguides. The RF transmission waveguides transport the RF
from the RF generator to the accelerating waveguide. These waveguides are
typically either evacuated to a near vacuum or pressurized with a dielectric gas.
The accelerating waveguide, as used in a LINAC, is an evolution of a basic
cylindrical waveguide. In its most generic form the LINAC accelerating
waveguide is a cylindrical waveguide with a series of discs spaced equidistant
along the cylinder dividing the cylinder into cavities. Each disc has a hole in its
center aligned on the central axis of the cylinder and in-line with the pencil beam
created by the electron gun, which is coupled to one end of the accelerating
waveguide. Once the waveguide has either been evacuated to near vacuum or filled
with a dielectric gas the electrons can be accelerated using the high power RF
waves. It should be noted that there are two main types of LINAC accelerating
waveguides, the traveling wave structure and the standing wave structure, the
Varian Trilogy used for data capture utilizes a standing wave guide.
Once the electron pencil beam has been accelerated it must strike a target
and/or other shaping devices to become a clinically useful beam. On many low
energy machines the target is attached directly to the accelerating waveguide.
However, in the case of the Varian Trilogy and all other high energy machines, the
electron pencil beam must go through the electron beam transport system. This
transport system is a complicated, interconnected group of components that
include, high voltage power supplies, electro-magnets, coils, and drift tubes. This
system must bend, steer, and focus the source beam into the LINAC head where the
actual clinical beam “production” takes place. The final piece of the beam
transport system is the window through which the electron beam must pass. The
window is typically made of Beryllium, which with its low atomic number Z,
minimizes the pencil beam scattering and bremstrahlung photon production.
As noted earlier, modern medical LINACs operate with multiple photon and
electron beam energies available for use in treatment. These multiple modalities
and energies create a special problem of each needing its own target and filter, or
7
foil to pass through to created the desired clinical beam. LINAC designers have
addressed this by creating moveable, typically pneumatically driven, targets, filters,
and foils. To create a clinical photon beam, the source electron beam passes
through a target, a tungsten alloy in the Varian Trilogy, then a flattening filter,
which results in a sufficiently large, flat, and symmetrical beam for clinical use.
For clinical electron beams, the target and flattening filter are retracted and a
scattering foil is driven into place to again create a sufficiently large, flat, and
symmetrical beam of the appropriate energy and modality to be used.
At this point the clinical beam is shaped to its maximum size by a fixed
primary collimator and is monitored by multiple internal ionization chambers.
Next, it continues on to multiple levels of beam shape modifiers. The modifiers
that a beam may pass through are specific to its modality. For a clinical photon
beam the general levels of beam shape modifiers, in order of passage by the beam,
are: (i) upper and lower collimator jaws; (ii) multi-leaf collimator; (iii) physical
wedge; (iv) physical field block; (v) physical compensating filter. For a clinical
electron beam the general levels of beam shape modifiers, in order of passage by
the beam, are: (i) upper and lower collimator jaws; (ii) physical electron cone; (iii)
physical field block; (iv) physical compensating filter.
The auxiliary systems attached to the LINAC provide critically important
resources and actions necessary to making a LINAC work. Some of these items
have been referenced above. There are many electric motors and tracking systems
for providing motion to the table, gantry, collimator, jaws, and mlc. Also needed is
a constant source of air pressure for driving targets, filters, and flattening foils. The
transport and accelerating waveguides also require either a source of dielectric gas
or a vacuum pump to provide the necessary conditions within the guides. Finally, a
water cooling system is needed to remove the massive amounts of heat being
created in parts of the machine such as accelerating waveguide and target.
A final detail to the understanding of how a LINAC works is how the
machine is calibrated in a clinical setting. Generally, the internal ion chambers
8
used the machine for measuring the beams are sealed and therefore not temperature
and pressure dependant.
This allows the machine to provide a consistent daily
output. The internally measured output of each beam is displayed in monitor units.
A LINAC is then calibrated such that a known number of monitor units will deliver
a known dose to given point in a homogeneous medium under certain setup
conditions. For example, a 6MV photon beam may be calibrated so that 100
monitor units delivered through a 10x10 cm2 field size to a point 1.6 cm deep in a
water phantom that is set to a measured distance of 100cm will result in the
measured dose of 100 cGy. The output for each beam must be calibrated in a
known set of similar conditions.
Treatment Planning
A course of treatment
It’s helpful to understand the general sequence of events in today’s planning
process, so here is a brief summary that begins after diagnosis and oncologist
consultations. A patient must first be imaged by CT and any additional modalities
needed to identify tumor(s). These study sets are transferred to a radiotherapy
treatment planning system (RTPS) where a dosimetrist will outline the patient’s
critical structures, and a physician will outline the tumor(s) to be targeted. Next the
physician prescribes a total dose to be delivered to the target volume(s) in a given
number of treatment fractions, along with any additional special details specific to
this course of treatment. The dosimetrist then, utilizing a RTPS, develops the best
way to deliver the prescribed dose to the target(s), while sparing critical structures.
Upon completion and review by the physician and physicist, the treatment plan is
considered ready for patient treatment. The patient will then come in the prescribed
number of days, where each day they will be precisely positioned and treated with
that day’s planned fraction.
A brief history
Treatment planning for external beam radiation therapy has evolved even
more than the LINACS used to deliver the therapy of the past 60 years. There is
9
one fundamental reason for this evolution, and that is to provide better patient care
and outcome. However, there are many supporting new/improved technologies and
expansions in knowledge that make this evolution in treatment planning and
improvement in treatment efficacy possible.
In the early days of treatment planning, all planning was performed in 2D.
This relied upon a pair of orthogonal x-ray films and physical clinical
measurements to provide the necessary information needed to perform the
calculations that determined the necessary number of monitor units needed for each
beam to deliver the desired dose to a prescribed point. These calculations were
performed by hand with the assistance of calculators.
The next major leap in treatment planning would not take place until
minicomputers allowed for CT scanners to become available for medical use and
greatly expanding processing capabilities. With these new computing and imaging
capabilities came the development of 3D RTP systems and 3D conformal therapy.
3D conformal therapy is just like 2D in that the goal is to hit a target while
minimizing harm to critical structures. However, now these RTP systems are
capable of prescribing, computing, and visualizing dose to a volume instead of a
single point. This method and these capabilities allow for much more complex
field shapes and overall plans
After 3D conformal planning came Intensity Modulated Radiation Therapy
(IMRT). IMRT utilized the ever increasing processing power of minicomputers
and a piece of new machine hardware known as the multi-leaf collimator (MLC).
The MLC is two opposing banks of “leaves”. The individual leaves are anywhere
from .25cm to 1cm wide and each bank is anywhere from 40 to 80 leaves. IMRT
plans still prescribe dose to a volume, just as 3D conformal, but now the planning
systems will create a series of segments which together make up a single beam.
Each segment of the beam has a different set of MLC leaves open to a different
pattern, creating final field where the intensity of the dose has been modulated over
the field to maximize target dose while minimizing dose to critical structures. To
10
create very highly modulated fields, the RTP system must use a large number of
small to very small segments. As noted in the introduction, this increasing reliance
upon small fields has largely driven the development of small measurement
chambers.
A few notes on other advances in planning and treatment. There are of
course on-going advances in planning and therapy, such as image guided radiation
therapy (IGRT) for more precise patient positioning and 4D planning/treatment that
will track and adjust beam on time to patient breathing cycle. Both of these build
on IMRT as an underlying principle of many small segments. Stereotactic
radiosurgery (SRS) is another form of advanced external beam radiation therapy
that also relies upon very small fields for treatment. SRS mainly differs from
traditional external beam therapy based on its exclusive use of small fields and a
much higher dose per fraction with a smaller number of fractions. SRS may be
delivered with some the same machines that can deliver traditional external beam
or it may be delivered with a specialty machine. I have not attempted to cover the
history or workings of dedicated SRS machines, but the following research is
relevant due their use of similar energies and field sizes.
Beam Measurement
Ionization Chambers
As stated by Knoll, Ion chambers in principle are the simplest of all gasfilled detectors. Their normal operation is based on collection of all the charges
created by direct ionization within the gas through the application of an electric
field. (Knoll, 2000) As simple as ion chambers may be, there have been entire
texts written them. In their most basic construction there are two conductive
materials with a gas between them and a constant dc voltage is applied. Once a
charged particle enters the chamber and interacts with a neutral molecule an ion
pair is formed. Ideally this freed electron is attracted to the positive electrode and
counted in the form of current. However, this electron may recombine, interact
again, or scatter entirely out of the detector.
11
Ion chambers used for beam measurements in radiation therapy have
traditionally been either of thimble or parallel plate constructions and open to the
atmosphere. This unsealed or vented construction means that the fill gas is not a
specific mixture, but rather just air, and the chamber readings must be corrected for
temperature and pressure variation. The chambers have needed to be robust and
respond consistently to a variety of poly-energetic beams from 4 to 25 megavolts.
By far the most common chamber traditionally used in radiation therapy is
the Farmer chamber. (see Figure 4) In 1955 a chamber was designed to provide a
stable and reliable secondary standard for x-rays and γ rays for all energies in the
therapeutic range. This chamber connected to a specific electrometer (to measure
ionization charge) and is known as the Baldwin-Farmer substandard dosimeter.
(Khan, 2010) The chamber came to be traditionally known as the Farmer chamber.
Later in 1972 the chamber was modified. By substituting pure graphite and pure
aluminum for the existing chamber materials the response curve over the
therapeutic range was flattened and made more consistent from one instrument to
another. (Aird & Farmer, 1972) Today’s Farmer chambers are minor evolutions of
the 1972 improvements.
Figure 4 – PTW 30013 Farmer Chamber
12
Compact chambers have arisen out of a need to measure more accurately
small fields and high dose gradient regions of beams. These chambers are also of
the thimble design, vented, and may or may not have the same wall and electrode
materials. The biggest difference is the physical dimensions of the chamber. The
Farmer chamber cavity is 23.1mm long and 6.1mm in diameter. Currently
available compact chamber cavities vary from 3 to 6mm long and 1 to 3mm in
diameter. (see Figure 5)
Figure 5 – IBA Welhoefer cc01 Compact Chamber
Using air filled chambers with walls made of air-equivalent material also
allows for dose to be computed and measured based on the Bragg-Gray principle.
Bragg-Gray states that the absorbed dose in a given material can be deduced from
the ionization produced in a small gas-filled cavity within that material. (Knoll,
2000) This same principle allows for the computation of dose in water when using
the same chambers in a water phantom.
Electrometers
Defined by Knoll, an electrometer indirectly measures the current by
sensing the voltage drop across a series resistance placed in the measuring circuit.
(Knoll, 2000) In a basic sense, and without getting into electrical circuits, the
13
electrometer performs two important functions. One, the electrometer provides a
consistent and stable 300 volts DC to the ion chamber. Two the electrometer is
able to measure the accumulated charge while the chamber is being exposed to the
beam.
3D Beam Scanning Systems
The ability to accurately measure and visualize what was happening in 3D
with a clinical beam is another problem that has been greatly alleviated with
advancements in computing technology. It’s also a subject whose history has not
been well documented in the literature. 3D beam scanning systems utilize large
water tanks, and a collection of motors, sensors, ion chambers, electrometers, and
computer control equipment. These systems allow for an ion chamber to be moved
and tracked in 3D through a field while keeping track of the relative dose readings
along the prescribed course of motion. The most common scans performed are
percent depth dose (PDD) scans and profile scans. PDD scans drive the ion
chamber to a given depth along the central axis (CAX), then measures the relative
dose along the CAX from that depth to the surface. A profile scan measures the
beam at a given depth along the in-plane or cross-plane direction from a few cm
outside one field edge to a few cm beyond the opposite field edge.
14
Equipment Used and Data Collection Conditions
Equipment/Parameters
LINAC: Varian Trilogy
3D scanning system: IBA Wellhofer Blue Phantom with CU500E controller
Ion Chambers: PTW Type 30013 Farmer; IBA Wellhofer cc13, cc04, and cc01 (see
Appendix 1 for technical specs)
Electrometer: Fluke Biomedical Systems Model 35040
LINAC rep rate: 400MU/min
SSD to water for scans: 100 cm
SSD to water for field size dependence: 90 cm
Depth for profile scans: Dmax for energy chosen (dmax is the point of maximum
dose deposition)
Depth for field size dependence: 10 cm
Data Collection Conditions
Due to the large amount of data collected, it was impossible to collect all
data in one session. Each set of scans to be inter-compared were captured back to
back in the same data collection session. Before each session the water tank was
leveled and centered. A centering routine was run within the scanning software to
verify centering for each energy before capturing the corresponding data set. All
scans were normalized to the given energy’s established dmax. No data is intercompared that was collected during different collection sessions.
For all scans the detector was horizontally placed with the cable pointing
towards the gantry. All percent depth dose (pdd) scans were captured from some
depth moving towards the surface. All profile scans compared are cross-plane
scans. The cross-plane direction sweeps the chamber in the left/right axis. For
each profile scan the direction of chamber motion was reversed. There were no
changes made to the chamber mounting or orientation during scanning.
Three scans or measurements were made with each detector at each speed for all of
the following samples. As necessary for inter-chamber or inter-speed comparison,
15
the three measurements were averaged, and each corresponding result is then used
for these comparisons. The detector speed chosen for inter-chamber comparison
was slow speed for small fields, and medium speed for all others.
Data Analysis Legend
For the purposes of data analysis we must define small, medium, and large field
sizes: Small fields < 3x3 cm2 <Medium fields < 20x20 cm2 < Large fields. When
comparing speed changes on graphs for a given chamber, fast scans are represented
red, medium scans green and slow scans blue. When comparing chamber changes
on graphs, the cc13 chamber is represented in red, the cc04 chamber in green, and
the cc01 chamber in blue.
Additional Notes on Data Collection
The scans chosen to be presented here are the result of analysis of many
data sets collected by many physicists on many different machines. The number of
combinations of energies and field sizes available for treatment is very large, and
it’s simply not feasible to compare all energies and field sizes in one study. An
attempt has been made to create reasonable sample for discussion of established
questions.
16
Data Presentation and Analysis
Detector Speed Change
As stated in the introduction, collecting data with a 3D scanning system can
be a very time consuming process. LINACs and RTP systems are large capital
expenditures made by the organizations that do no good sitting idle. Whether it is a
new installation or verifying a machine after major repair, it is important to both
patients and administrators that the LINAC be brought to a clinical status in the
most efficient and accurate way possible.
The ability to change the detector motion is not entirely new for scanning
systems, but it has been made much easier to manipulate in the more recent
versions. The IBA Wellhofer Blue Phantom scanning system used for these scans
offered three pre-defined speeds (slow, medium, and fast) for detector motion
during scans. The scanning software is programmed to attempt a minimum number
of counts relative to its speed of motion. The result is that the speed of detector
motion is relative to the rep rate of the LINAC, i.e. the slow speed with the LINAC
running at 100MU/min is slower than the slow speed with the LINAC running at
400MU/min and the size of the chamber being used if there is a large enough
difference in collection volume. The most common rep rates used in clinical
treatment are 300MU/min and 600MU/min, which led the choice of 400 MU/min
for this data set. With the LINAC running at 400 MU/min the variable speed
results were: slow = .17cm/sec; medium = .51cm/sec; fast = 1.36cm/sec.
Small fields – Data
The small field chosen to sample for comparing speed change with different
chambers was a 6MV photon field of size .5x.5cm2. The IBA Welhoefer cc13 was
excluded from the comparison due to its size relative to the field size.
IBA Welhoefer - cc01
Figures 17 thru 24 in the appendix show the graphs and statistics for all
cc01 scans for this field size. Figures 25 thru 32 show the graphs and statistics for
all cc04 scans for this field size
17
Small fields - Analysis
Regardless of which compact ion chamber was used the analytical results
are the same. Tables 1 thru 13 show the cc01 and cc04 data statistics for profile
and pdd scans. There was little difference in the spread of results, and
unexpectedly for profiles, the slow speed scans did not necessarily give the tightest
statistical data spread. However, the medium and slow speed scans do have the
appearance of being smoother overall. With that being said there are some
differences between pdds and profiles.
Table 1 – 6 MV Photons, .5x.5cm2, cc01, profile, fast
Chamber:
2
.5x.5 cm
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Speed:
fast
Scan 1 Scan 2 Scan 3 Spread
2.0
3.1
2.4
1.1
21.8
21.7
23.0
1.3
.21:.22 .22:.22 .21:.21 0.01
0.46
0.47
0.46
0.01
Table 2 – 6 MV Photons, .5x.5cm2, cc01, profile, medium
Chamber:
.5x.5 cm2
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Speed:
med
Scan 1 Scan 2 Scan 3 Spread
5.1
2.3
2.1
2.9
22.2
22.2
22.1
0.1
.21:.20 .21:.21 .22:.21 0.01
0.45
0.45
0.45
0
Table 3 – 6 MV Photons, .5x.5cm2, cc01, profile, slow
Chamber:
2
.5x.5 cm
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Speed:
slow
Scan 1 Scan 2 Scan 3 Spread
2.0
3.8
2.3
1.8
22.1
22.6
22.0
0.6
.19:.20 .20:.20 .21:.20 0.02
0.46
0.45
0.45
0.01
18
Table 4 – 6 MV Photons, .5x.5cm2, cc01, profile, fast
Chamber:
2
.5x.5 cm
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
fast
Scan 1 Scan 2 Scan 3 Spread
2.3
1.7
2.2
0.6
22.0
22.8
22.4
0.8
.27:.26 .27:.27 .28:.27 0.01
0.56
0.56
0.55
0.01
Table 5 – 6 MV Photons, .5x.5cm2, cc01, profile, medium
Chamber:
2
.5x.5 cm
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
med
Scan 1 Scan 2 Scan 3 Spread
2.7
2.0
2.4
1.7
22.9
21.0
22.5
1.9
.27:.26 .26:.26 .27:.26 0.01
0.55
0.55
0.56
0.01
Table 6 – 6 MV Photons, .5x.5cm2, cc01, profile, slow
Chamber:
2
.5x.5 cm
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
slow
Scan 1 Scan 2 Scan 3 Spread
2.9
2.2
3.2
1.0
21.8
22.9
21.9
1.1
.26:.26 .26:.25 .26:.26 0.01
0.54
0.56
0.54
0.02
Table 7 – 6 MV Photons, .5x.5cm2, cc01, pdd, fast
Chamber:
2
.5x.5 cm
R100 (dmax,cm):
D100 (dose at 10cm,%):
cc01
Scan 1
1.06
55.9
Scan 2
1.23
55
Speed:
fast
Scan 3
1.06
54.1
Spread
0.17
1.8
19
Table 8 – 6 MV Photons, .5x.5cm2, cc01, pdd, medium
Chamber:
2
.5x.5 cm
R100 (dmax,cm):
D100 (dose at 10cm,%):
cc01
test
Speed:
med
Scan 1
1.07
53.8
Scan 2
1.15
54
Scan 3
1.07
53.2
Spread
0.08
0.8
Table 9 – 6 MV Photons, .5x.5cm2, cc01, pdd, slow
Chamber:
2
.5x.5 cm
R100 (dmax,cm):
D100 (dose at 10cm,%):
cc01
Scan 1
1.05
53.9
Scan 2
0.95
54.1
Speed:
slow
Scan 3
1.04
53.9
Spread
0.1
0.2
Table 10 – 6 MV Photons, .5x.5cm2, cc01, pdd, fast
Chamber:
cc04
.5x.5 cm2
R100 (dmax,cm):
D100 (dose at 10cm,%):
Scan 1
1.2
56.3
Scan 2
1.13
57.2
Speed:
fast
Scan 3
1.25
56.7
Spread
0.12
0.9
Table 11 – 6 MV Photons, .5x.5cm2, cc01, pdd, medium
Chamber:
cc04
.5x.5 cm2
R100 (dmax,cm):
D100 (dose at 10cm,%):
Scan 1
1.03
57.6
Scan 2
1.14
56.3
Speed:
med
Scan 3
1.09
56.3
Spread
0.11
1.3
Table 12 – 6 MV Photons, .5x.5cm2, cc01, pdd, slow
Chamber:
2
.5x.5 cm
R100 (dmax,cm):
D100 (dose at 10cm,%):
cc04
Scan 1
1.18
56.7
Scan 2
1.11
56.1
Speed:
slow
Scan 3
1.05
56.4
Spread
0.13
0.6
20
For profiles, there is not a question that the slow speed scans for both
chambers appear to give the best results. Figure 6 shows the cc01 profile scans for
all speeds. In this graph the smoother shape of the slower speed scans can plainly
be seen. There is also very small time penalty to be paid for scanning small field
profiles at slow speed, as the width of scans is typically only the field width + a 1-3
cm margin to capture the penumbra. To scan one cross-plane profile of a .5x.5cm2
field, total width with margin of 3x3cm2, at slow speed takes 18 seconds, while at
medium speed takes 6 seconds. If the margin is shrunk to a total field width of
2x2cm2, slow speed takes 12 seconds, and medium speed takes 4 seconds. By
optimizing the total profile scan width the time penalty can also be made as
minimal as possible.
Figure 6 – 6 MV Photons, .5x.5cm2, cc01, profile, all speeds
However, for pdds, the appearance of the slow speed scans is very noisy in
its raw, unsmoothed form, but still gave the overall best data results. The medium
speed scans also provided very consistent statistical results, and the time savings is
fairly significant. Figure 7 shows the cc01 profile scans for all speeds. Photon pdd
21
scans are typically made from a depth of 30cm moving the detector towards the
surface regardless of field size. A slow speed photon pdd scan for any field size,
therefore takes 2 minutes and 56 seconds. At medium speed the same pdd scan
takes 59 seconds, a savings of nearly 2 minutes.
Figure 7 – 6 MV Photons, .5x.5cm2, cc01, pdd, speeds
Small fields - Recommendation
Run profiles at slow speed and pdds at medium speed.
Medium fields – Data
The medium field chosen to sample for comparing speed change with
different chambers was a circular 6MeV electron field of diameter 3cm. An
electron field was chosen due to the fact that ion chambers are generally more
sensitive to perturbation effects in electron fields than in photon fields. As a
general rule a 3 cm circle or square is the smallest electron field scanned and
modeled for treatment. Smaller electron fields may be used, but special dosimetric
measurements in water must be performed on the fields before they can be
computed for dose to a patient.
22
Figures 33 thru 40 in the appendix show the graphs and statistics for all
cc01 scans for the 6 MeV electrons, 3cm field. Figures 41 thru 48 show the graphs
and statistics for all cc04 scans for the 6 MeV electrons, 3cm field. Figures 49 thru
56 show the graphs and statistics for all cc13 scans for the 6 MeV electrons, 3cm
field. Figure 57 in the appendix shows the 6 MeV electrons, 10x10 cm2, all
chambers, pdd.
Medium fields – Analysis
As with the small fields, regardless of which compact ion chamber was used
the analytical results are the same. Tables 13 thru 30 show the cc01, cc04, and
cc13 data statistics for both profile and pdd scans. The medium field analysis does
break down to some small differences based on beam modality.
Table 13 – 6 MeV electrons, 3cm circle, cc01, profile, fast
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Speed:
Scan 1
Scan 2
Scan 3
1.8
2.1
1.8
19.4
19.1
19.5
1.17:1.10 1.16:1.11 1.14:1.11
3.23
3.26
3.26
fast
Spread
0.3
0.4
0.03
0.03
Table 14 – 6 MeV electrons, 3cm circle, cc01, profile, medium
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Speed:
Scan 1
Scan 2
Scan 3
1.7
2.3
2.1
20.0
19.7
19.3
1.25:1.18 1.25:1.17 1.24:1.18
3.26
3.25
3.26
med
Spread
0.6
0.7
0.01
0.01
23
Table 15 – 6 MeV electrons, 3cm circle, cc01, profile, slow
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Speed:
Scan 1
Scan 2
Scan 3
2.0
2.0
2.1
19.0
19.5
18.9
1.20:1.11 1.17:1.10 1.16:1.09
3.22
3.23
3.24
slow
Spread
0.1
0.6
0.04
0.02
Table 16 – 6 MeV electrons, 3cm circle, cc04, profile, fast
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
Scan 1
Scan 2
Scan 3
1.8
1.3
1.1
20.2
19.8
19.3
1.26:1.24 1.27:1.22 1.26:1.21
3.25
3.27
3.26
fast
Spread
0.7
0.9
0.03
0.02
Table 17 – 6 MeV electrons, 3cm circle, cc04, profile, medium
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
Scan 1
Scan 2
Scan 3
1.3
1.8
1.2
19.9
20.5
19.9
1.27:1.22 1.28:1.23 1.29:1.24
3.26
3.26
3.25
med
Spread
0.6
0.6
0.02
0.01
Table 18 – 6 MeV electrons, 3cm circle, cc04, profile, slow
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
Scan 1
Scan 2
Scan 3
1.1
0.9
1.4
19.9
20.2
19.6
1.26:1.21 1.26:1.22 1.27:1.20
3.27
3.27
3.26
slow
Spread
0.5
0.6
0.01
0.01
24
Table 19 – 6 MeV electrons, 3cm circle, cc13, profile, fast
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc13
Speed:
Scan 1
Scan 2
Scan 3
1.7
1.1
1.9
19.5
19.4
19.6
1.19:1.13 1.19:1.14 1.20:1.13
3.22
3.23
3.22
fast
Spread
0.8
0.2
0.01
0.01
Table 20 – 6 MeV electrons, 3cm circle, cc13, profile, medium
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc13
Speed:
Scan 1
Scan 2
Scan 3
2.1
1.6
1.9
19.2
19.4
19.6
1.20:1.13 1.20:1.15 1.20:1.14
3.23
3.24
3.23
med
Spread
0.5
0.4
0.02
0.01
Table 21 – 6 MeV electrons, 3cm circle, cc13, profile, slow
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc13
Speed:
Scan 1
Scan 2
Scan 3
1.4
1.4
1.7
19.4
19.3
19.7
1.21:1.15 1.21:1.14 1.22:1.15
3.23
3.23
3.22
slow
Spread
0.3
0.4
0.01
0.01
Table 22 – 6 MeV electrons, 3cm circle, cc01, pdd, fast
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc01
Scan 1
1.20
2.29
Scan 2
1.21
2.29
Speed:
Scan 3
1.22
2.30
fast
Spread
0.02
0.01
25
Table 23 – 6 MeV electrons, 3cm circle, cc01, pdd, medium
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc01
Scan 1
1.09
2.30
Scan 2
1.10
2.29
Speed:
Scan 3
1.11
2.30
med
Spread
0.02
0.01
Table 24 – 6 MeV electrons, 3cm circle, cc01, pdd, slow
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc01
Scan 1
1.12
2.29
Scan 2
1.07
2.29
Speed:
Scan 3
1.08
2.29
slow
Spread
0.04
0
Table 25 – 6 MeV electrons, 3cm circle, cc04, pdd, fast
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc04
Scan 1
1.08
2.29
Scan 2
1.05
2.28
Speed:
Scan 3
1.1
2.27
fast
Spread
0.05
0.02
Table 26 – 6 MeV electrons, 3cm circle, cc04, pdd, medium
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc04
Scan 1
0.94
2.28
Scan 2
1.12
2.28
Speed:
Scan 3
1.02
2.26
med
Spread
0.16
0.02
Table 27 – 6 MeV electrons, 3cm circle, cc04, pdd, slow
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc04
Scan 1
1.11
2.27
Scan 2
1.04
2.29
Speed:
Scan 3
1.09
2.28
slow
Spread
0.07
0.02
26
Table 28 – 6 MeV electrons, 3cm circle, cc13, pdd, fast
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc13
Scan 1
1.05
2.27
Scan 2
0.98
2.26
Speed:
Scan 3
1.2
2.27
fast
Spread
0.22
0.01
Table 29 – 6 MeV electrons, 3cm circle, cc13, pdd, medium
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc13
Scan 1
0.98
2.28
Scan 2
1.12
2.29
Speed:
Scan 3
1.06
2.28
med
Spread
0.14
0.01
Table 30 – 6 MeV electrons, 3cm circle, cc13, pdd, slow
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc13
Scan 1
1.08
2.29
Scan 2
0.99
2.28
Speed:
Scan 3
1.12
2.28
slow
Spread
0.13
0.01
Unexpectedly, the cc01 and cc04 medium speed electron pdd scans show
two odd humps in the graph, as seen on figures 34 and 42 in the appendix, at 2.5
cm deep and 2.7 cm deep. The cc13 scan, figure 50, does not show the same
humps, probably due to the volume averaging effects of the larger chamber. The
humps do not appear as pronounced on the high speed scans, but this can be
attributed to the stretching/smoothing effect on the pdd of running the chamber at
high speed. Only the slow speed pdd scans appear to capture enough data to fully
represent these beam scans accurately.
The time penalty for slow speed electron pdd scans is energy dependant.
Unlike photons, the depth of the pdd scan must increase in proportion to the
electron energy. As a general rule, the electron pdd is 10% when the depth is equal
to half of the energy. Therefore to capture a full electron pdd the total depth must
27
equal half the energy, in cm, plus 1-3 cm. So, for 6 MeV electrons the total pdd
depth is typically 4-5 cm, but for 21e, the highest available electron energy, the
total pdd depth is typically 12-13 cm. At this highest available electron energy, the
slow speed scan time is 1 minute and 16 seconds, assuming the total depth of 13
cm. A medium speed scan takes 26 seconds, and a fast speed scan takes 10
seconds. This time penalty is not insignificant, but necessary to obtain the
necessary quality of scans.
Electron profiles did not differ much as a function of detector speed change.
The slow speed scans generally produced the tightest groups of data statistics, with
the medium and fast speed each close behind. Electron profiles must also include a
margin to capture the penumbra just as with photon profiles, therefore also have a
field size dependent time penalty. The medium speed appears to be a good
compromise of time and repeatability of data.
For photon fields, it was already established with the small field analysis
that photon pdds can be run at medium speed. There were not a selection of
medium size photon field profiles run, but instead we can infer the best speed based
on the electron analysis. Since the perturbation effect is generally accepted to be
smaller for photon fields, it’s reasonable to conclude that the photon pdds can also
be run at medium speed to again, reach a good compromise of time and
repeatability of data.
Medium fields – Recommendation
Electron pdds should be run at slow speed. Photon pdds should be run at
medium speed. Electron and photon profiles should be run at medium speed.
Large Fields – Data
For examining the speed change on large fields, as 6 MeV electrons
25x25cm2 field was chosen. Figures 58 thru 65 in the appendix show the graphs
and statistics for all cc01 scans for this field size. Figures 66 thru 73 show the
graphs and statistics for all cc04 scans for this field size. Figures 74 thru 81 show
the graphs and statistics for all cc13 scans for this field size.
28
Large Fields – Analysis
As with the small and medium fields, regardless of which compact ion
chamber was used the analytical results are the same. The profiles and pdds did not
differ much as a function of detector speed change. Again, the slow speed scans
generally produced the tightest groups of data statistics, with the medium and fast
speed scans each giving a slightly wider grouping with the each increase in speed. .
Tables 31 thru 48 show the cc01, cc04, and cc13 data statistics for both profile and
pdd scans. As the field sizes continue to increase so do the time penalties for using
the slow or medium speeds relative to the fast speed.
Table 31 – 6 MeV electrons, 25x25 cm2, cc01, profile, fast
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Scan 1
2.1
2.1
1.01:1.00
25.63
Scan 2
2.7
1.8
.99:1.00
25.63
Speed:
Scan 3
1.8
1.9
.98:.99
25.63
fast
Spread
0.9
0.3
0.03
0
Table 32 – 6 MeV electrons, 25x25 cm2, cc01, profile, medium
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Scan 1
2.2
1.8
1.02:1.01
25.62
Scan 2
3.1
2.3
.99:1.02
25.65
Speed:
Scan 3
2.7
2.1
1.00:.99
25.63
med
Spread
0.9
0.5
0.03
0.03
29
Table 33 – 6 MeV electrons, 25x25 cm2, cc01, profile, slow
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Scan 1
1.7
1.6
1.00:1.01
25.63
Scan 2
2.0
2.0
.98:1.02
25.63
Speed:
Scan 3
2.0
1.7
.99:.99
25.63
slow
Spread
0.3
0.4
0.03
0
Table 34 – 6 MeV electrons, 25x25 cm2, cc04, profile, fast
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
Scan 1
Scan 2
Scan 3
1.7
1.9
1.9
1.8
1.9
1.7
1.06:1.08 1.08:1.09 1.07:1.09
25.65
25.63
25.66
fast
Spread
0.2
0.2
0.02
0.03
Table 35 – 6 MeV electrons, 25x25 cm2, cc04, profile, medium
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
Scan 1
Scan 2
Scan 3
1.5
1.7
1.8
1.5
1.8
1.6
1.07:1.09 1.08:1.09 1.08:1.07
25.63
25.64
25.64
med
Spread
0.3
0.3
0.02
0.01
Table 36 – 6 MeV electrons, 25x25 cm2, cc04, profile, slow
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
Scan 1
Scan 2
Scan 3
2.1
2.6
2.5
1.7
1.9
1.9
1.07:1.08 1.07:1.07 1.08:1.09
25.65
25.65
25.63
slow
Spread
0.5
0.2
0.02
0.02
30
Table 37 – 6 MeV electrons, 25x25 cm2, cc13, profile, fast
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc13
Scan 1
1.8
1.6
.95:.95
25.58
Scan 2
2.1
1.4
.94:.94
25.59
Speed:
Scan 3
1.8
1.3
.95:.94
25.61
fast
Spread
0.3
0.3
0.01
0.03
Table 38 – 6 MeV electrons, 25x25 cm2, cc13, profile, medium
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc13
Scan 1
1.6
1.7
.95:.94
25.59
Speed:
Scan 3
1.5
1.5
.93:.95
25.58
Scan 2
1.7
1.5
.93:.94
25.58
med
Spread
0.2
0.2
0.02
0.01
Table 39 – 6 MeV electrons, 25x25 cm2, cc13, profile, slow
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc13
Scan 1
1.6
1.4
.93:.95
25.61
Scan 2
1.6
1.5
.93:.94
25.59
Speed:
Scan 3
1.7
1.6
.94:.94
25.59
slow
Spread
0.1
0.2
0.01
0.02
Table 40 – 6 MeV electrons, 25x25 cm2, cc01, pdd, fast
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc01
Scan 1
1.30
2.27
Scan 2
1.18
2.28
Speed:
Scan 3
1.32
2.28
fast
Spread
0.14
0.01
31
Table 41 – 6 MeV electrons, 25x25 cm2, cc01, pdd, medium
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc01
Scan 1
1.29
2.27
Scan 2
1.15
2.27
Speed:
Scan 3
1.28
2.27
med
Spread
0.14
0
Table 42 – 6 MeV electrons, 25x25 cm2, cc01, pdd, slow
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc01
Scan 1
1.29
2.28
Scan 2
1.23
2.27
Speed:
Scan 3
1.18
2.28
slow
Spread
0.11
0.01
Table 43 – 6 MeV electrons, 25x25 cm2, cc04, pdd, fast
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc04
Scan 1
1.24
2.28
Scan 2
1.31
2.27
Speed:
Scan 3
1.27
2.26
fast
Spread
0.07
0.02
Table 44 – 6 MeV electrons, 25x25 cm2, cc04, pdd, medium
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc04
Scan 1
1.25
2.26
Scan 2
1.24
2.26
Speed:
Scan 3
1.3
2.26
med
Spread
0.06
0
Table 45 – 6 MeV electrons, 25x25 cm2, cc04, pdd, slow
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc04
Scan 1
1.22
2.27
Scan 2
1.26
2.27
Speed:
Scan 3
1.26
2.27
slow
Spread
0.04
0
32
Table 46 – 6 MeV electrons, 25x25 cm2, cc13, pdd, fast
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc13
Scan 1
1.24
2.29
Scan 2
1.34
2.31
Speed:
Scan 3
1.31
2.31
fast
Spread
0.1
0.02
Table 47 – 6 MeV electrons, 25x25 cm2, cc13, pdd, medium
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc13
Scan 1
1.22
2.32
Scan 2
1.33
2.32
Speed:
Scan 3
1.34
2.33
med
Spread
0.12
0.01
Table 48 – 6 MeV electrons, 25x25 cm2, cc13, pdd, slow
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc13
Scan 1
1.23
2.33
Scan 2
1.3
2.32
Speed:
Scan 3
1.28
2.32
slow
Spread
0.07
0.01
Margins for profiles must also increase with field size, which further
increases the time penalty for slower speed scans. An example 25x25cm2 field
needs a total width of 32x32cm2 to completely capture the needed profile
information. A slow speed scan takes 3 minutes and 14 seconds. The same scan at
medium speed takes 1 minute and 3 seconds, and a fast scan 24 seconds.
Large fields – Recommendation
If a LINAC is being spot checked after repair to verify that there have not
been major changes, then running scans at high speed is sufficient. However, if
collecting data for beam model commissioning, the medium speed is a better choice
for the best compromise of time versus quality of data.
33
Physical Detector Change
Studying small chamber profile scanning and output measurements is not a
new endeavor. There have been numerous studies done and journal articles written
on the subjects. (Das, Ding, & Ahnesjo, 2007) (Fox C. , et al., 2010) (Klein, Tailor,
Archambault, Wang, Therriault-Proulx, & Beddar, 2010) (Sauer & Wilbert, 2007)
(Pappas E. , et al., 2008) (Agostinelli, Garelli, Piergentili, & Foppiano, 2008)
(Sahoo, Kazi, & Hoffman, 2008) (Fox C. , Simon, Li, Palta, Liu, & Simon, 2007)
(Pappas T. G., et al., 2006) (Dawson, Schroeder, & Hoya, 1985) (Sibata, Mota,
Bedder, Higgins, & Shin, 1991) The lessons and recommendations of these studies
can be brought together into several relevant points. One, use a detector that is as
minimally perturbing as reasonably achievable that has the appropriate response
characteristics for the field size being measured. Two, the cc01 and other small
diode detectors are known to over-respond to large fields. Three, small fields need
as small a detector as possible and the cc01 is good down to a .5x.5cm2 field. Four,
the measured penumbra width varies linearly with detector radius. Five, the stem
effect is fairly significant for farmer chambers, but not nearly as significant for
compact chambers. Six, the near surface response of detectors improves with
decreasing measurement volume dimensions.
For the following comparisons, three measurements were made with each
detector. As necessary when putting all chambers on one graph, the three
measurements were averaged, and each corresponding result used for that
comparison.
Chamber response to field size change – Data
As was just noted, the cc01 is known to over-respond to large fields.
Agostinelli et al., studied this phenomenon and also established that the overresponse could be eliminated by applying a polarity correction. (Agostinelli,
Garelli, Piergentili, & Foppiano, 2008) To examine the response characteristics for
each compact chamber a series of field size dependence measurements were made
with each chamber and all measurements normalized to the 10x10 cm2 reading of
34
that particular chamber. These were then compared against the traditional Farmer
chamber. As the over-response only becomes significant at the largest field sizes, it
is only necessary to run these measurements for photon fields. Figure 8 shows the
graphs of field size dependence for 6MV photons, and figure 9 shows the same for
18MV photons.
Normalized Output
0.345
0.299
0.204
0.067
cc04
cc13
Farmer
0.5
cc01
0.000
0.200
0.400
0.600
0.800
1.000
1.200
1.400
0.253
0.577
0.638
0.670
1
0.616
0.776
0.782
0.778
2
0.809
0.825
0.827
0.818
3
0.858
0.860
0.862
0.853
4
0.919
0.919
0.920
0.911
6
0.960
0.964
0.964
0.959
8
1.000
1.000
1.000
1.000
10
1.029
1.029
1.030
1.028
12
1.062
1.064
1.065
1.067
15
20
1.106
1.106
1.106
1.115
Figure 8 – Field Size Dependence – 6 MV photons
1.131
1.138
1.135
1.152
25
1.162
1.164
1.160
1.183
30
1.180
1.184
1.177
1.205
35
1.197
1.200
1.189
1.221
40
35
Normalized Output
0.477
0.248
0.178
cc13
Farmer 0.059
0.525
0.246
cc04
0.550
1
0.297
0.5
cc01
0.000
0.200
0.400
0.600
0.800
1.000
1.200
0.599
0.732
0.744
0.757
2
0.813
0.833
0.837
0.835
3
0.877
0.886
0.887
0.884
4
0.942
0.943
0.943
0.940
6
0.970
0.977
0.976
0.975
8
1.000
1.000
1.000
1.000
10
1.018
1.019
1.019
1.019
12
1.037
1.038
1.038
1.041
15
1.061
1.061
1.061
1.067
20
Figure 9 – Field Size Dependence – 18 MV photons
1.075
1.079
1.078
1.087
25
1.094
1.095
1.093
1.106
30
1.101
1.108
1.106
1.120
35
1.117
1.119
1.116
1.132
40
36
37
Chamber response to field size change – Analysis
The results of these output readings support previous work done on the
subject. The cc01 begins to over-respond at 20x20 cm2 for 6MV photons, and
25x25 cm2 for 18MV photons. As expected based on its size, the Farmer chamber
begins to under-respond at 4x4 cm2 for both photon energies. Also of
consideration are the responses of the compact chambers. For 6 MV photons the
cc01, cc04, and cc13 all agree down to a 2x2 cm2 field size. For 18 MV photons,
they only agree to a 3x3 cm2 field size. Below the noted small field sizes only the
cc01 gives accurate output readings to field sizes down to .5x.5 cm2. (Sauer &
Wilbert, 2007)
Small fields – Data
Again, a 6 MV photon, .5x.5 cm2 field was chosen for the small field
comparison. Figures 82 thru 85 in the appendix show the pdds for each chamber,
then all chambers combined on one graph. Figures 86 thru 89 show the same for
profiles.
Small fields - Analysis
The pdd data presented here is strictly for the interest of discussion. As
already referenced from other papers and shown with the field size dependence
graphs, the only valid chamber for this field size is the cc01. All of the chambers
were normalized to the same depth, so that does make the combined pdd graph ,
figure 10 below and 85 in appendix, a little interesting. Up to the established
energy dmax of 1.6 cm the cc01 records a higher relative dose. However, beyond
dmax the graph lines cross and both the cc04 and cc13 begin to record more dose.
The amount of dose discrepancy increases with depth and is larger for the cc13 than
the cc04. The reason for this discrepancy was not study, but for a quick hypothesis,
I would suggest that it is caused by the increased scatter component at depth
combined with the larger collection volume creating this effect. Further study
would be interesting, but not clinically significant as it has already been determined
that the cc04 and cc13 are too large for this field size.
38
Figure 10 – 6 MV Photons, .5x.5cm2, cc01(blue)/cc04(green)/cc13(red), pdd
With all of the profiles graphed together, as shown in figure 11 below and
89 in the appendix, and normalized to 100% and field center, one can see the field
widening effect of the larger volume chambers. Also apparent from the statistics
(cc13 = 3 mm, cc04 = 2.6 mm, cc01 = 2 mm) is the penumbra widening effect of
the increasing radius of the ion chambers. Again, all of this data merely reinforces
studies already cited.
39
Figure 11 – 6 MV Photons, .5x.5cm2, cc01(blue)/cc04(green)/cc13(red), profile
Small field – Recommendation
For small fields down to the size sampled the cc01 is clearly the most
appropriate chamber of those tested for use.
Medium field – Data
The same 6 MeV, 6x6 cm2 cone with 3 cm diameter circle cutout electron
field was used for chamber size change comparison. In addition pdd scans for a 6
MeV electron and 6 MV photon 10x10 cm2 fields are included. Figures 90 thru 93
in the appendix show the pdds for each chamber, then all chambers combined on
one graph. Figures 94 thru 96 show the additional 10x10 cm2 pdd scans. Figures
97 thru 100 show the data and statistics for profiles.
Medium field – Analysis
When analyzed individually there is virtually no statistical difference in the
grouping of results. Tables 49 thru 54 show the cc01, cc04, and cc13 data statistics
for both profile and pdd scans. However, the raw appearance of the larger chamber
graphs at this speed is smoother. This smoother appearance can be attributed to the
volumetric smoothing effect of the larger chamber. As noted in the speed change
40
analysis the bumps that appear at depths 2.5 cm and 2.75 cm with the smaller
chambers can be avoided by scanning at a slower speed.
Table 49 – 6 MeV electrons, 3cm circle, cc01, profile
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Speed:
Scan 1
Scan 2
Scan 3
1.7
2.3
2.1
20.0
19.7
19.3
1.25:1.18 1.25:1.17 1.24:1.18
3.26
3.25
3.26
med
Spread
0.6
0.7
0.01
0.01
Table 50 – 6 MeV electrons, 3cm circle, cc04, profile
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
Scan 1
Scan 2
Scan 3
1.3
1.8
1.2
19.9
20.5
19.9
1.27:1.22 1.28:1.23 1.29:1.24
3.26
3.26
3.25
med
Spread
0.6
0.6
0.02
0.01
Table 51 – 6 MeV electrons, 3cm circle, cc13, profile
Chamber:
3 cm circle
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc13
Speed:
Scan 1
Scan 2
Scan 3
2.1
1.6
1.9
19.2
19.4
19.6
1.20:1.13 1.20:1.15 1.20:1.14
3.23
3.24
3.23
med
Spread
0.5
0.4
0.02
0.01
Table 52 – 6 MeV electrons, 3cm circle, cc01, pdd
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc01
Scan 1
1.09
2.30
Scan 2
1.10
2.29
Speed:
Scan 3
1.11
2.30
med
Spread
0.02
0.01
41
Table 53 – 6 MeV electrons, 3cm circle, cc04, pdd
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc04
Scan 1
0.94
2.28
Scan 2
1.12
2.28
Speed:
Scan 3
1.02
2.26
med
Spread
0.16
0.02
Table 54 – 6 MeV electrons, 3cm circle, cc13, pdd
Chamber:
3 cm circle
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc13
Scan 1
0.98
2.28
Scan 2
1.12
2.29
Speed:
Scan 3
1.06
2.28
med
Spread
0.14
0.01
The biggest and most interesting difference in the pdds is the shallow depth
response difference of the compact chambers. The 3 cm circle cutout pdds
responded in unexpected way, as shown in figure 12. Previous studies and research
would suggest that the cc01 chamber would provide the highest relative near
surface dose. This characteristic is supported by the 6 MeV electrons and 6 MV
photons 10x10cm2 pdd graphs and shown in figures 13 and 14. For both of these
fields the cc01 provides a more accurate near surface response. However, for the
3cm circle cutout, the cc13 provided the highest relative near surface dose
response. Further study is needed to completely explain this response, however it
should be noted that the cerrobend used to make the 3 cm cutout is known to
provide a hard to quantify energy and field size dependant photon scatter
contamination component. I would suggest this additional photon scatter
component combined with the larger collection volumes should largely compensate
for this unexpected result.
42
Figure 12 – 6 MeV electrons, 3cm circle, cc01/cc04/cc13, pdd
Figure 13 – 6 MeV electrons, 10x10cm2, cc01/cc04/cc13, pdd
43
Figure 14 – 6 MV photons, 10x10cm2, cc01/cc04/cc13, pdd
For the profile scans there is no statistically significant difference in any
parameter as a result of the analysis. Interestingly, the cc13, which has the largest
radius, actually reported the narrowest penumbra. However, for practical purposes,
all chambers reported essentially the same penumbral width, as the variation in
measurements was only .2 mm.
Medium fields – Recommendation
The recommendations for this set of fields are not quite as clear. Generally,
the cc01 shows to be a solid performing chamber. However, as field sizes move up
from the smallest in this range, there is no statistical difference among any of the
compact chambers tested except near the surface on pdd scans, and any could be
considered a satisfactory chamber for use. A special area of consideration for these
field sizes are the small electron cutouts. Further study needs to be done to further
explain and characterize the near surface response of the compact chambers, before
one can be suggested as the best for use.
44
Large fields – Data
As with the speed change comparison, a 6 MeV, 25x25 cm2, electron field
was used for chamber size change comparison. Also shown is a 6 MV, 40x40 cm2
photon field. Figures 101 thru 104 in the appendix show the pdds for each
chamber, and then all chambers combined on one graph. Figure 105 shows the
photon pdd graph. Figures 106 thru 109 show the data and statistics for the profile
scans.
Large fields – Analysis
Changing the chamber had virtually no effect on the largest electron field
pdd. All compact chambers provided comparable results. Tables 55 thru 60 show
the statistics for profile and pdd scans for the 25x25 cm2 electron scans. The only
exception was as expected in the near surface regions where the smaller chambers
provide a more accurate response. The 40x40 cm2 photon field provides further
evidence of the cc01 over-response to large fields. For the largest photon field,
both the cc04 and cc13 provided equivalent results.
Table 55 – 6 MeV electrons, 25x25 cm2, cc01, profile
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc01
Scan 1
2.2
1.8
1.02:1.01
25.62
Scan 2
3.1
2.3
.99:1.02
25.65
Speed:
Scan 3
2.7
2.1
1.00:.99
25.63
med
Spread
0.9
0.5
0.03
0.03
Table 56 – 6 MeV electrons, 25x25 cm2, cc04, profile
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc04
Speed:
Scan 1
Scan 2
Scan 3
1.5
1.7
1.8
1.5
1.8
1.6
1.07:1.09 1.08:1.09 1.08:1.07
25.63
25.64
25.64
med
Spread
0.3
0.3
0.02
0.01
45
Table 57 – 6 MeV electrons, 25x25 cm2, cc13, profile
Chamber:
Symmetry (%)
Flatness (%)
Penumbra (cm)
Field Width (cm)
cc13
Scan 1
1.6
1.7
.95:.94
25.59
Scan 2
1.7
1.5
.93:.94
25.58
Speed:
Scan 3
1.5
1.5
.93:.95
25.58
med
Spread
0.2
0.2
0.02
0.01
Table 58 – 6 MeV electrons, 25x25 cm2, cc01, pdd
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc01
Scan 1
1.29
2.27
Scan 2
1.15
2.27
Speed:
Scan 3
1.28
2.27
med
Spread
0.14
0
Table 59 – 6 MeV electrons, 25x25 cm2, cc04, pdd
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc04
Scan 1
1.25
2.26
Scan 2
1.24
2.26
Speed:
Scan 3
1.3
2.26
med
Spread
0.06
0
Table 60 – 6 MeV electrons, 25x25 cm2, cc13, pdd
Chamber:
R100 (dmax,cm):
R50 (depth of 50% dose,cm):
cc13
Scan 1
1.22
2.32
Scan 2
1.33
2.32
Speed:
Scan 3
1.34
2.33
med
Spread
0.12
0.01
46
Figure 15 – 6 MV photons, 40x40 cm2, cc01/cc04/cc13, pdd
Observation of the profile scans show that the cc01, shown in figure 16
below and 106 in the appendix, is very noisy and upon analysis, results in the
widest data spread of all the chambers. It is reasonable to conclude that this poor
quality of data is because this field size is just beyond the upper limit of field sizes
where it has been established that the cc01 can accurately be used. The larger cc04
and cc13 chambers proved to be better for this field size. Both provided smoother
and more consistent data. There was no statistical difference between the cc04 and
cc13.
47
Figure 16 – 6 MeV, 25x25cm2, cc01, profile
Large fields – Recommendation
For the large field spectrum, the use of either the cc04 or cc13 would
provide adequate results across the entire range.
48
Conclusion
The results of this study further reinforce the importance of choosing the
correct chamber for the chosen field size. Whether the mistake is choosing too
large of a chamber for a small field size, or choosing a chamber that over-responds,
an incorrect modeling of beam parameters will result in incorrect calculations of
dose to be delivered to patients. Scanning at the most appropriate speed has also
been shown to affect both the quality and reproducibility of data acquired.
Generally, the smallest, minimally perturbing chamber available and
appropriate for the field sizes to be measured should be used for scanning. The
smallest chamber in these tests consistently, until it reached it maximum usable
field size, provided to the most consistent data. However, if field sizes are
restricted to the 4x4 cm2 to 20x20 cm2, then any of the compact chambers tested
can be used for data acquisition with a high degree of confidence in the data. The
cc13 chamber does begin to show some apparent volume averaging, but it is not
significant enough to disqualify its results. The penumbra widening effect was also
not an issue when comparing all but the smallest of field sizes.
The use of the slowest scanning speed also showed to be very important
when scanning small field sizes. The effect of detector speed change diminished
with field size, but still provided measurable changes. This is fortunate, since as
the penalty of the slower speed increases, the detector speed can be increased with
decreasing penalty. To maximize efficiency of time, it is also important to
optimize the field scan parameters. Scanning needlessly beyond field edges and
maximum depths of dose delivery can easily add more time than is saved by
increasing the speed.
49
Bibliography
Agostinelli, S., Garelli, S., Piergentili, M., & Foppiano, F. (2008). Response to high-energy
photons of PTW31014 PinPoint ion chamber with a central aluminum electrode. Medical
Physics , 3293-3302.
Aird, E. G., & Farmer, F. T. (1972). The design of a thimble chamber for the Farmer
dosemeter. Physics in Medicine and Biology , 169.
Das, I. J., Ding, G. X., & Ahnesjo, A. (2007). Small Fields: Nonequilibrium radiation
dosimetry. Medical Physics , 206-215.
Dawson, D. J., Schroeder, N. J., & Hoya, J. D. (1985). Penumbral measurements in water
for high energy photons. Medical Physics , 101-105.
Fox, C., Simon, T., Li, J., Palta, J., Liu, C., & Simon, W. (2007). Determination of a
Chamber Independent Profile. Medical Physics , 2433-2434.
Fox, C., Simon, T., Simon, B., Dempsey, J. F., Kahler, D., Palta, J. R., et al. (2010).
Assessment of the setup dependence of detector response functions for mega-voltage linear
accelerators. Medical Physics , 477-484.
Ginzton, E. L. (2010). An Early History: Invention of the Klystron. Retrieved 10 2, 2010,
from Varian Inc.: http://www.varianinc.com/cgibin/nav?corp/history/klystron&cid=KJLJIMJQFN
Greene, D., & Williams, P. (1997). Linear Accelerators for Radiation Therapy 2nd Ed.
New York: Taylor and Francis Group.
Khan, F. M. (2010). The Physics of Radiation Therapy 4th Ed. Baltimore: Lippincott,
Williams, and Wilkins.
Klein, D. M., Tailor, R. C., Archambault, L., Wang, L., Therriault-Proulx, F., & Beddar, A.
S. (2010). Measuring output factors of small fields formed by collimator jaws and multileaf
collimator using plastic scintillation detectors. Medical Physics , 5541-5549.
Knoll, G. F. (2000). Radiation Detectoin and Measurement 3rd Ed. Hoboken: John Wiley
& Sons, Inc.
Pappas, E., Maris, T. G., Zacharopoulou, F., Papadakis, A., Manolopoulos, S., Green, S., et
al. (2008). Small SRS photon field profile dosimetry performed using a PinPoint air ion
chamber, a diamond detector, a novel silicon-diode array (DOSI), and polymer gel
dosimetry. Analysis and intercomparison. Medical Physics , 4640-4648.
50
Pappas, T. G., Maris, A., Papadakis, A., Zacharopoulou, F., Damilakis, J., Papinakolaou,
N., et al. (2006). Experimental determination of the effect of detector size on profile
measurements in narrow photon beams. Medical Physics , 3700-3711.
Podgorsak, E. (2010). Radiation Physics for Medical Physicists 2nd Ed. Springer:
Springer-Verlag.
Podgorsak, E., Metcalf, P., & Van Dyk, J. (1999). Medical Accelerators - The Modern
Technology in Radiation Oncology: A Copendium for Medical Physicists and Radiation
Oncologists. Madison: Medical Physics Publishiing.
Sahoo, N., Kazi, A., & Hoffman, M. (2008). Study of the Effect of Size of the Beam
Scanning Diodes On Measurement of Beam Profiles of the Clinical High Energy Photon
Beams. Medical Physics , 2772-2773.
Sauer, O., & Wilbert, J. (2007). Measurement of output factors for small photom beams.
Medical Physics , 1983-1988.
Scandale, W. (2006). Introduction to Particle Accelerators. Rome: CERN-AT Department.
Sibata, C. H., Mota, H. C., Bedder, A. S., Higgins, P. D., & Shin, K. H. (1991). Influence
of detector size in photon beam measurements. Physics in Medicine and Biology , 621-628.
3
0.01
3.6
1
Shonka C552
0.4
Steel
1
cc01
0.04
3.6
2
Shonka C552
0.4
C552
1
cc04
0.13
5.8
3
Shonka C552
0.4
C552
1
Description
Commonly used air equivalent plastic
Polymethyl methacylate (acrylic)
Graphite
Material
Shonka C552
PMMA
Graphite
Farmer
0.6
23
3.05
PMMA/graphite
.335 PMMA / .09 graphite
Aluminum (99.98%)
1.1
1.76
1.19
1.85
Density (g/cm3)
Table 62 – Ion Chamber Wall Material Information
Cavity Volume (cm )
Cavity Length (mm)
Cavity Radius (mm)
Wall Material
Wall Thickness (mm)
Center Electrode Material
Diameter Center Electrode (mm)
Specification
Chamber
cc13
Table 61 – Ion Chamber Technical Details
Appendix
51
Figure 17 – 6 MV photons, .5x.5cm2, cc01, fast pdd
52
Figure 18 – 6 MV photons, .5x.5cm2, cc01, medium pdd
53
Figure 19 – 6 MV photons, .5x.5cm2, cc01, slow pdd
54
Figure 20 – 6 MV photons, .5x.5cm2, cc01, all speeds, pdd
55
Figure 21 – 6 MV photons, .5x.5cm2, cc01, fast profile
56
Figure 22 – 6 MV photons, .5x.5cm2, cc01, medium profile
57
Figure 23 – 6 MV photons, .5x.5cm2, cc01, slow profile
58
Figure 24 – 6 MV photons, .5x.5cm2, cc01, all speeds profile
59
Figure 25 – 6 MV photons, .5x.5cm2, cc04, fast pdd
60
Figure 26 – 6 MV photons, .5x.5cm2, cc04, medium pdd
61
Figure 27 – 6 MV photons, .5x.5cm2, cc04, slow pdd
62
Figure 28 – 6 MV photons, .5x.5cm2, cc04, all speeds pdd
63
Figure 29 – 6 MV photons, .5x.5cm2, cc04, fast profile
64
Figure 30 – 6 MV photons, .5x.5cm2, cc04, medium profile
65
Figure 31 – 6 MV photons, .5x.5cm2, cc04, slow profile
66
Figure 32 – 6 MV photons, .5x.5cm2, cc04, all speeds profile
67
Figure 33 – 6 MeV electrons, 3cm circle, cc01, fast pdd
68
Figure 34 – 6 MeV electrons, 3cm circle, cc01, medium pdd
69
Figure 35 – 6 MeV electrons, 3cm circle, cc01, slow pdd
70
Figure 36 – 6 MeV electrons, 3cm circle, cc01, all speeds pdd
71
Figure 37 – 6 MeV electrons, 3cm circle, cc01, fast profile
72
Figure 38 – 6 MeV electrons, 3cm circle, cc01, medium profile
73
Figure 39 – 6 MeV electrons, 3cm circle, cc01, slow profile
74
Figure 40 – 6 MeV electrons, 3cm circle, cc01, all speeds profile
75
Figure 41 – 6 MeV electrons, 3cm circle, cc04, fast pdd
76
Figure 42 – 6 MeV electrons, 3cm circle, cc04, medium pdd
77
Figure 43 – 6 MeV electrons, 3cm circle, cc04, slow pdd
78
Figure 44 – 6 MeV electrons, 3cm circle, cc04, all speeds pdd
79
Figure 45 – 6 MeV electrons, 3cm circle, cc04, fast profile
80
Figure 46 – 6 MeV electrons, 3cm circle, cc04, medium profile
81
Figure 47 – 6 MeV electrons, 3cm circle, cc04, slow profile
82
Figure 48 – 6 MeV electrons, 3cm circle, cc04, all speeds profile
83
Figure 49 – 6 MeV electrons, 3cm circle, cc13, fast pdd
84
Figure 50 – 6 MeV electrons, 3cm circle, cc13, medium pdd
85
Figure 51 – 6 MeV electrons, 3cm circle, cc13, slow pdd
86
Figure 52 – 6 MeV electrons, 3cm circle, cc13, all speeds pdd
87
Figure 53 – 6 MeV electrons, 3cm circle, cc13, fast profile
88
Figure 54 – 6 MeV electrons, 3cm circle, cc13, medium profile
89
Figure 55 – 6 MeV electrons, 3cm circle, cc13, slow profile
90
Figure 56 – 6 MeV electrons, 3cm circle, cc13, all speeds profile
91
Figure 57 – 6 MeV electrons, 10x10 cm2, all chambers, medium pdd
92
Figure 58 – 6 MeV electrons, 25x25 cm2, cc01, fast pdd
93
Figure 59 – 6 MeV electrons, 25x25 cm2, cc01, med pdd
94
Figure 60 – 6 MeV electrons, 25x25 cm2, cc01, slow pdd
95
Figure 61 – 6 MeV electrons, 25x25 cm2, cc01, all speeds pdd
96
Figure 62 – 6 MeV electrons, 25x25 cm2, cc01, fast profile
97
Figure 63 – 6 MeV electrons, 25x25 cm2, cc01, med profile
98
Figure 64 – 6 MeV electrons, 25x25 cm2, cc01, slow profile
99
Figure 65 – 6 MeV electrons, 25x25 cm2, cc01, all speeds profile
100
Figure 66 – 6 MeV electrons, 25x25 cm2, cc04, fast pdd
101
Figure 67 – 6 MeV electrons, 25x25 cm2, cc04, med pdd
102
Figure 68 – 6 MeV electrons, 25x25 cm2, cc04, slow pdd
103
Figure 69 – 6 MeV electrons, 25x25 cm2, cc04, all speeds pdd
104
Figure 70 – 6 MeV electrons, 25x25 cm2, cc04, fast profile
105
Figure 71 – 6 MeV electrons, 25x25 cm2, cc04, med profile
106
Figure 72 – 6 MeV electrons, 25x25 cm2, cc04, slow profile
107
Figure 73 – 6 MeV electrons, 25x25 cm2, cc04, all speeds profile
108
Figure 74 – 6 MeV electrons, 25x25 cm2, cc13, fast pdd
109
Figure 75 – 6 MeV electrons, 25x25 cm2, cc13, med pdd
110
Figure 76 – 6 MeV electrons, 25x25 cm2, cc13, slow pdd
111
Figure 77 – 6 MeV electrons, 25x25 cm2, cc13, all speeds pdd
112
Figure 78 – 6 MeV electrons, 25x25 cm2, cc13, fast profile
113
Figure 79 – 6 MeV electrons, 25x25 cm2, cc13, medium profile
114
Figure 80 – 6 MeV electrons, 25x25 cm2, cc13, slow profile
115
Figure 81 – 6 MeV electrons, 25x25 cm2, cc13, all speeds profile
116
Figure 82 – 6MV photons, .5x.5cm2, cc01, pdd
117
Figure 83 – 6MV photons, .5x.5cm2, cc04, pdd
118
Figure 84 – 6MV photons, .5x.5cm2, cc13, pdd
119
Figure 85 – 6MV photons, .5x.5cm2, all chambers pdd
120
Figure 86 – 6MV photons, .5x.5cm2, cc01, profile
121
Figure 87 – 6MV photons, .5x.5cm2, cc04, profile
122
Figure 88 – 6MV photons, .5x.5cm2, cc13, profile
123
Figure 89 – 6MV photons, .5x.5cm2, all chambers, profile
124
Figure 90 – 6 MeV electrons, 3cm circle, cc01, pdd
125
Figure 91 – 6 MeV electrons, 3cm circle, cc04, pdd
126
Figure 92 – 6 MeV electrons, 3cm circle, cc13, pdd
127
Figure 93 – 6 MeV electrons, 3cm circle, all chambers pdd
128
Figure 94 – 6 MeV electrons, 10x10 cm2, all chambers, pdd
129
Figure 95 – 6 MV photons, 10x10 cm2, all chambers, pdd
130
Figure 96 – 6 MV photons, 10x10 cm2, all chambers, pdd (near surface zoom)
131
Figure 97 – 6 MeV electrons, 3cm circle, cc01, profile
132
Figure 98 – 6 MeV electrons, 3cm circle, cc04, profile
133
Figure 99 – 6 MeV electrons, 3cm circle, cc13, profile
134
Figure 100 – 6 MeV electrons, 3cm circle, all chambers, profile
135
Figure 101 – 6 MeV electrons, 25x25cm2, cc01, pdd
136
Figure 102 – 6 MeV electrons, 25x25cm2, cc04, pdd
137
Figure 103 – 6 MeV electrons, 25x25cm2, cc13, pdd
138
Figure 104 – 6 MeV electrons, 25x25cm2, all chambers pdd
139
Figure 105 – 6 MV photons, 40x40cm2, all chambers, pdd
140
Figure 106 – 6 MeV electrons, 25x25cm2, cc01, profile
141
Figure 107 – 6 MeV electrons, 25x25cm2, cc04, profile
142
Figure 108 – 6 MeV electrons, 25x25cm2, cc13, profile
143
Figure 109 – 6 MeV electrons, 25x25cm2, all chambers, profile
144