Small Field Dosimetry for IMRT and Radiosurgery
Jan Seuntjens, Ph.D., FAAPM, FCCPM
Professor & Director Medical Physics
McGill University
Canada
Thursday April 7
SEAAPM 2011
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Overview – “Small field” radiotherapy
• Background
– Issues and definitions
• Measurement physics of small fields
• Measurement physics of IMRT (composite) fields
• New dosimetry of small and IMRT fields –
status report
• Conclusions
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Acknowledgments – small field committees
• IAEA committee
–
–
–
–
–
–
–
–
–
–
Palmans (Chair)
Andreo
Huq
Mackie Ulrich
Kilby
Izewska
Capote
Alfonso
Seuntjens
Thursday April 7
• AAPM Committees
– TG‐178 (Goetsch et al)
– TG‐155 (Das et al)
– WGDPCB
• ICRU Report Committee
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Background
• Technological improvements in conventional Linacs have improved mechanical accuracy and stability as well as dosimetric control
• Increased availability of tertiary MLCs on conventional accelerators (Brainlab M3, Novalis
Tx)
• Specialized machines for IGRT using small fields or combination of small fields (GammaKnife, CyberKnife, TomoTherapy, Vero)
Has technology gotten ahead of comprehension of basic dosimetry principles?
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Summary of some challenges in small‐
field dosimetry
• Definition of field size?
• Difficulties in accurate measurements
• Modeling of small field dose calculations in TPSs
• Calibration protocol reference conditions cannot be achieved
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IMRT versus SRS/SRT – some nomenclature • nonstandard (a.k.a. non‐compliant) fields: radiation fields for which reference dosimetry
cannot be reliably performed using the existing protocols (AAPM TG‐51 or IAEA TRS‐398)
– Small fields (static)
• Reference conditions cannot be met (10 x 10 cm2 is not available) – Composed fields (IMRT, step‐and‐shoot or dynamic)
• Delivery conditions are far removed from calibration conditions
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Large differences in Output Factors
among users/machines
Statistics of 45 Output Factors for 6 mm and 18 mm square fields Novalis, SSD = 100 cm, depth = 5 cm, various detectors)
factor of 2 in dose determination!
From Wolfgang Ullrich (BrainLab)SEAAPM 2011
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Physics of small fields
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What constitute 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
Thursday April 7
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What constitute 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
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Lateral charged particle loss broad photon field
narrow photon field
volume
volume
A small field can be defined as a field with a size smaller than the “lateral range” of charged particles
is a measure of the degree of equilibrium or transient equilibrium
ICRU, September 2008
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Loss of lateral charged particle equilibrium
Concept of rLEE
Li et al. 1995 Med Phys 22: 1167‐
70
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Source occlusion
Large field conditions
Thursday April 7
Small field conditions
SEAAPM 2011
(IPEM Report 103)
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Overlapping of beam penumbras
• definition of field size?
From Das et al. 2008 Med Phys 35: 206‐15
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What constitute 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
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Detector dependence of output factor
Thursday April 7
From Doblado et al. 2007 Phys Med 23:58‐66
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Detector issues in small field dosimetry
• Energy dependence of the response
• Perturbation effects
– Central electrode
– Wall effects
– Fact that cavity is air‐filled instead of water
– Volume averaging
• Interaction of these effects with beam focal spot size
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Detector‐related small field condition
Meltsner et al., Med Phys 36:339 (2009)
Exradin A16 outer diameter
Exradin A16 inner diameter
One could claim that the GammaKnife 18 or 14 mm diameter fields are not small (quasi point source + electron equilibrium length about 6 mm)
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Measurements with small‐field detectors
Sauer & Wilbert
Med Phys 34, 1983‐88 (2007)
IC = PTW 31010 (0.125 cm3)
PiP = PTW 31006 (Pinpoint) SES = size of equivalent square Thursday April 7
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Why do we worry about CPE or TCPE?
Valid for CPE or TCPE.
Correction factor kQ well‐known for:
Q1 = Co‐60; Q2 = linac beam with field
10 x 10 cm2; z = 10 cm; SSD or SAD 100 cm
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Why do we worry about CPE or TCPE?
For the measurement of absorbed dose:
• In regions of CPE and TCPE: SPR conversion is accurate
– Dosimetry according to TG‐51 or TRS‐398
• In regions of non‐CPE: SPR conversion is not accurate and additional, sometimes large, corrections are needed, i.e., – Dosimetry in narrow stereotactic fields
• In regions of “recomposed‐CPE” (IMRT, Tomotherapy, etc): we don’t know (since composition of CPE may be disturbed by detector, or may not be perfect)
– Dosimetry of intensity modulated fields
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Small field dosimetry
0.3% effect
Sanchez‐Doblado et al Phys. Med. Biol. 48 2081‐2099 (2003)
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Stopping power ratio water to air
Very small effects!
Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)
Thursday April 7
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Narrow 1.5 mm field
Ratio of dose to water to dose to air averaged over cavity volume
1.70
1.60
1.40
Dw/Dair
Collecting electrode diameter: 1.5 mm
Separation: 1 mm
1.50
1.30
Stopping power ratio w/air
1.20
60%1.10
1.00
0.90
0.80
0
2
4
6
Off‐axis distance (mm)
Paskalev, Seuntjens, Podgorsak (2002) AAPM Proc. Series 13, Med. 24
Phys. Publishing, Madison, Wi, 298 –
318.
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Role of different perturbation factors in small fields
080915
PP31006 and PP31016 chambers
Crop et al., Phys Med Biol 54:2951 (2009)
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080915
Off‐axis behaviour of correction factors
8 mm x 8 mm field, 10 cm depth (0.6 mm, 2 mm spot sizes)
Very small effects!
Crop et al., Phys Med Biol 54:2951 (2009)
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080915
Off‐axis behaviour of correction factors
8 mm x 8 mm field, 10 cm depth (0.6 mm, 2 mm spot sizes)
Very large effects!
Crop et al., Phys Med Biol 54:2951 (2009)
Thursday April 7
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Summary of measurement issues in small fields
• Beam dependent issues
– Beam focal spot size plays a role
– Lateral disequilibrium
– How do we measure beam quality in practice?
• Detector effects
– There is no ideal detector
– Volume averaging and perturbation effects
– Corrections depend on beam focal spot size
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Physics of IMRT fields
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Composite nonstandard fields
• Composed of multiple small fields
• Thus: same dosimetric issues as in small fields
– Lateral charged particle disequilibrium
– Partial occlusion of source
– Detector response problems, volume averaging, perturbations
• How can dosimetric accuracy be affected?
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Dose measurements in composite fields
x NE = 0.973
s ( x NE ) = 0.022
xIC10 = 0.963
x PP = 0.944
s ( xIC10 ) = 0.024
s ( x PP ) = 0.035
Fraser et al (2009) JACMP 10 (4): 241‐51 Thursday April 7
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Why do we worry about CPE or TCPE?
For the measurement of absorbed dose:
• In regions of CPE and TCPE: SPR conversion is accurate
– Dosimetry according to TG‐51 or TRS‐398
• In regions of non‐CPE: SPR conversion is not accurate and additional, sometimes large, corrections are needed, i.e., – Dosimetry in narrow stereotactic fields
• In regions of “recomposed‐CPE” (IMRT, Tomotherapy, etc): we don’t know (since composition of CPE may be disturbed by detector, or may not be perfect)
– Dosimetry of intensity modulated fields
Thursday April 7
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Field name
CIMRT,6 MVmeasured
CIMRT,6 MVcalculated
Difference
Static #1
1.019
± 4.0%
0.950
6.8%
Static #2
1.173
± 6.1%
1.150
1.9%
Static #3
1.124
± 3.2%
1.094
2.7%
Static #4
1.274
± 6.0%
1.233
3.2%
Static #5
1.172
± 2.5%
1.141
2.7%
Dynamic #1
1.139
± 3.0%
1.143
-0.3%
Dynamic #2
1.169
± 2.5%
1.161
0.7%
Dynamic #3
1.089
± 3.9%
1.004
7.8%
Dynamic #4
1.007
± 3.2%
1.031
-2.4%
Dynamic #5
0.920
± 5.4%
0.885
3.8%
Dynamic #6
1.583
± 5.9%
1.605
-1.4%
Dynamic #7
1.077
± 5.7%
1.014
5.9%
Dynamic #8
1.079
± 5.9%
1.005
6.9%
Bouchard &Seuntjens, Med Phys 31: 2453‐2464 (2004)
Thursday April 7
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Exradin A14
Exradin A12
Gradient correction due to volume averaging is dominating Thursday April 7
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Issue with reference dosimetry
conditions
Modality
IMRT,
SRS/SRT
Most relevant
static calibration
field size
10 x 10 cm2
S&S or dynamic
capabilities?
Yes
TomoTherapy 5 x 20 cm2
Yes
CyberKnife
6 cm diameter
Yes
GammaKnife
16 mm / 18 mm
diameter
Yes
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New dosimetry of small and IMRT fields
Status Report
Thursday April 7
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Several working groups and TG’s
• AAPM TG 155 – small field relative dosimetry
• AAPM TG 178 – GammaKnife dosimetry
• IAEA small field committee – liaised with AAPM WGDPCB
• ICRU report committee on prescribing and reporting of stereotactic radiation therapy
• IPEM – Report 103
• DIN – small field subcommittee
Thursday April 7
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Upcoming guidelines and recommendations
• New formalism
• Data for new formalism
• Practical issues
– Small fields:
•
•
•
•
Beam quality
Suitable detectors
Correction factors
etc
– Composite fields IMRT:
•
•
•
•
•
Thursday April 7
Beam quality
Suitable detectors
Reference field criteria
Correction factors
etc
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New formalism
• Two related routes for Dw in non‐conventional reference conditions, both requiring an extension of concept of reference field and modified reference conditions • Small static field dosimetry
– intermediate machine‐specific‐reference field (msr)
– Recommendations for correction factors to measured output factors
• Composite field – IMRT 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
Alfonso et al Med Phys 35: 5197 (2008)
Thursday April 7
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Static small fields
Thursday April 7
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1
REFERENCE DOSIMETRY
fmsr
w,Qmsr
D
=M
fmsr
Qmsr
Broad beam
reference field fref
RELATIVE DOSIMETRY
fmsr , fref
Qmsr ,Q
ND,w,Q0 kQ,Q0 k
fclin
w,Qclin
D
=D
fmsr
w,Qmsr
Machine specific
reference field fmsr
Ω
fclin, fmsr
Qclin,Qmsr
Clinical
fclin
Radiosurgical collimators
∅ 1.8 cm
f msr , f ref
Qmsr ,Q
k
ND,w,Q0 kQ,Q0
Hypothetical
reference field fref
f msr , f ref
Qmsr ,Q
BrainLAB
micro MLC
10cmx10cm
CyberKnife 6 cm
GammaKnife
∅ 1.6/1.8 cm
Ω
f clin , f msr
Qclin , Qmsr
=
M
M
f clin
Qclin
f msr
Qmsr
clin , f msr
⋅ kQf clin
, Qmsr
k
≡ Ionization
chamber
Thursday April 7
Tomotherapy
5 cm x 20 cm
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How to specify beam quality?
Multiple beams from BJR Suppl 25
Sauer (2009) Med. Phys. 36: 4168
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Beam quality specifier for Tomotherapy
AAPM TG‐148 (Langen et al. 2010 Med Phys 37:4817‐53): “dd(10)x[HT‐ref]”
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Data for • correction factors are small for the larger field msr
Thursday April 7
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Data for (cont’d) Tomotherapy msr field
Sterpin et al (2011 ‐ preliminary data)
• correction factors are small for the larger field msr
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Field output factors
, f msr
ΩQf clin
=
clin ,Qmsr
clin , f msr
kQf clin
,Qmsr
Dwf clin
,Qclin
Dwf msr
,Qmsr
f clin
f clin
clin
⎡ Dwf clin
⎤
M Qf clin
M
M
Qclin
Qclin
,Qclin
f clin , f msr
= f msr ⋅ ⎢ f msr
=
⋅
k
Qclin ,Qmsr
f msr ⎥
f msr
M Qmsr ⎢⎣ Dw,Qmsr M Qmsr ⎥⎦ M Qmsr
f msr
Dwf clin
M
Qmsr
,Qclin
= f msr ⋅ f clin
Dw,Qmsr M Qclin
clin , f msr
kQf clin
,Qmsr (1)
msr
clin
M Qf msr
(1) M Qf clin
(2) M relf clin,Qclin (2)
= f clin ⋅ f msr
=
f clin , f msr
kQclin ,Qmsr (2) M Qclin (1) M Qmsr (2) M relf clin,Qclin (1)
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Field output factors ‐ CyberKnife: Pantelis et al. 2010 Med Phys 37:2369
1.050
0.950
0.800
0.70
0.750
0.700
1.150
1.100
1.050
1.000
M clin / M ref
0.650
0.600
0
0.65
0.950
5
10
15
20
0
5
diameter / mm
10
diam eter / m m
0.60
1.15
0.55
Diode 60008
1.05
Diode 60012
EDGE
1.00
0.50
TLD
ExrA16
0.95
PinPoint EBT film
SHD
Polymer gel
USD
ratio of correction factors (MC or vol)
1.300
1.10
f clin , f msr
clin , f msr
kQfclin
, Q msr (1) kQ clin , Q msr ( 2)
1.200
( M clin / Mref) * kclin,msr
(M/M 60)2/(M/M 60) 1
0.850
PinPoint
Diode 60008
Diode 60012
EDGE
Alanine
TLD
EBT film
Polymer gel
1.250
0.75
0.900
M / M60
1.300
A16
PinPoint
Diode 60008
Diode 60012
EDGE
Alanine
TLD
EBT film
Polymer gel
1.000
15
20
PinPoint
1.250
Diode 60008
1.200
Diode 60012
EDGE
1.150
Alanine
1.100
EDGE
alanine
TLD
EBT
GEL
1.050
detector
1.000
0.90
0.950
Polymer gel
EBT film
TLD
Alanine
EDGE
Diode 60012
PinPoint
A16
080915
Diode 60008
0.85
47/25
0
5
10
diam eter / m m
15
20
Getting output factor data for multiple detector types
Thursday April 7
Sauer and Wilbert 2007 SEAAPM 2011
Med Phys 34:1983‐8
48
Output factors – CyberKnife
coupling of beam spot size and detector correction factors
Different FWHM primary source
f clin , f msr
Qclin ,Qmsr
f clin , f msr
clin , f msr
Ω
kQf clin
=
a
⋅
,Qmsr
Qclin ,Qmsr + b
k
k
f clin , f msr
Qclin ,Qmsr
M msr
clin , f msr
=
⋅ Ω Qfclin
,Qmsr
M clin
msr = 60 mm collimator
Ω Qf
clin , f msr
clin ,Qmsr
Francescon et al 2008 Med Phys 35:504‐13
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IMRT fields
Thursday April 7
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New formalism
• Two related routes for Dw in non‐conventional reference conditions, both requiring an extension of concept of reference field and modified reference conditions • Small static field dosimetry
– intermediate machine‐specific‐reference field (msr)
– Recommendations for correction factors to measured output factors
• Composite field – IMRT 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
Alfonso et al Med Phys 35: 5197 (2008)
Thursday April 7
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2
REFERENCE DOSIMETRY
f pcsr
w,Qpcsr
D
=M
Broad beam
reference field fref
f pcsr
Qpcsr
RELATIVE DOSIMETRY
f pcsr, fref
Qpcsr,Q
ND,w,Q0 kQ,Q0 k
f pcsr, fref
Qpcsr,Q
D
k
Ω
fclin, f pcsr
Qclin,Qpcsr
Clinical
fclin
Plan‐class
specific reference field
fpcsr
(e.g. IMRT Linac)
300º
340º
20º
60º
260º
ND,w,Q0 kQ,Q0
Hypothetical
reference field
=D
f pcsr
w,Qpcsr
fclin
w,Qclin
100º
220º
fmsr
e.g. 9‐field prostate pcsr
Ω
f clin , f pcsr
Qclin ,Q pcsr
=
clin
M Qfclin
M
f pcsr
Q pcsr
180º
⋅k
140º
f clin , f pcsr
Qclin ,Q pcsr
f pcsr , f msr
Qpcsr ,Qmsr
k
f msr , f ref
Qmsr ,Q
Thursday April 7
k
(e.g. Tomotherapy
5cm x 20cm)
SEAAPM 2011
≡
Ionization
chamber
52
Plan class specific reference fields
Dynamic 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. 2009
Med. Phys. 36:3891‐3896 Thursday April 7
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pcsr field correction factors
1.03
1.02
Bailat et al.
Bailat
et al.
2009
2009
Chung et al.
Chung
et al.
2010
2010
k pcsr,ref
1.01
1.00
k=2
0.99
0.98
Rosser
Rosser and
and
Bedford
Bedford 2009
2009
0.97
pcsr field
Suitable IMRT calibration fields?
Chung et al 2011
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What are possible criteria for suitable IMRT calibration fields?
Chung et al 2011
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Conclusions
• Small and IMRT field dosimetry can be complex
– There are hefty perturbation effects that can have significant impact on reference dosimetry procedures and output factors
– Comparison between different detectors provides valuable information
• In small field dosimetry
– Machine‐specific reference fields defined
– Data on correction factors is being collected
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Conclusions (cont’d)
• In IMRT / composite field dosimetry
– Measurement uncertainties, reference detectors
– Practical criteria for the reference field definition are being developed
• New documents will be coming out providing guidelines on how to better deal with these issues
Continued education is needed and can prevent complacency Thursday April 7
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