Bruce J. Gerbi, Ph.D.
TGTG-70: Clinical electron beam dosimetry:
supplement to TGTG-25
Bruce J. Gerbi, Ph.D.
University of Minnesota, Minneapolis, MN
John Antolak,
Antolak, Ph.D.
David Followill,
Followill, Ph.D.
Michael Herman,
Herman, Ph.D.
Dimitris Mihailidis,
Mihailidis, Ph.D.
Ellen Yorke,
Yorke, Ph.D.
Ph.D.
F. Christopher Deibel,
Deibel, Ph.D.
Patrick D. Higgins, Ph.D.
M. Saiful Huq,
Huq, Ph.D.
David. W. O. Rogers, Ph.D.
TGTG-70
Task Group of the Radiation Therapy
Committee of the AAPM
Formally charged in April, 2001
Inception date: July 22, 2001
Sunset date: December, 2005
Consultants:
Consultants:
Faiz Khan, Ph.D.
Kenneth Hogstrom, Ph.D.
TGTG-70: Goal of the Task Group
Maintain the original intent of TGTG-25
– provide a useful set of procedures and
processes for the practicing clinical
physicist for the use of clinical electron
beams in the energy range from 55-25
MeV
– Not simply a rere-write of TGTG-25
• TGTG-25 was very well written and extensive
• Much of the information is still very useful
TG-51 Electron Beam Calibration
TGTG-70 Goals (continued)
To define clearly the tasks that a physicist
needs to perform with regards to highhighenergy electrons
To supplement the material of TGTG-25
To cover topics that are new since
TGTG-25 or that were not fully developed in
that report
1
Bruce J. Gerbi, Ph.D.
TGTG-70: Table of Contents
I. INTRODUCTION
II. NOTATION AND DEFINITIONS
III. DOSE MEASUREMENTS
A. Calibration protocol, TGTG-51
B. Electron beam quality specification
C. Dosimetry equipment
1. Ionization chambers
2. Phantoms
D. Measurement of central axis percentage depth dose in water
1. Measurements using cylindrical ionization chambers
2. Measurements using planeplane-parallel ionization chambers in water
3. Measurements using diodes in water
4. Water phantom considerations
TGTG-70: Table of Contents (2)
E. Output factors for electron beams
F. Dose determination in small, irregular electron fields
G. Non-water phantoms: Conversion of relative dose
measurements from non-water phantoms to water
1. Measurements using cylindrical ionization chambers in nonwater phantoms
2. Measurements using plane-parallel ionization chambers in
non-water phantoms
3. Film dosimetry
4. Measurement of central axis percentage depth dose using
non-water phantoms
IV. ELECTRON BEAM ALGORITHMS
V. ICRU 71 – Prescribing, Recording, and Reporting electron beam
therapy
TGTG-70: Table of Contents (3)
VI. CLINICAL APPLICATIONS OF ELECTRON BEAMS
A. Heterogeneities in electron treatments
B. The use of bolus in electron beam treatments
C. Electron field abutment
VII. LIBRARY OF CLINICAL EXAMPLES
A. Intact Breast- Tangent plus Electrons (IMC), Electron Boost
B. Chest Wall - Tangent plus Electrons, Electrons only, Conformal Bolus
C. Electron Arc
D. Total Scalp
E. Parotid
F. Nose
G. Eye – Eyelid, Retinoblastoma, Orbit
H. Posterior Cervical Nodes
I. Craniospinal
J. Intraoperative
K. Total Skin Electron Treatment (TSET)
L. Total Limb
III. A. Calibration Protocol, TGTG-51
TGTG-51 accomplished its two main objectives:
1. Incorporate the new absorbed dose to water standard
– Absorbed dose is more robust than the Air Kerma
Standard
– Dose to water is closer to the dose to tissue
2. Simplify the calibration formalism (as much as
possible)
Defines dose at one point, the reference point
VIII. REFERENCES
TG-51 Electron Beam Calibration
2
Bruce J. Gerbi, Ph.D.
TGTG-51
TGTG-51: Calibration equation for electrons
60
III.B. Electron beam quality specification
Electron beam quality specified by R50 , the 50% depth of dose
maximum instead of E0
R50 can be obtained:
– from the depth ionization curve in terms of I50
• Correct the raw depth ionization curve for depth by offoff-setting
the depth by 0.5 rcav toward surface for cylindrical chamber, no
offset for p-p chamber.
• Then use the following equations:
– R50 = 1.029 I50 – 0.06 (cm) 2
I50 10 cm
I50 > 10 cm
– R50 = 1.059 I50 – 0.37 (cm)
• The measurement must be made at 100 cm SSD.
DwQ = MPgrQ k R' 50 kecal N D ,Co
w [Gy ]
60
Co
Uses the new absorbed dose standard, N D ,w
Full calibration to be done in water only
Reference depth, dref = 0.6R
0.6R50 - 0.1 cm
rather than dmax
Protocol uses realistic electron beam data
for stopping powers of water to air
Uses new factors, k’R50 and kecal
DwQ = dose to water at beam quality, Q
M = corrected meter reading
PgrQ = gradient correction
k R' 50 = energy/chamber dependent factor
k ecal = factor tha t maps dose at 60Co energy into dose at reference
electron energy
60
N D ,Co
Co energy
w = absorbed dose to water chamber calibration factor at
60
III.C. Equipment, TGTG-70
Ionization chambers – calibration and relative
measurements
- Both cylindrical and planeplane-parallel chambers are
acceptable
Diodes – for measurement of %dd
%dd data
(checked v. ion chambers for accuracy)
Phantom material
–
–
–
directly from % Depth Dose curve using diode system
(checked for agreement with ion chamber data)
TG-51 Electron Beam Calibration
Water is preferred whenever possible
NonNon-water materials are allowed (but not for absolute
calibration, as per TGTG-51)
3
Bruce J. Gerbi, Ph.D.
PlanePlane-Parallel Chambers
PlanePlane-parallel chambers are recommended to
calibrate electrons of energy R50 ≤ 2.6 cm
Not many waterproof pp-p chambers
– Markus, NACP, Memorial, Exradin
–
p-p chambers cannot be waterproofed easily
Interface problems between pp-p chamber and
surrounding water medium have not been
addressed
Backscatter from p-p chamber material
Cylindrical Ion Chambers
Commonly available
Used routinely: calibration & automated
scanning systems
Gradient correction required
Fluence correction required
Corrected Meter Reading, M
Make measurement with chamber at dref to
obtain Mraw
– Correct Mraw for Pion, PTP , Pelec , Ppol to get M
M = Pion PTP Pelec Ppol M raw
– So, of course, you need these correction
factors
TG-51 Electron Beam Calibration
4
Bruce J. Gerbi, Ph.D.
Gradient Correction, Pgr
Pgr
Electron Beam Dosimetry, k´R50
Based on TGTG-21 formalism
(remember Prepl = PgrPfl)
– depends on the user’
user’s beam
– must be measured for each beam
k 'R50 =
M raw (d ref + 0.5rcav )
k ' R50 =
PgrQ =
M raw (d ref )
= 1.00
)
)
L water
ρ air
L water
ρ air
(cylindrical)
cylindrical)
• Prepl • Pcel |evaluated at arbitrary electron quality R50
• Prepl • Pcel |evaluated at electron quality R50 =7.5 cm
The energy/chamber dependent factor which relates dose at
an arbitrary electron energy, expressed as R50, to the
reference energy, R50 = 7.5 cm.
-Uses L/ρ for an electron spectrum representative of
realistic electron beams
(plane(plane-parallel)
-The values of k’R50 are appropriate only at dref
Determination of k´R50
Electron Beam Dosimetry, kecal
From available figures, or using analytical
fits (to within 0.2%)
Farmer cylindrical chambers, 2 ≤ R50 ≤ 9 cm:
k R' 50 (cyl ) = 0.9905 + 0.0710e (
− R50
3.67
)
PlanePlane-parallel chambers, 2 ≤ R50 ≤ 20 cm:
k R' 50 ( pp ) = 1.2239 − 0.145(R50 )
0.214
TG-51 Electron Beam Calibration
)
L water
k ecal =
ρ air
)
• Prepl • Pcel |evaluated at electron quality R50 =7.5 cm
L water
ρ air
• Prepl • Pwall • Pcel |evaluated at cobalt energy
-Factor that maps dose at CoCo-60 energy into dose at a reference
electron energy (that energy whose depth dose falls to 50% at
7.5 cm depth, ~18 MeV)
-Allows a specific electron chamber calibration factor for
cylindrical chambers (at a later date, when/if available)
-Allows the calibration of pp-p chambers by intercomparison
with a cylindrical chamber in a highhigh-energy electron beam
-kecal is independent of energy
5
Bruce J. Gerbi, Ph.D.
Determination of kecal
Cylindrical chambers
– look up the value in available table
PlanePlane-parallel chambers
– Can use tabled values but crosscross-calibration
versus a cylindrical chamber is recommended
kecal , PlanePlane-Parallel Ionization Chambers
kecal determined from crosscross-calibration with
cylindrical chamber at dref in water using highhighenergy electrons (recommended):
(k
60
Co
ecal N D , w
)
pp
=
=
TGTG-51: Dose Equation for Electrons
DwQ = MPgrQ k R' 50 kecal N D ,Co
w [Gy ]
(Dw )cyl
(Mk )
pp
'
R50
(MP k
Q '
gr R50 ecal
k
60
N D ,Co
w
(Mk )
)
cyl
pp
'
R50
TGTG-51 Calibration
60
DwQ = dose to water at beam quality, Q
M = corrected meter reading
PgrQ = gradient correction
k R' 50 = energy/chamber dependent factor
So now you have the calibration, or the
dose rate, at one point, dref
However, according to ICRU
specifications, the prescription dose is to
be reported at the dmax point
k ecal = factor tha t maps dose at 60Co energy into dose at reference
electron energy
60
N D ,Co
Co energy
w = absorbed dose to water chamber calibration factor at
60
TG-51 Electron Beam Calibration
6
Bruce J. Gerbi, Ph.D.
Absorbed Dose at dmax from dref
Determine the dose at dmax from that at dref
using clinical %dd data
%dd requires stoppingstopping-power ratios
– In TGTG-51, realistic stoppingstopping-power ratios (SPRs
(SPRs)) have
been used instead of monomono-energetic SPRs as in TGTG25
– Expression from Burns et al. as a function of R50
should be used
– Prepl (= Pfl X Pgr) is also required (TG21 & TG25)
Burns formula for clinical work
Accurate enough for clinical work
Realistic StoppingStopping-Power Ratios for Water
From Burns et al.
al. (in water)
L
ρ
w
(R50 , z ) =
air
a + b(ln R50 ) + c (ln R50 ) + d ( z R50 )
2
1 + e(ln R50 ) + f (ln R50 ) + g (ln R50 ) + h( z R50 )
2
3
c = 0.088670
g = 0.003085
d = - 0.08402
h = - 0.12460
Where:
a = 1.0752
e = - 0.42806
b = - 0.50867
f = 0.064627
These coefficients give an rms deviation of 0.4% and a max.
deviation of 1.0% for z/R50 between 0.02 and 1.1. The max. deviation
increases to 1.7% if z/R50 values up to 1.2 are considered.
Measurement of CA %dd
%dd Curve
Usually done using automated water scanning
system
– Exception of magnetically swept beams
– Regions close to the surface
– Can use cylindrical or plane-parallel chambers
– Each type has its advantages & disadvantages OR
corrections that are needed
Problem: if corrections are not applied in the scanner
software, it is difficult to access data and apply
corrections
Rogers DW, Med. Phys. 2004
TG-51 Electron Beam Calibration
7
Bruce J. Gerbi, Ph.D.
Cylindrical chamber corrections
Apply gradient correction to raw depth
ionization data
Need to apply Realistic Stopping Power
data to depth ionization data
Need to apply fluence correction as a
function of electron energy at depth
II. B. Cone Factors for Electron Beams
Should be determined at dmax
– to minimize perturbations
– to minimize positioning uncertainty
Potential problems
– significant contamination by low energy electrons
moving dmax toward surface
– dmax is broad for high energy electrons
Possible solution (IPEMB, 1996)
PlanePlane-parallel chamber corrections
No gradient correction
Need to apply Realistic Stopping Power
data to depth ionization data
No fluence correction except for Markus &
Capintec chambers (TG39)
Pwall small correction (1-2%)
Ppol small effect but needs to be checked
II. C. Measurements in nonnon-water phantoms
Water is recommended for absolute calibration
Output checks can be done in plastics
Plastic phantoms may be more convenient in
certain situations
• low energy electron beams
• use of planeplane-parallel chambers
There is added complexity to convert dose in
plastic to dose in water using ion chambers
– Use dmax or 0.5R
0.5R50 , whichever is greater
TG-51 Electron Beam Calibration
8
Bruce J. Gerbi, Ph.D.
Use of nonnon-water phantoms
Water substitutes should mimic water across the whole
electron energy range
– mainly in stopping and scattering powers
• thus, both the electron density and the effective atomic
number should be matched to water
• in practical phantoms, this is difficult to achieve (due to the
carbon in plastics)
– offoff-thethe-shelf material can have large variations in
density and scattering power
• Must be careful when using these materials
• Check the density of the plastic being used, composition is
more difficult to verify
Measurements of Absorbed Dose in nonnonwater Phantoms
The SSD and field size are not to be
scaled
Chamber must be positioned with its
effective point of measurement at the
equivalent scaled reference depth in the
plastic phantom
TG-51 Electron Beam Calibration
Use of nonnon-water Phantom Materials
Depths need to be scaled
Chamber readings need to be multiplied
by an appropriate fluencefluence-ratio correction
StoppingStopping-power ratios should be taken at
the scaled depth
Charge storage effects should be kept in
mind using polystyrene and PMMA
III. D. Dose Determination in Small/Irregular Fields
Inherent Problems in Dosimetry of Small
Electron Fields
– depth of dmax becomes shallower
– the output factor may be significantly
different than the cone factor if the field size
is smaller than ~ E (MeV) /2.5 cm.
– isodose coverage is reduced in all directions
as the field shrinks
9
Bruce J. Gerbi, Ph.D.
Change in %DD v. Field Size
9 MeV Fractional Depth Dose
vs Field Size
When is special dosimetry required?
16 MeV Fractional Depth Dose
vs Field Size
1.2
1.0
1.0
0.8
10x10 open
0.8
3.4 cm diam
4 cm diam
fdd
3.4cm diam
fdd
10x10 open
0.6
5 cm diam
0.6
When the minimum field dimension is less
than the minimum radius of a circular field
that produces lateral scatter equilibrium
Req = 0.88 E p , 0
4 cm diam
0.4
0.4
where
0.2
0.2
E p , 0 = 0.22 + 1.98 R p + 0.0025R p2
0.0
0.0
0
1
2
3
4
5
6
0
depth (cm)
2
4
6
8
10
depth (cm)
12
Khan F & Higgins PD, PMB 44 (1999), N77N77-N80
Possible Methods for Small Field Dosimetry
Models for Small Field Dosimetry
Measurements of output and depth dose
data
Several publications give calculational
methods to approximate output
– film dosimetry
– ionization chamber dosimetry
Model to describe change in output, depth
of dmax, and isodose changes
TG-51 Electron Beam Calibration
– Square Root Method (Phys
Med Biol, 1981))
(
– Khan Model using Lateral Buildup Ratios
(LBRs)
LBRs) (Phys Med Biol. 1998 43:274143:2741-54, Phys
Med Biol. 44:N7744:N77-N80, Phys Med Biol. 2001
46:N946:N9-14)
– Jones approach (Br J Radiol.
Radiol. 1990 63:5963:59-64)
– Jursinic approach (Med Phys. 1997 24:176524:1765-9)
10
Bruce J. Gerbi, Ph.D.
Square Root Method
Rectangular electron field %dd determination
Take the geometric mean of the percent depth
doses for a square field of length (L) and width (W)
% D (d ; LxW ) = % D(d ; LxL) ⋅ % D(d ;WxW
Khan Model - Lateral Buildup Ratios (LBRs
(LBRs))
Using this approach and a sector integration
description for LBR, the depth dose, and dose
per monitor unit for irregular shaped electron
fields can be determined
Uses Lateral Buildup Ratios (LBRs
), and
(LBRs),
Pencil Beam Model parameters
σ r ( d ) = effective spread ( width )
Hogstrom KR et al., Phys Med Biol, 1981
Basic Procedure
Separate Cone Factors from inin-phantom
scatter contributions by measuring
percentage depth doses (%
(%dd),
dd),
normalized to the surface
Measure %dds as a function of field size,
ranging from very small (e.g. 1 cm radius)
to “infinitely”
infinitely” large fields (e.g. 20x20)
TG-51 Electron Beam Calibration
Basic Procedure - LBR
For each depth, divide the %dd of the
small field by that for the large (reference)
field
This ratio = Lateral Buildup Ratio (LBR),
or
LBR =
% dd x (rx , d )
% dd ∞ (r∞ , d )
11
Bruce J. Gerbi, Ph.D.
LBR v. depth
LBR vs Depth
20x20 Cone
16 MV
1.2
Result
1.0
2 cm
0.8
LBR
3 cm
4 cm
6 cm
0.6
8 cm
0.4
Use the values of σ r (d ) for 2 cm diameter
cutout,
Then, for any radius, the value of LBR can
be calculated using:
σ r2 (d ) = r 2 / ln
0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1
1 − LBR(0, d )
Z/Rp
Small or Irregular Electron Beams
New methods of modeling electron depth
dose distributions may enable irregular
field calculations to become an option,
option
eliminating the need for many clinical
measurements.
IV. Electron Beam Algorithms
Short history of electron algorithms
Description of how data should be entered into
treatment planning computers
What should be done to commission these
algorithm (being consistent with TG53)
– Describe some of the pitfalls and limitations of
electron algorithms
– Discuss normalization of dose distributions for
electron algorithms
• Restricted field
• Extended treatment distance
• Plans involving inhomogeneities
TG-51 Electron Beam Calibration
12
Bruce J. Gerbi, Ph.D.
V. ICRU 71 – Prescribing, Recording,
and Reporting electron beam therapy
Regular treatments
Intra Operative treatments
Total Skin Electron treatments
VI. Clinically Relevant Topics
Inhomogeneities in electron treatments
– The effects of inhomogeneities on dose distributions
– Computer representation of the effects of dose
inhomogeneities
Use of bolus
Field abutment
–
–
–
VII. Library of Clinical Examples
Extensive list of clinical applications
Each section to detail the following:
ElectronElectron-electron, same & different energies
ElectronElectron-photon, standard & extended distances
Tertiary shielding for field abutment
TGTG-70: Conclusion
Task Group report to be finished by end of
2005
– Introduction and Purpose of the treatment
– History and Description
– Pre-requisites: special equipment,
measurements
– Treatment Planning & Delivery
– Quality Assurance
TG-51 Electron Beam Calibration
13