iaea/aapm code of practice for the dosimetry of static...
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IAEA/AAPM Code of Practice for the
Dosimetry of Static Small Photon
Fields
Jan Seuntjens
McGill University
Montréal, Canada
-
Acknowledgements
• IAEA/AAPM small and composite field working group: Hugo Palmans (Chair), Rodolfo Alfonso, Pedro Andreo, Roberto Capote, Saiful Huq, Joanna Izewska, Jonas Johansson, Warren Kilby, T Rock Mackie, Ahmed Meghzifene, Karen Rosser, Jan Seuntjens, Wolfgang Ullrich
• Edmond Sterpin, Mania Aspradakis, Simon Duane, Hugo Palmans, Pedro Andreo for discussions on a variety of aspects related to this effort.
-
Disclosures
• My work is supported in part by the Canadian Institutes
of Health Research, the Natural Sciences and
Engineering Research Council, Canada through
operating grants and training grants.
• Sun Nuclear Corporation provided untied funding to
support the graphite probe calorimeter project.
• Some brand names of commercial products are
mentioned in this presentation. This does not represent
any endorsement of one product or manufacturer over
another
3
-
Learning Objectives
• Review the problems of small field dosimetry
and the solutions that have been identified
• Learn about the IAEA-AAPM recommendations
and data for small field dosimetry
-
Overview
• The problems in small-field dosimetry
• The IAEA dosimetry formalism
• Conclusions
-
What constitutes small-field conditions?
• Beam-related small-field conditions
– the existence of lateral charged particle disequilibrium
– partial geometrical shielding of the primary photon
source as seen from the point of measurement
• Detector-related small-field condition
– detector size compared to field size
-
Lateral charged particle loss broad photon field
volume volume
narrow photon field
A small field can be defined as a field with size smaller
than the “lateral range” of charged particles
is a measure of the degree of charged particle
equilibrium or transient equilibrium
-
Concept of rLCPE
Lateral charged particle loss
MC calculations, Seuntjens (2013)
-
Detector size relative to field size
• Small field conditions exist when one of the
edges of the sensitive volume of a detector is
less then a lateral charged particle equilibrium
range (rLCPE) away from the edge of the field
(Li et al. 1995 Med Phys 22, 1167-1170)
rLCPE (in cm) = 5.973•TPR20,10 – 2.688
Slide courtesy: H. Palmans
-
Source occlusion
Large field conditions Small field conditions
(Figure courtesy M.M. Aspradakis et al, IPEM Report 103)
-
Overlapping of beam penumbras
Das et al. 2008 Med Phys 35: 206-15
definition
of field
size is not
unique
-
Detector-related small field condition
Based on criterion 1, one could claim that the GammaKnife
18 or 14 mm diameter fields are not small (quasi point
source + electron equilibrium length about 6 mm).
Meltsner et al., Med
Phys 36:339 (2009)
Exradin A16 inner
diameter
Exradin A16 outer
diameter
-
Detector dependence of output factor
From Sanchez-Doblado et al. 2007 Phys Med 23:58-66
-
Detector issues in small field dosimetry
• Energy dependence of the response
• Perturbation effects
– Central electrode
– Wall effects
– Fact that cavity is different from water, fluence perturbation
– Volume averaging
• These effects depend somewhat on the beam spot size
-
Dosimetry protocol values (e.g., TG-51) of these factors are
applicable usually only in TCPE and only for the
conditions:
10 x 10 cm2; zref = 10 cm; SSD or SAD 100 cm
Detector issues in small field dosimetry
15
-
Stopping power ratio water to air
Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)
Very
small
effects!
0.5% effect
Andreo&Brahme PMB 8:839 (1986)
-
Role of different perturbation factors
080915
Crop et al., Phys Med Biol 54:2951 (2009)
PP31006 and PP31016
chambers
-
Magnitude of correction factors on and
off-axis
080915
Crop et al., Phys Med Biol 54:2951 (2009)
8 mm x 8 mm field, 10 cm depth (0.6 mm, 2 mm spot sizes)
Very large effects!
Very large effects! Relatively small effects!
-
Correction factors for ionization
chambers
Benmahklouf and Andreo (2013)
-
Diodes for small field dosimetry
Sauer and Wilbert 2007
Med Phys 34:1983-8
-
Shielded and unshielded diodes
Benmahklouf and Andreo (2013)
-
Benmahklouf and Andreo (2013) 22
-
Summary of issues leading to dosimetric
uncertainties in small fields
• Beam dependent issues
– Beam focal spot size
– Lateral disequilibrium
– How do we measure beam quality in practice?
• Detector effects
– There is no ideal detector
– Volume averaging and fluence perturbation effects
– Corrections depend on beam spot size
-
What are the single set of two largest contributors to correction
factors and their uncertainties for commercial air-filled ionization
chambers in small photon fields?
3%
17%
75%
5%
1% 1. The stopping power ratio and the central electrode
effect
2. The stopping power ratio and the chamber wall
effect
3. The fluence perturbation effect and the volume
averaging effect
4. The stopping power ratio and the volume averaging
effect
5. The ionization chamber wall effect and the stem
effect
-
• Correct answer: 3 The fluence perturbation effect and the volume
averaging effect
• Discussion: The field size dependence of stopping power ratios is
0.5% or less. For most ionization chambers the field size
dependence of wall corrections is limited to a few percent. The
volume averaging and fluence perturbation corrections are
potentially very large (on the order of 10-30% or more depending on
the situation)
• Reference:
– Crop et al (2009) Phys Med Biol 54 2951-2969
– Bouchard et al (2009) Med Phys 36 (10), 4654-4663
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Which two competing effects lead to field size dependent correction factors of unshielded diode detectors?
1. Intrinsic energy dependence
of Si in photon beams and
volume averaging
2. Intrinsic energy dependence
of Si in photon beams and
perturbation effects
3. Polarity effect and
recombination
4. Polarity effect and
electrometer calibration
5. Recombination effect and
diode doping 1. 2. 3. 4. 5.
14%
78%
5%0%
3%
-
• Correct answer: 2 Intrinsic energy dependence of Si in photon
beams and electron fluence perturbation effects
• Discussion: Volume averaging is usually small in diodes because of
the small size of the sensitive volume. Diodes are not polarized by
an external bias, so there is no polarity effect. Recombination effects
and diode doping are not relevant in this context.
• References:
– Francescon et al 2011, Med Phys 38: 6513
– Benmakhlouf et al 2014, Med Phys 41: 041711
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IAEA TECDOC small field dosimetry
• Code of Practice / working document
• Physics relevant to reference and relative dosimetry
• Formalism
• Instrumentation
• Practical implementation
– Machine-specific reference dosimetry
– Relative dosimetry
• Data
-
Ch. 2 - Physics of small fields
e.g. Small field conditions
LCPE source occlusion detector size
0.2
0.4
0.6
0.8
1.0
0.0 0.5 1.0 1.5 2.0 2.5
beam radius / cm
rati
o o
f d
ose
to
ke
rma
Co-60
6 MV
10 MV
15 MV
Meltsner et al. 2009
Med Phys 36:339-50
Aspradakis et al 2010
IPEM Report 103
Seuntjens
MC
-
0 0
,
, , , , ,msr refmsr msr
msr msr msr
f ff f
w Q Q D w Q Q Q Q QD M N k kmsrclin
msrclin
msr
msr
clin
clin
ff
QQ
f
Qw
f
Qw DD,
,,, Machine specific
reference field fmsr Clinical field
fclin
Tomotherapy
5 cm x 20 cm
REFERENCE DOSIMETRY RELATIVE DOSIMETRY
GammaKnife
d = 1.6/1.8 cm
CyberKnife
6 cm
Ionization chamber
Broad beam reference field
fref
00 ,,, QQQwDkN
Hypothetical
reference field fref
micro MLC
10 cm x 10
cm
refmsr
msr
ff
QQk,
,
Radiosurgica
l collimators
d = 1.8 cm
refmsr
msr
ff
QQk,
,
msrclin
msrclinmsr
msr
clin
clinmsrclin
msrclin
ff
QQf
Q
f
Qff
QQ kM
M,
,
,
,
30
Reference Fields Small Fields
-
Ch3. – Formalism (Alfonso et al) / Dw in
machine specific reference (msr) fields
• Chamber calibrated specifically for the msr field
• Chamber calibrated for the conventional reference field and
generic correction factors are available
• Chamber calibrated for the conventional reference field and
generic correction factors not available
msr
msr
msr
msr
msr
msr
f
QwD
f
Q
f
Qw NMD ,,,
refmsr
msr
refmsr
msr
msr
msr
ff
QQ
f
QwD
f
Q
f
Qw kNMD,
,,,, 00
refmsr
msr
refrefmsr
msr
msr
msr
ff
QQ
f
QQ
f
QwD
f
Q
f
Qw kkNMD,
,,,,, 00
fref==10 x 10 cm2
Q0= 60Co
-
Equivalent square fields - msr
WFF beams:
BJR 25 - equivalent
field size is energy
independent
FFF beams:
equivalent field size is
energy dependent;
Tables are provided
for 6 MV and 10 MV
-
Ch 3. – Formalism / equations for
beam quality in non-standard
reference fields
for TPR20,10(10) = TPR20,10
(Palmans 2012 Med Phys 39:5513)
)(
)()()( ,,
sd
sdsTPRTPR
101
1010
1020
1020
0.55
0.60
0.65
0.70
0.75
0.80
0.85
2 4 6 8 10 12
s / cm
TP
R2
0,1
0(s
)
(b)4 MV
10 MV
8 MV
6 MV
5 MV
25 MV21 MV18 MV15 MV12 MV
-
Ch 3. – Formalism / equations for
beam quality in non-standard
reference fields for PDD10X(10) = %dd(10)X
11
1
1010
2
10
110
10
t
s
t
s
ec
ecsPDD
PDD
)(
)(
07510020102671
075101010
1010
1010
10.)(,.)(.
.)(),()(
PDDPDD
PDDPDDPDD x
(Palmans 2012 Med Phys 39:5513-9) 55
60
65
70
75
80
85
2 4 6 8 10 12
s / cmP
DD
10(s
)
4 MV
10 MV
8 MV
6 MV
5 MV
25 MV
21 MV
18 MV
15 MV
12 MV
(d)
(TG-51)
-
Note about volume averaging in FFF
beams
Pantelis et al. 2009 Med Phys 37:2369
-
Volume averaging in FFF beams
-
Ch3. – Formalism / determination of
field output factors
• Field output factor relative to reference field (ref stands here for a
conventional reference or msr field)
• Field output factor relative to reference field using intermediate field or ‘daisy chaining’ method
where
refclin
refclinref
ref
clin
clinrefclin
refclin
ff
QQf
Q
f
Qff
QQ kM
M,
,
,
,
refclin
refclinref
ref
clin
clinrefclin
refclin
ff
QQf
Q
f
Q
f
Q
f
Qff
QQ KICM
ICM
M
M ,,
,
,)(
)(
(det)
(det) intint
int
int
)((det),
,
,
,
,
,int
int
int
detICkkK ref
ref
clin
clin
refclin
refclin
ff
QQ
ff
QQ
ff
QQ
-
Ch 4 – Instrumentation
• Required equipment, detectors, phantoms for
msr dosimetry
• Required equipment, detectors, phantoms for
relative dosimetry
-
Ch 5 – Practical implementation msr
dosimetry
• Reference conditions for beam quality and msr
dosimetry
• Overall correction factors for ionization
chambers
• Correction for influence quantities
• Measurement in plastic phantoms and cross-
calibration
-
Ionization chambers,
recombination, polarity
LeRoy et al., PMB 56:5637-51 (2011)
Agostinelli et al., Med Phys 35:3293-301 (2008)
-
Note on the use of plastic phantoms
(Seuntjens et al 2005, Med. Phys. 32: 2945)
-
Ch 5 – Practical implementation msr
dosimetry / availability data
refmsr
refmsr
ff
QQk,
,
-
Francescon et al: Phys. Med. Biol. 57 (2012) 3741–3758
Correction factor data (cont’d)
-
Ch 6 – Practical implementation
relative dosimetry
• Required equipment, detectors, phantoms
• Measurements of profiles and field output factors
• Correction factors for determination of output
factors
-
Ch 6 – Practical implementation
relative dosimetry / correction
factors for OF
• Examples of different sources of correction
factors will be further discussed in the next
presentation (I. Das)
• IAEA-AAPM code of practice data tables is
based on a vetted set of correction factor from
the literature
• Uncertainty analysis has been performed
-
Field output factors – correction
factors - example
• PTW-60012 – unshielded diode
0.92
0.93
0.94
0.95
0.96
0.97
0.98
0.99
1
1.01
1.02
0.0 2.0 4.0 6.0 8.0 10.0
co
rre
ctio
n fa
cto
r w
ith
re
f fin
t
square field size / cm
Cranmer Sargison 2011 / Elekta
Cranmer Sargison 2011 / Varian
Krauss 2008
Schwedas 2007
Griesbach 2005
Ralston et al 2011 - cones
Ralston et al 2011 - microMLC
Fit (exp)
Fit (exp + MC)
-
Field output factors – correction
factors - diode
• IBA SFD – unshielded diode
0.950
0.960
0.970
0.980
0.990
1.000
1.010
1.020
1.030
1.040
1.050
0.0 2.0 4.0 6.0 8.0 10.0
co
rrection f
acto
r w
ith r
ef fint
square field size / cm
Azangwe 2014 Lechner 2013 WFF Lechner 2013 FFF Bassinet 2013 /Novalis+muMLC Cranmer Sargison 2011 / Elekta Cranmer Sargison 2011 / Varian Sauer 2007 Ralston et al 2011 - cones Ralston et al 2011 - microMLC Fit (exp) Fit (exp + MC)
-
Field output factors – correction
factors • PTW-31006 - Pinpoint
-
Uncertainty in correction factor
introduced due to field size definition
Cranmer Sargison et al Med. Phys. 38, 6592–6602 (2011) Benmakhlouf et al Med. Phys. 41, 041711 (2014)
-
Output factors – validation
methodology
)1(
)2(
)2(
)2(
)1(
)1(
)2(
)1(
,
,
,
,
,
,
clin
clin
clin
clin
msr
msr
clin
clin
clin
clin
msr
msr
msrclin
msrclin
msrclin
msrclin
f
Qrel
f
Qrel
f
Q
f
Q
f
Q
f
Q
ff
QQ
ff
QQ
M
M
M
M
M
M
k
k
msrclin
msrclinmsr
msr
clin
clin
msr
msr
msr
msr
clin
clin
clin
clin
msr
msr
clin
clin
msr
msr
clin
clinmsrclin
msrclin
ff
QQf
Q
f
Q
f
Q
f
Qw
f
Q
f
Qw
f
Q
f
Q
f
Qw
f
Qwff
QQ kM
M
MD
MD
M
M
D
D,
,
,
,
,
,,
,
clin
clin
msr
msr
msr
msr
clin
clinmsrclin
msrclin f
Q
f
Q
f
Qw
f
Qwff
QQM
M
D
Dk
,
,,
,Where:
-
Output factors – example CyberKnife
0.600
0.650
0.700
0.750
0.800
0.850
0.900
0.950
1.000
1.050
0 5 10 15 20
diameter / mm
M /
M60
A16PinPointDiode 60008Diode 60012
EDGEAlanineTLDEBT filmPolymer gel
0.950
1.000
1.050
1.100
1.150
1.200
1.250
1.300
0 5 10 15 20
diameter / mm
(M/M
60) 2
/(M
/M6
0) 1
PinPoint
Diode 60008
Diode 60012
EDGE
Alanine
TLD
EBT film
Polymer gel
0.950
1.000
1.050
1.100
1.150
1.200
1.250
1.300
0 5 10 15 20
diameter / mm
rati
o o
f co
rre
cti
on
fac
tors
(M
C o
r vo
l) PinPoint
Diode 60008
Diode 60012
EDGE
Alanine
) 2
( ) 1
( , ,
, , m
sr
clin
msr
clin
msr
clin
msr
clin
f f
Q
Q
f f
Q
Q
k
k
0.85
0.90
0.95
1.00
1.05
1.10
1.15
Diode 60008
Diode 60012
EDGE
TLD
EBT film
Polymer gel
A16
PinP
oint
Diode 60008
Diode 60012
EDGE
Alanine
TLD
EBT film
Polymer gel
0.50
0.55
0.60
0.65
0.70
0.75
detector
Mc
lin /
Mre
f
(M
clin
/ Mre
f )* k
clin
,ms
r
ExrA16 PinPoint SHD USD EDGE alanine TLD EBT GEL
Pantelis et al.
2010 Med Phys
37: 2369
Slide courtesy:
H. Palmans
-
For what purpose is the measurement of the
beam quality specifier required?
1. To specify the correction factors to be applied to the output ratios measured in small fields
2. To specify small field output factors
3. To specify the beam quality correction factor in the msr field
4. To ensure the beam is of adequate quality
5. To specify the absorbed dose calibration coefficient for a small field 1. 2. 3. 4. 5.
22%
6%
13%
1%
58%
-
• Correct answer: 3 To specify the kQmsr,Qref beam quality correction
factor in the msr field
• Discussion: In general, no beam quality measurement is performed
in small fields, only in msr fields.
• References:
– Palmans 2012 Med Phys 39: 5513
-
Conclusions
• Solutions to most small-field dosimetry problems
have been described and translated in
formalised procedures
• The IAEA CoP will be coming out in the very
near future – timeline < 6 months
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