Leksell Gamma Knife® Perfexion™
Perfexion™
QA Considerations
Paula L. Petti, Ph.D.
Taylor McAdam Bell Neuroscience Institute
Washington Hospital Healthcare System
Fremont, CA
1
QA for LGK Perfexion™
Perfexion™: Follow NRC licensing
guidelines (10
(10 CFR 35.1000)
Basic Tests and Measurements:
1) Coincidence of the mechanical isocenter of the patient-positioning
system (PPS) with the radiation-focal point (RFP)
2) Agreement of measured beam profiles with Leksell GammaPlan
calculations for all collimator sizes in the XY, YZ and XZ planes
3) Measurement of the absolute dose-rate calibration for largest
collimator
4) Confirmation of the relative output factors (ROFs
(ROFs)) for smaller
collimators.
2
Leksell Gamma Knife® Perfexion™
•
•
•
•
•
•
The patient is positioned via precise couch motions
A frame adapter attaches the Leksell coordinate frame (affixed to the
patient’s skull) to the treatment couch
Collimation system is built into the unit
3 Collimator sizes: 16-mm, 8-mm and 4-mm
192 60Co sources
60Co sources not fixed in space: they reside on 8 moveable sectors
Collimator
Patient Frame
Adapter
Patient Couch: 3-axis
positioning system
3
1
Perfexion™
Perfexion™ Sector Design
Sectors slide back and forth
on outside of collimator
8 independent,
identical sectors
24 sources per
sector
Sector Drives
Sources are arranged in 5 rings
The sources in each sector can be aligned with a different
collimator size or blocked completely
4
Coincidence of PPS and RFP for
Perfexion™
Perfexion™
• No helmets or microswitches to check
• Collimator settings (4, 8 and 16) are independent
• Therefore, we must check the coincidence of the
PPS and RFP for each collimator
• Elekta uses:
• Master Diode Tool
• New Film Holder Tool
• We use:
• Diode Test Tool (4-mm collimator only)
• New or old Film Holder
5
Alignment of Patient-Positioning System
(PPS) with Radiation Focal Point (RFP)
Master
Diode Tool:
Tool:
Service
instrument used
during bi-annual
preventative
maintenance
New Film
Holder:
Holder
Service and Field
instrument used
for annual QA and
acceptance testing
Old-Style
Film
Holder:
Holder
Field
instrument:
used for
annual QA
and
acceptance
testing
Diode
Tool: Field
instrument
used for
routine
checks (at
least monthly,
but is usually
done6 more
often)
2
Master Diode Tool:
Tool:
Used by Elekta during commissioning to align the RFP to the PPS
Attaches directly to the PPS
Accomodates up to 5 diodes
There are programmed scannning sequences for all three collimators
Diode Test Tool:
Tool:
Used by GK physicist for routine QA
Attaches to patient frame adapter, which then attaches to the PPS
One central diode
There is one programmed scannning sequence for the 4-mm
collimator
No scanning sequences for other collimators.
7
Film Holders
New Model
Old Model
Dosimetry
adapter
More precise than old-style film holder because:
1) Uses the same frame adapter that is used for
patient treatments (not shown above) which
is machined to very high tolerance
2) Has larger film compartment – easier to
interpret 16-mm films
1) Requires special dosimetry adapter, which
is not machined to the same high tolerances
as the patient frame adapter
2) Provides QA for RFP reproducibility, but
does not provide QA for patient setup, since
patient adapter is not employed
8
EBT GAFCHROMIC Film
• Manufactured by International Specialty
Products (ISP, Wayne, NJ)
• Excellent white paper available at the
manufacturer’s website:
http://online1.ispcorp.com/_layouts/Gafchromic/index.html
• Main advantage over earlier GAFCHROMIC
films for LGK dosimetry: More sensitive to
lower radiation doses: EBT films can measure
doses in the range of 0.01Gy to 8 Gy
• Low energy dependence (<5% between MeV
and keV photons)
• Film density growth complete within < 2 hrs
9
3
Basic Requirements for EBT
Film Analysis
Peak absorbance of EBT film is 636 nm, ⇒ response
enhanced by measurement with red light (match absorbance
of film with spectral response of scanner)
• Flatbed scanners:
– Epson and Microtek scanners are designed to
scan color film
– According to the ISP white paper, Epson scanners
produce the optimum results with EBT films
10
Basic Requirements for EBT
Film Analysis
• Software
– ImageJ: Available for free download at the
website: http://rsb.info.nih.gov/ij/
– Microsoft Excel
– More sophisticated software can be
employed
11
EBT Film Handling
• Must keep track of film orientation: Before cutting
film into appropriate size pieces, number the same
corner on each sub-section of film:
When analyzing films with flatbed
scanner, the amount of scattered light
depends on the orientation of the active
particles in the film, which depends on
the direction in which the film was
coated.
1
2
3
4
5
6
7
8
9
10
11
12
Film coating
for EBT film is
parallel to the
short edge of
film
• For RFP – PPS coincidence measurements, cut film
to fit holder
• For Profile and ROF measurements cut film into
pieces between about 6.0 cm and 6.5 cm
12
4
EBT Film Handling (continued)
• Place film in holder such that the marked
corner is in the same place for all films
• When scanning, place films in the same area of
the flatbed scanner with the marked corner of
the film in the same place
• Avoid fingerprints and mechanical stress
• Avoid long exposures to daylight
13
Recommended Procedures for GK
EBT Film Measurements
• To obtain background subtraction for each
piece of cut film, pre-scan each film on flatbed
scanner prior to irradiation.
• Scanning parameters:
– Scan as “positive film”
– Use either 200 or 400 dpi to obtain resolution of
the order of 0.1 mm (0.127 mm for 200 dpi and
0.064 mm for 400 dpi)
– 48-bit color
– Save as PNG files (or equivalent lossless format)
14
RFP - PPS Coincidence Measurements
• Not necessary to convert readings to absolute dose
(i.e., calibration curve not required), since we are only
looking at symmetry about the pin-point.
• If using ImageJ software to analyze films
– Extract red channel (Image  color  RGB split)
– Invert image (Edit  Invert) (so that profile will be
positive - personal preference)
– Using line drawing tool, draw line through center of
scan
– Extract Profile (Analyze  Plot Profile)
– List Profile Data, select it and copy to Excel (or
other) worksheet
• Subtract background
15
5
Coincidence of RFP and PPS: Film Analysis
with ImageJ Software
Red Channel
Extracted
Raw Data
Red Channel
Inverted
4-mm
irradiation
Old-style
film holder
XZ Plane
Background
Reading for
Film #4
16
Coincidence of RFP and PPS: Film Analysis with
ImageJ Software Reading for Background
Subtraction
Using “Ellipse” Tool,
outline central portion
un-irradiated film
Under Analyze options,
select “Histogram”
Background reading for this film =
mean value in central region of unirradiated film
17
Coincidence of RFP and PPS: Film
Analysis with ImageJ Software
Use “Line” tool to
indicate axis for profile
Sup
Under “Analyze” select
“Plot Profile”
R
L
Inf
Note: Draw line in the direction of
increasing LGK coordinate, in this
case from superior to inferior
List, select and copy the
data to Excel (or other)
18
worksheet
6
Coincidence of RFP and PPS: Film
Analysis with ImageJ Software
• Identify channel containing pin point
• By definition, this corresponds to X, Y or Z = 100 in LGK
coordinate system
• Assign LGK coordinates to other channels by the
formula:
LGK Coord =
100 " (C100 " C) # P
Where,
C 100 = the channel that contains the pin point
C = the channel in question
!
P = conversion factor, e.g., 0.0635 mm/pixel for films
scanned at 400 dpi
19
4-mm Z Axis: Coincidence of PPS and RFP
200
Counts (Background Subtracted)
4mm Z axis from XZ film
PPS Center
Central Channel (pin
point) defines center of
LGK coordinate system
(magenta line)
180
160
140
120
Center of FWHM
FWHM
Compute the
channel value for
the center of the
FWHM from the
endpoints of the
horizontal line
defining the
FWHM (green line)
100
80
60
40
! Z = -0.03 mm
= 100 - (center of FWHM)
20
0
85
90
95
100
105
110
115
Z Coordinate of PPS (mm)
20
Example of Pin-Point Films to Check RFP PPS Coincidence for 16-mm Collimator
New-Style Film Holder
Old-Style Film Holder
Y
X
Z
Y
21
7
New Film Holder: Example
Pin-Point Test of Coincidence of PPS and RFP, New Film
Holder
Reading (background subtracted)
140
120
16mm X Axis
FWHM
PPS Center
RFP Center
100
80
60
40
!X = 0.02 mm
20
0
80
85
90
95
100
105
110
115
120
-20
22
X Coordinate of PPS (mm)
Old Film Holder: Example
16-mm X Axis (XY Plane): Coincidence of PPS and RFP
200
Counts (Background subtracted)
180
16-mm X Axis XY Film
PPS Center
RFP Center
80% Width
160
140
120
100
80
60
!x = 0.13 mm
40
20
0
80
85
90
95
100
105
110
115
120
X Coordinate of PPS (mm)
23
Test Specifications and
Frequency
• Elekta’s specification for the 4-mm collimator is that
Δx, Δy and Δz are all ≤ 0.3mm and that
"r = "x 2 + "y 2 + "z 2 ! 0.4mm
Frequency of tests:
1) Master Diode test is done bi-annually by Elekta service
engineers as part preventative maintenance
2) Films are usually irradiated annually by the on-site GK
physicist
24
8
Example of Results using Different Tools to
Check PPS and RFP Coincidence for
WHHS Perfexion™
Perfexion™ GK
!r = !x 2 + !y 2 + !z 2
(mm)
Collimator =
4-mm
8-mm
16-mm
Master - Diode Tool
0.098
0.16
0.16
New-Style Film Holder
0.09
0.15
0.30
Old-Style Film Holder
0.11
0.20
0.38
25
Profile Measurements using
EBT film
• Same film handling requirements as for
pin-point films
• Measurements performed in spherical
phantom (80 mm radius)
• Must measure dose versus film
response calibration curve
26
Beam Profiles (film): Measurement Tools
New-style Spherical Phantom
Old-style Spherical Phantom
1) Attaches to patient frame adapter
1) Attaches to dosimetry adapter
2) Film positioned between 2 rods
2) Film positioned in central insert
3) Can irradiate a 3D stack of films
3) Can irradiate only one film at a time
4) Composed of certified Therapy Grade Solid
Water®
4) Presumably composed of polystyrene
5) 3 adapters provided for ion chambers or other
detectors
5) Additional inserts are supplied for ion
chambers and other detectors 27
9
EBT Film Calibration
•
•
•
Calibration curve obtained by irradiating films at several energy levels
between 0.5 Gy and 8 Gy with the 16-mm collimator
Pre-scan all films to obtain background subtraction
Dose on the horizontal axis equals the dose at (100, 100, 100) plus the
transit dose for the 16-mm collimator.
Film Reading Calibration Curve
Reading minus Background (inverted)
160
Film reading
in red
channel
140
120
100
80
60
Calibration Curve from
16mm Data
40
20
0
0
1
2
3
4
5
6
7
8
9
28
Dose (Gy) at (100, 100, 100)
Transit Dose for 16-mm Collimator
Blocked source position
between 4- and 8-mm
collimators
Sources move to
the blocked
position at the
initiation of a
treatment and
between shots.
Therefore, to
achieve a 16-mm
shot, the sources
must pass over
the 4-mm
collimator, and
thereby deliver a
so-called “transit
dose.”
Sources
Sector
16
4 B 8
Collimator
29
Transit Dose for 16-mm Collimator
• One way to measure 16-mm transit dose is to deliver the
same prescribed dose via two treatment plans to
isocenter in the spherical phantom, one plan consisting
of a single 16-mm shot, and the second plan consisting
of four 16-mm shots. The transit dose is then:
difference in measured dose
between the two runs
Transit Dose =
D4 ! D1
3
Gy/(16-mm shot)
difference in the number of times
the source travels back and forth
over the 4-mm collimator
30
10
Transit Dose for 16-mm Collimator
• Using ion-chamber measurements as
described on previous slide, transit
dose for 16-mm collimator = 0.03
Gy/shot for WHHS Perfexion™ in July
2008
• Note that the transit dose changes as a
function of time as the Co-60 sources
decay.
31
Determination of Transit Dose by Film
Measurements
Irradiate 1st film with 1 shot, dose = 5 Gy
Irradiate 2nd film with 18 shots, dose = 5 Gy
•
•
Number of shots:



•
•
•
•
large enough s.t. dose difference in measurable,
small enough s.t. irradiation times are valid for LGK PFX
Subtract background reading determined from unirradiated
film
Use Dose-to-reading calibration curve to convert readings
to dose
Transit dose = (D18 - D1 )/17
Results (July 2008):
16-mm Collimator: 0.03 Gy/shot
Gy/shot (same as result obtained with ion
chamber!)
8-mm and 4-mm Collimators: 0.01 Gy/shot
Gy/shot
32
Determining Beam Profiles using ImageJ
Using “Line” Tool, draw line in
direction of increasing
coordinate value
Under Analyze options, select
“Profile”
List profile data, Select it, and
Copy to Excel (or similar)
33
program for analysis
11
Beam Profile Analysis
•
•
Subtract background
Apply dose calibration to data
points
Identify FWHM
Define X (or Y or Z) = 100 at
center of FWHM (note that we
have already confirmed that
the center of the RFP is at
(100,100,100) with the pinpoint measurement.)
Define X (or Y or Z) coordinate
value on horizontal axis (400
dpi)
Calculate FWHM in mm
•
•
Raw Data (from
ImageJ,
ImageJ, red
channel, inverted)
•
•
34
16-mm Y Profile from YZ Film
7
16 mm Y Profile from YZ film
Center
FWHM
LGP Data
6
Dose (Gy)
5
4
3
2
1
0
60
70
80
90
100
110
120
130
140
LGP Y Coordinate (mm)
Raw Data Converted to Plot of Dose vs LGP Coordinate (example):
Measured FWHM = 21.7 mm,
Diff = 0.4 mm
35
LGP FWHM = 21.3 mm,
(where “LGP” stands for Leksell GammaPlan)
Simplified Procedure to Check
only the FWHM
Film Reading Calibration Curve
160
Reading minus Background (inverted)
Use Calibration curve to calculate
R1/2 , the film reading (with
background subtracted) that
corresponds to 50% of maximum
profile dose
140
120
R1/2
100
80
60
20
0
0
Locate R1/2 on the raw profile curve
(background subtracted), and
compute:
FWHM = (C2 – C1) × P
Calibration Curve from
16mm Data
½ Max
Dose
40
1
2
3
4
5
6
7
8
9
Dose (Gy) at (100, 100, 100)
Raw Data from ImageJ
R1/2
Where, P = conversion factor, e.g.,
0.0635 mm/pixel for films scanned at 400
dpi
C1
C2
36
12
Specifications and Frequency for
Beam Profile measurements
Specification:
According to Elekta: Measured and LGP values for
FWHM should be within ± 1 mm of each other
Frequency:
Beam profiles should be measured upon acceptance
of the GK unit and annually thereafter
37
Results from last Annual QA:
• All FWHM were between ± 0.1 mm and
± 0.4 mm of Leksell GammaPlan
38
16-mm Dose Rate Measurement
• Currently no official calibration protocol
specific to the LGK
• Charges of AAPM Task Group 178:
– Suggest a protocol for calibration with
ionization chambers calibrated at an ADCL
that can be used with all Gamma
Stereotactic Radiosurgery (GSR) devices
– Work with the working group on dosimetry
calibration protocol for beams that are not
compliant with TG-51
39
13
16-mm Dose Rate Measurement
Performed in spherical phantom
Calibration Protocols:
• TG-21: Ion chamber calibrated 60Co in-air
– Can be used for various phantom materials
– Can be used for various geometrical setups
• TG-51: Ion chamber calibrated for 60Co in water.
– Designed to facilitate linear accelerator calibration and
QA
– phantom must be water,
– 10 cm × 10 cm field size
40
LGK Dose Rate Measurement:
Some Recent Publications
• R Drzymala R Wood, J Levy: Calibration of
the Gamma Knife using a new phantom
following AAPM TG51 and TG21 protocols,
Med. Phys. 35:514-521;2008
– Compared the 2 protocols in the Elekta old-style
spherical phantom and in a newly designed water
phantom
– TG-51 in water phantom results were 1.4% lower
than TG-21 in polystyrene phantom
41
LGK Dose Rate Measurement:
Some Recent Publications
• S Griffin Meltsner and LA DeWerd: Air
Kerma based dosimetry calibration for the
Leksell Gamma Knife, Med Phys 36:339-
350;2009
– Proposes an air-kerma-based dosimetry protocol
using either an in-air or in-acrylic phantom
measurement
– Modified version of TG-21 specific to LGK
calibration geometry
– With new protocol, measured dose rates were
between 1.5% and 2.9% higher than those used
clinically by at 7 LGK sites (Models B and C)
42
14
16-mm Dose Rate Measurement: Practical
Issues: Choice of new or old-style phantom
Old-style phantom requires
dosimetry adapter
Bhatnagar, et al. (Med Phys
36:1208-1211;2009):
The dosimetry adapter
attenuates some beams in
the lateral (3 and 7) sectors
of the Perfexion™ unit,
causing the overall 16-mm
dose rate to be
underestimated by
approximately 1%.
43
Choice of phantom
New spherical phantom
• Does not require
dosimetry adapter
• Attaches to patient
adapter
Patient Frame
Adapter
– More precise
– Provides better
check of entire
system
44
16-mm Dose Rate Measurement:
Practical Issues
• Checking measured dose rate: compare
results to
– TLD: either in-house or outside service (e.g.,
RPC SRS phantom)
– EBT GafChromic
45
15
Practical Method for Checking GK
Dose Rate Calibration using EBT Film
Irradiate films in 6-MV linear
accelerator beam at 3 dose
levels, e.g. 4, 5 and 6 Gy to
obtain a mini calibration curve
6-MV Mini-Calibration
Net Counts in Red
Channel
(Background subtracted)
140
Note that EBT film exhibits very little
energy dependence
4.0 Gy
130
120
110
100
6-MV Mini-Calibration
90
80
3
4
5
6
7
Dose (Gy)
5.0 Gy
6.0 Gy
Red channel
extracted,
Background
subtracted,
film intensity
46
inverted
Practical Method for Checking Dose
Rate Calibration
6-MV Mini-Calibration
Irradiate film with 16-mm
collimator, 5.0 Gy @ maximum
Net Counts in Red
Channel
(Background subtracted)
140
130
120
110
100
6-MV Mini-Calibration
90
80
3
4
5
6
7
Dose (Gy)
Determine dose at center of
peak region from minicalibration curve
Compare to expected value
(e.g. 4.96 Gy)
47
Expect ± 2% to 3% agreement
Relative Output Factors (ROF)
for the 8- and 4-mm Collimators
• Relative output
factor for Gamma
Knife is defined as:
dDC / dt
dD16 / dt
(100 ,100 ,100 )
i.e., ROF = dose rate of
collimator C relative to 16mm collimator, where both
are measured at isocenter
= (100,100,100) in
spherical, 80-mm radius
phantom
48
16
Unique Features of Perfexion™
Perfexion™ Geometry
1 2
3 4
5
Co-60 sources
distributed in 5
rings for each
identical sector
Ring
Number
Number of
Sources
1
2
3
4
5
48
32
40
32
40
49
ROF: Unique Feature of Perfexion™
Perfexion™ Geometry
Each collimator within each ring
has a different beam geometry
16
4
8
50
Beam Geometry:
There are, therefore, 15 distinct beam geometries: 5 rings
multiplied by 3 beam-on positions per ring.
This is in contrast to previous LGK designs for which all of
the beam channels were identical, and there was only one
type of beam.
Each of the 15 beam types has a different:
1) Virtual source-to-isocenter distance (SAD)
2) Output factor
Elekta determined these values by fitting a beam model to
Monte Carlo generated data.
51
17
Fitted Values for Relative Output Factor (ROF)
and Virtual Source-to-focus distance (R
(Rvsf)
Collimator
Size
Ring
ROF
Rvsf
(mm)
Collimator
Size
Ring
ROF
Rvsf
(mm)
4
1
0.799
521
8
4
0.808
480
4
2
0.815
546
8
5
0.730
522
4
3
0.792
533
16
1
0.961
481
4
4
0.725
595
16
2
1.000
459
4
5
0.663
607
16
3
0.986
455
8
1
0.957
431
16
4
0.920
488
8
2
0.946
437
16
5
0.851
519
8
3
0.901
468
52
Relative Output Factor for Each Collimator Size
• We cannot measure the 15 ROFs individually
• The ROF for the 8- and 4-mm collimators relative to the
16-mm collimator is determined from the equation:
5
" n ! OF (c )
i
ROF (c ) =
i
,ni = ( 48,32,40,32,40)
i =1
5
" n ! OF (c = 16mm)
i
i
i =1
Where the sum is taken over all 5 rings
ni represents the number of sources in each ring
ROF(8mm) = 0.924
ROF(4mm) = 0.805
53
Two Ways to Determine ROF
Measure dose, Dc, delivered by each
collimator at (100, 100, 100) for:
• A given treatment time:
ROFc = Dc/D16
• The same prescription dose:
ROFc =
ROFc "
!
Dc / Tc
D16 / T16
Dc
# ROFcno min al
D16
Tc is the irradiation time for
each collimator
The approximately
equal sign is replaced
by an equality if the
dose is prescribed to
(100,100,100) instead
54
of the point of
maximum dose
!
18
Relative Output Factors:
Measurement Techniques
•
•
•
•
Pin-point ion chamber
GafChromic Film
TLDs (rods and LiF microcubes)
Glass Rods (Perks, et al.)
Some References:
•
•
•
Mack et al., Precision dosimetry for narrow photon beams
used in radiosurgery - determination of Gamma Knife® output
factors, Med. Phys. 29: 2080-9; 2002
Perks et al., Glass rod detectors for small field, stereotactic
radiosurgery dosimetric audit, Med. Phys. 32:726-32; 2005
Novotny et al. Measurement of relative output factors for the 8
and 4 mm collimators of the Leksell Gamma Knife Perfexion
by film dosimetry, Med Phys. 36:1768-1774;2009
55
ROF Meas:
Meas: EBT Film and Fixed Dose
 Cut and mark Films
 Scan un-irradiated films to obtain background correction
 Irradiate 2 films at the 3 dose levels, e.g., 4.5, 5.0 and 5.5 Gy for 16 collimator to
obtain mini-calibration curve (choose either axial, coronal or sagittal plane). It is
reasonable to assume that the calibration curve is piece-wise linear between
measured points (6 films)
 Irradiate 2 films the 4- and 8-mm collimators to a dose in the middle of the minicalibration range in the same plane as calibration films (4 films)
 “Process” films with ImageJ: extract red channel, invert intensity values, subtract
background
56
 Sample intensity values in center of films, convert to dose, calculate ROF
My Results: Average of 7 sets of ROF
Measurements
8-mm Collimator: 0.888 ± 0.012 (∼3.9% lower than Elekta)
4-mm Collimator: 0.792 ± 0.007 (∼2.5% lower than Elekta)
Used constant time for 3 sets of measurements, constant
dose for 4 sets
Used different measurement planes (axial, coronal or
sagittal)
Did not change values in LGP
57
19
Results Reported in Literature: Average
of 5 sets of ROF Measurements
Collimator
EDR 2 Film EBT Film
MD-V2-55
Film
8 mm
0.904 ± 0.012
(-2.1%)
0.917 ± 0.014
(-0.8%)
0.906 ± 0.018
(-2.0%)
4 mm
0.769 ± 0.010
(-4.5%)
0.810 ± 0.007
(+0.6%)
0.819 ± 0.009
(+1.7%)
Novotny et al., Med. Phys.2009
58
Error levels in ROF Measurement
0.03Gy
= 0.6%
5Gy
• Neglecting transit dose:
• Neglecting the
difference between
maximum dose and
dose at (100,100,100)
< 1%
• Standard deviation in
film pixel values around
point of measurement
Between 0.5
and 1 count
59
What order of magnitude error in film
reading causes a 4% error in dose?
Film Reading Calibration Curve
Reading minus Background (inverted)
160
140
120
100
Slope in vicinity of 5 Gy ~ 7 counts/Gy
80
Y-intercept ~ 85 counts
60
Calibration Curve from
16mm Data
40
20
0
0
1
2
3
4
5
6
7
8
9
Dose (Gy) at (100, 100, 100)
60
20
What order of magnitude error in film
reading causes a 4% error in dose?
From calibration curve on previous slide:
5 Gy ⇒ a reading of 120 counts
4.8 Gy ⇒ a reading of 118.6 counts
∴ A difference of only 1.4 counts results
in a 4% dose discrepancy
61
Summary
• The AAPM is working towards a calibration
protocol for GSR units based on ADCL
calibrated ionization chambers
• PPS/RFP coincidence and beam profile
measurements are facilitated by using EBT
GAFCHROMIC film
• ROF measurements remain the most
challenging aspect of QA for GSR devices
62
Thank you for your attention!
63
21