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International Dosimetry Symposium, IAEA, Vienna 9-12 Nov 2010
Small and composite field dosimetry: the problems and recent progress
H. Palmans
National Physical Laboratory, Teddington, UK
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2/25International Dosimetry Symposium, IAEA, Vienna 9-12 Nov 2010
Acknowledgements
• IAEA organisers• IAEA/AAPM small and composite field working
group (Rodolfo Alfonso, Pedro Andreo, Roberto Capote, Saiful Huq, Joanna Izewska, Jonas Johansson, Per Kjäll, Warren Kilby, Rock Mackie, Ahmed Meghzifene, Karen Rosser, Jan Seuntjens, Ken Shortt, Wolfgang Ullrich, Stan Vatnitsky)
• IPEM small field working party (Mania Aspradakis, John Byrne, John Conway, Simon Duane, Karen Rosser and Jim Warrington)
• DIN small field subcommittee (Gregor Bruggmoser, Dietrich Harder, Harald Feist, Simon Foschepoth, Ralph-Peter Kapsch, Martin Janich, Edmund Schüle, Oliver Dohm, Kai Henkel, Otto Sauer, Björn Poppe, Harald Treuer and Bernhard Rhein)
• Many others…
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A general thought
• The ideal dosimeter is the patient• Maybe even more ideal is a phantom which
is an identical copy of the patient• Then we can make ranking:
– Anthropomorphic phantoms– Simple phantoms + anatomical features– Simple phantom + patient-like outer dimensions– 30 cm x 30 cm x 30 cm water phantom
QA Ref Dosimetry
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Overview
• Problems and proposed solutions of small field dosimetry
• Problems and proposed solutions of composite field dosimetry
• Dosimetry formalism• Availability of data
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• Loss of lateral charged particle equilibrium
• Partial occlusion of the primary source
• Detector size too largeLi et al. 1995 Med Phys 221167-70
What is a small field – three criteria
From Meltsner et al. 2009 Med Phys 36:339-50
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Small field size – apparent field widening
From Das et al. 2008 Med Phys 35:206-15
From Doblado et al. 2007 Phys Med 23:58-66
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Beam quality
• Problem: no reference conditions• Solutions:
– Sauer 2009 Med Phys 36:4168-72: TPR20,10(smaller field)
– AAPM TG-148 (Langen et al. 2010 Med Phys 37:4817-53): “dd(10)x[HT-ref]”
– Duane 2010: DAP ratio (poster 221, also: Djouguela 2006 Z Med Phys 16:217 & Sanchez-Doblado 2007 Phys Med 23:58-66)
DAP20,10 – Elekta SL25 6MV
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Detector response – ionization chambers
• sw,air:Eklund and Ahnesjö 2008, Phys Med Biol 53:4231-47
Crop et al. 2009 Phys Med Biol 54:2951-69
• pQ:
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Detector response - diodes
Sauer and Wilbert 2007 Med Phys 34:1983-8
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IMRT field dosimetry
From Roberto Capote, IAEA
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Composite field dosimetry
Output measurementref field
Small field OF and profiles
TPS: clinical delivery sequence
Dose calculation in patient
Dose calculation in verification phantom
Verification
TPS: pcsr delivery sequence
Dose calculation in pcsr phantom
Output measurementpcsr field
1 2 3
Treatment delivery
Output measurementref field
Small field OF and profiles
TPS: clinical delivery sequence
Dose calculation in patient
Dose calculation in verification phantom
Verification
TPS: pcsr delivery sequence
Dose calculation in pcsr phantom
Output measurementpcsr field
1 2 3
Treatment delivery
20º 60º
100º
140º180º220º
260º
300º 340º
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Criteria for pcsr fields
• Dose homogeneous over volume surrounding ion chamber
• Simplicity (ref conditions) versus clinically relevant (as close as possible to clinical plan) (Poster 321: Öhrman et al)
• Based on homogeneity index (Chung, last talk of this session)
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Dose at a point (cfr. ICRU Report 83)
• Two other concepts:– For full CPE:
– Dose to ion chamber calculation
Bouchard et al. 2009Med Phys 36:4654-63
ref
refpcsr
pcsr fgr
ffQQ P
k 1,, =
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Ion recombination in dynamic fieldsRecombination in an IMRT sequence (Helical
TomoTherapy)
phantom
cavity
Fan beam
( )[ ]
( )V
t L
Lpulseair
t L
Lpulseair
pulsed
V
satVion e
dxdttxD
dxdttxD
VB
VA
QQep γγ −
+
−
+
−− ⋅
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛
⋅++≈=⋅
∫ ∫
∫ ∫
0
2
2
0
2
2
2
1 /
/,
/
/,**
,
,
0.00
0.02
0.04
0.06
0.08
-4 -2 0 2 4
chamber offset / cm
∫Dai
rdx/
L / e
V g-1
0.000
0.001
0.002
0.003
0.004
∫( Dair ) 2dx/∫ D
air dx / eV g-1
Model + MC
0.00
0.02
0.04
0.06
0.08
-4 -2 0 2 4
chamber offset / cm
∫Dai
rdx/
L / e
V g-1
0.000
0.001
0.002
0.003
0.004
∫( Dair ) 2dx/∫ D
air dx / eV g-1
Model + experiment
0.00
0.01
0.02
0.03
-4 -2 0 2 4
chamber offset / cm
∫Dai
rdx/
L / e
V g-1
0.000
0.001
0.002
0.003 ∫ (Dair ) 2dx/∫ D
air dx / eV g-1
0.00
0.01
0.02
0.03
-4 -2 0 2 4
chamber offset / cm
∫Dai
rdx/
L / e
V g-1
0.000
0.001
0.002
0.003 ∫ (Dair ) 2dx/∫ D
air dx / eV g-1
V1/V2 = 250/100V1/V2 = 250/50V1/V2 = 100/50
Palmans et al. 2010 Med. Phys. 37:2876
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Formalism proposed in Alfonso et al 2008, Med Phys 35:5179
• Two related routes for Dw in non-conventional reference conditions, both requiring extension of concept of reference field and modified reference conditions
• Small static field dosimetry: intermediate machine-specific-reference field (msr)
• Composite field dosimetry: plan-class specific reference field (pcsr) – A pcsr field can be a 3-D irradiated volume or a 4-D
delivery sequence. – The pcsr should be as close as possible to a class of
clinical plans of interest, and provide a uniform dose over a region exceeding the dimensions of a reference detector
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0 0
,, , , , ,
msr refmsr msr
msr msr msr
f ff fwQ Q D wQ Q Q Q QD M N k k= msrclin
msrclin
msr
msr
clin
clin
ffQQ
fQw
fQw DD ,
,,, Ω=Machine specific
reference field fmsr Clinicalfclin
Tomotherapy5cm x 20cm
REFERENCE DOSIMETRY RELATIVE DOSIMETRY
GammaKnife∅ 1.6/1.8 cm
CyberKnife6 cm
1
≡ Ionizationchamber
Broad beamreference field
fref
00 ,,, QQQwD kNHypothetical
reference field fref
BrainLABmicro MLC10cmx10cm
refmsr
msr
ffQQk ,,
Radiosurgicalcollimators∅ 1.8 cm
refmsr
msr
ffQQk ,,
msrclin
msrclinmsr
msr
clin
clinmsrclin
msrclin
ffQQf
Q
fQff
QQ kMM ,
,,, ⋅=Ω
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refpcsr
pcsr
ffQQk,,
refpcsr
pcsr
pcsr
pcsr
pcsr
pcsr
ffQQQQQwD
fQ
fQw kkNMD ,
,,,,, 00= pcsrclin
pcsrclin
pcsr
pcsr
clin
clin
ffQQ
fQw
fQw DD ,
,,, Ω=Broad beam
reference field fref
Plan-classspecific reference
fieldfpcsr
00 ,,, QQQwD kN
Clinicalfclin
msrpcsr
msrpcsr
ffQQk ,,
Hypotheticalreference field e.g. 9-field prostate
pcsr
(e.g. IMRT Linac)
refmsr
msr
ffQQk ,,
fmsr
(e.g. Tomotherapy5cm x 20cm)
REFERENCE DOSIMETRY RELATIVE DOSIMETRY
20º 60º
100º
140º180º220º
260º
300º 340º
2
≡ Ionizationchamber
pcsrclin
pcsrclinpcsr
pcsr
clin
clinpcsrclin
pcsrclin
ffQQf
Q
fQff
QQ kM
M ,,
,, ⋅=Ω
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A few data on refmsr
refmsr
ffQQk ,
,
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Examples – msr/pcsr: TomoTherapy
Sterpin et al (2009)
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Field output factors – Monte Carlo calculated correction factors CyberKnife
Francescon et al 2008 Med Phys 35:504-13
msr
clin
msr
clin
ff
QQk
, ,
msrclin
msrclin
ffQQ,,Ω
collimator mm 60
,,
,,
,,
,,
=
⋅=
+⋅=
msr
MMk
bak
msrclin
msrclin
msrclin
msrclin
msrclin
msrclin
msrclin
msrclin
ffQQ
clin
msrffQQ
ffQQ
ffQQ
Ω
Ω
Different FWHM primary source
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Field output factors:
)1()2(
)2()2(
)1()1(
)2()1(
,
,,,
,,
clin
clin
clin
clin
msr
msr
clin
clin
clin
clin
msr
msr
msrclin
msrclin
msrclin
msrclin
fQrel
fQrel
fQ
fQ
fQ
fQ
ffQQ
ffQQ
MM
MM
MM
kk
=⋅=
msrclin
msrclinmsr
msr
clin
clin
msr
msr
msr
msr
clin
clin
clin
clin
msr
msr
clin
clin
msr
msr
clin
clinmsrclin
msrclin
ffQQf
Q
fQ
fQ
fQw
fQ
fQw
fQ
fQ
fQw
fQwff
QQ kMM
MDMD
MM
DD ,
,,
,
,
,,, ⋅=
⎥⎥⎦
⎤
⎢⎢⎣
⎡⋅==Ω
clin
clin
msr
msr
msr
msr
clin
clinmsrclin
msrclin fQ
fQ
fQw
fQwff
QQ MM
DD
k ⋅=,
,,,
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Field output factors - CyberKnife: Pantelis et al. 2010 Med Phys 37:2369
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 60012EDGEAlanineTLDEBT f ilmPolymer 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/M
60) 1
PinPointDiode 60008Diode 60012EDGEAlanineTLDEBT 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
ratio
of c
orre
ctio
n fa
ctor
s (M
C o
r vol
) PinPoint
Diode 60008
Diode 60012
EDGE
Alanine
)2()1(
, ,, ,
msr
clin
msr
clin
msr
clin
msr
clin
ff
ff
kk
0.85
0.90
0.95
1.00
1.05
1.10
1.15
Diode 60008
Diode 60012
EDGE
TLD
EBT film
Polymer gel
16 nPoi
nt
ode
6000
8
ode
6001
2
DG
E
anin
e
LD BT fi
lm
olym
er g
el
0.50
0.55
0.60
0.65
0.70
0.75
detector
Mcl
in /
Mre
f
(Mclin / M
ref )* kclin,m
sr
ExrA16 PinPoint SHD USD EDGE alanine TLD EBT GEL
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Plan class specific reference fieldsDynamic IMRT H&N – Chung et al. 2010 Med. Phys. 37:2404-13
VMAT – Rosser and Bedford 2009 Phys. Med. Biol. 54:7045-7061
TomoTherapy – Bailat et al. 2009Med. Phys. 36:3891-3896
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0.97
0.98
0.99
1.00
1.01
1.02
1.03
pcsr field
kpc
sr,re
f
Bailat et al. 2009
Rosser and Bedford 2009
Chung et al. 2010
pcsr field correction factors
0.97
0.98
0.99
1.00
1.01
1.02
1.03
pcsr field
kpc
sr,re
f
Bailat et al. 2009
Rosser and Bedford 2009
Chung et al. 2010
k=2
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Conclusions
• Small field dosimetry: proposed solutions for definition, field size, beam quality, msr field, correction factors,…
• Composite field dosimetry: continue exploration of ideas, proven for TomoTherapy (AAPM TG-148) to other modalities; step-and-shoot and dynamic IMRT, RapidArc, GammaKnife, …
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