MV Photon Dosimetry:
Small Field Considerations
Hugo BouchardAssistant ProfessorPhysics DepartmentUniversity of MontrealMontreal, Canada
January 2016
Special thanks to the Hugo Palmans from the IAEA-AAPM workgroup for letting me use some of their valuable definitions
2
Overview
1. The physics of small fields1. What is a small field?
2. The 4 main effects
3. Quiz
2. The IAEA-AAPM formalism1. Evolution of dosimetry protocols
2. Definitive calibration in standard reference conditions
3. The IAEA-AAPM formalism
4. Definitive calibration in machine specific reference conditions
5. Output factors in nonstandard conditions
3
1. THE PHYSICS OF SMALL FIELDS
4
What is a small field?
• 3 physical conditions determine if an external photon beam can be designated small (IAEA-AAPM CoP) :
1. There is a loss of lateral charged particle equilibrium
2. There is partial occlusion of the primary photon source by the collimating devices
3. The size of the detector is large compared to the beam dimensions.
5
Lack of LCPE
• Dose-to-kerma ratio is an indicator of the lack of CPE since the ratio is 1 when CPE exists
• Study by Li et al. (1995)
6
Partial source occlusion
• Overlap of penumbra over the detector
Broad beam Small field 7
Size of detector vs field size
• Several perturbation effects occur when modulation or dose gradients exist over the chamber volume
• The most obvious one is volume averaging
Detector volume Detector volume
8
Why do small fields require correction factors?
• There a 4 main effects related to the characteristics of the detector
1. The density of the sensitive volume of the detector
2. The atomic properties of the sensitive volume
3. The presence of extracameral components
4. Volume averaging
9
1. Density perturbation effects
• The most important perturbation effect in small fields is usually caused by the difference in mass (or electronic) density with respect to water
• Fano’s theorem can be used to explain this effect
10
Fano’s theorem
• In a medium of uniform properties irradiated by uniform photon fluence, the fluence of charged particles is uniform and independent of the mass density distribution in space.
Water cavity Vapor cavity Dense-water cavity
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Electrons enter cavity sidewaysSome electrons escape the cavity
Electrons entering cavity mainly from frontElectrons are produced in cavitySome electrons escape the cavity
Water cavityBeam crossing cavity
Beam not crossing cavity
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Number of electrons produced in cavity is less than waterElectron range in cavity is higherElectrons can exit cavity more easilyFluence is lowerDose is smaller than water cavity
Number of electrons cavity enter is slightly higher (less backscattering)Electron path is higherFluence is higherDose is higher than water cavity
Sparse cavityBeam crossing cavity
Beam not crossing cavity
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Number of electrons produced in cavity is higher Electron range in cavity is lower (mostly absorbed locally)Electrons can exit cavity less easilyFluence is higherDose is higher than water cavity
Electron path is smallerNumber of electrons entering cavity is lower (more backscattering)Fluence is lowerDose is lower than water cavity
Dense cavityBeam crossing cavity
Beam not crossing cavity
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Non-CPE conditions
• Monte Carlo simulation of cavity dose response to 1.25 MeV photon pencil beams in water cavities made of different density
Air-water: short for “air-density” waterSilicon-water: short for “silicon-density” water 15
2. Atomic properties
• Assuming the detector to be a cavity with the electron density of water, relevant atomic properties are
– The atomic number (photo-electric effect, pair production)
– The I-value (stopping power)
– The density effect parameter (stopping power)
• All have an effect on the interaction cross sections (i.e., electronic cross sections)
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2. Atomic properties
• Monte Carlo simulations of cavity dose response to 1.25 MeV photon pencil beams in cavity of identical electron density
Water-air: short for “water-density” airWater-silicon: short for “water-density” silicon 17
3. Extracameral components• Components such as wall, electrodes, stems, etc.,
can affect the detector dose response
• Dose response to 1.25 MeV photon pencil beams when the cavity is surrounded by a wall (MC)
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4. Volume averaging
• This effect can be described mathematically using the profile shape and detector geometry
• Some examples of this effect in nonstandard fields
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In summary
• In general, the effects are1. The density of the sensitive volume of the detector
• Low/high-density: under/over-responds
2. The atomic properties of the sensitive volume• Low/high Z: under/over-responds
• Low/high I-value: over/under-responds
• Low/high : over/under-responds
3. The presence of extracameral components• High-Z materials can increase reponse
4. Volume averaging• In small beams, under-response
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I invite you to read…
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Quiz
• Discuss the 4 main effects in the following detector irradiated by a 6 MV photon field of size:
A. 1 x 1 cm2
B. 4 x 4 cm2
C. 40 x 40 cm2Wall density: 1.76Wall material: Air-equivalent plasticWall thickness: 1 mm
6 mm
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Quiz
• Discuss the 4 main effects in the following detector irradiated by a 6 MV photon field of size:
A. 1 x 1 cm2
B. 4 x 4 cm2
C. 40 x 40 cm2
plastic
epoxy
water-equivalent
silicon (diam=0.6 mm)
6 mm
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2. THE IAEA-AAPM FORMALISM
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Before we start, Simon says…
• In the beginning was the IPEM 1990 code
Nice and simple to read and to use, but…
• Small field dosimetry is more complicated
How can we get there?
“If I were you, I wouldn’t start from here…”
…start from Alfonso et al., who start from TRS-398
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Evolution of dosimetry protocols
• Generation 1– HPA 1960 (γ), 1964 (γ), 1969 (γ),
1971 (e-), 1975 (e-)– AAPM 1966 (e-), 1971 (γ), 1975
(γ)– NACP 1972 (γ + e-)– *ICRU #10b 1962 (γ), #14 1969
(γ), ICRU #21 1972 (e-)
• Generation 2– HPA 1980 (γ + e-)– AAPM TG-21 1983 (γ + e-)– IAEA TRS-277 1987 (γ + e-)– *ICRU 1984 (e-)
• Generation 3– IPEM 1990 (γ)– AAPM 1999 (γ + e-)– IAEA 2001 (γ + e- + p+)– IPEM 2003 (e-)
• Generation 4– IAEA-AAPM 2015 (small γ fields)
First protocols
Air-kerma standards
Global perturbation factor
Tables for energy dependence (C, CE, SPR)
5% uncertainty
Air-kerma standards
Accounting for chamber-specific perturbation
3-4% uncertainty
Dose-to-water standards
1-2% uncertainty
Dose-to-water standards
Nonstandard beams
*Strictly speaking not a protocol26
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Definitive calibration in standard referenceconditions
0
00
0
0 ,,,
f
QQwD
f
Qw MND refref f
QQwD
f
Qw MND ,,,
dref(5 or 10 cm)
SSD/SAD = 80/100 cm
H2O
60Co
10 x 10 cm2
dref(10 cm)
SSD/SAD = 100 cm
H2O
Linac
10 x 10 cm2
0
0
,
,
ff
QQrefk
IAEA TRS-398 reference
dosimetry protocol
*NPL gives you this coefficient directly27
The IAEA-AAPM formalism
• Nonstandard beam protocols (generation #4)
• Generalized absorbed dose to water-based approach• Alfonso et al, A new formalism for reference dosimetry of small and nonstandard fields, Med. Phys.
35 (11), 2008
0
00
0
0 ,,,
f
QQwD
f
Qw MND
Machine specific reference fieldStandard-lab field
msr
msrmsr
msr
msr
f
QQwD
f
Qw MND ,,,
0
0
,
,
ff
QQmsr
msrk
clin
clinclin
clin
clin
f
QQwD
f
Qw MND ,,,
Clinical field
Definitive calibration
msrclin
msrclin
ff
QQk,
,
Beam characterization or QA
or
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The IAEA-AAPM formalism
• Remember the following rule:
1
11
1
1 ,,,
f
QQwD
f
Qw MND
Field 1 with beam quality 1
2
22
2
2 ,,,
f
QQwD
f
Qw MND
1,,
,,,
,212
12
QwD
QwDff
QQN
Nk
Field 2 with beam quality 2
29
Specifying beam quality in MSR
• Ideally, we would want the factor to be tabulated directly
• Data is available for standard reference conditions (TRS-398) or even direct calibration coefficients (e.g., NPL)
• Some machines cannot provide standard reference conditions, hence the need for defining a Machine Specific Reference (MSR) field
• The concept of hypothetical field defined by Jeraj et al. (2005) has been useful for Tomotherapy
0
0
,
,
ff
QQmsr
msrk
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The concept of the hypothetical field
• It is helpful to define a hypothetical field to use
0
00
0
0 ,,,
f
QQwD
f
Qw MND
Hypothetical reference fieldStandard-lab field
ref
refref
ref
ref
f
QQwD
f
Qw MND ,,,
0
0
,
,
ff
QQrefk refmsr
msr
ff
QQk,
,
Machine specific reference field
Definitive calibration
msr
msrmsr
msr
msr
f
QQwD
f
Qw MND ,,,
0
0
0
0
,
,
,
,
,
,
ff
ff
ff
QQrefrefmsr
msr
msr
msrkkk 31
The concept of the hypothetical field
• Data available in TRS-398
0
00
0
0 ,,,
f
QQwD
f
Qw MND
Hypothetical reference fieldStandard-lab field
ref
refref
ref
ref
f
QQwD
f
Qw MND ,,,
0
0
,
,
ff
QQrefk
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The concept of the hypothetical field
• Data simulated with Monte Carlo and available in future protocols (for Tomotherapy already in TG-148)
• Not very sensitive to model and close to unity
Hypothetical reference field
ref
refref
ref
ref
f
QQwD
f
Qw MND ,,,
refmsr
msr
ff
QQk,
,
msr
msrmsr
msr
msr
f
QQwD
f
Qw MND ,,,
Machine specific reference field
33
• Example for Tomotherapyusing TG-148:– Step1 : you determine %dd(10)x[HT,ref]
with a MSR
– Step 2: you convert the beam specifier %dd(10)x[HT,ref] to %dd(10)x[HT,TG-51] with fig. 19 and eq. 5 of TG-148
– Step 3: you obtain the global quality correction factor from table 1, which combines TG-51 data and Monte Carlo simulations
Beam quality in MSR
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35
36
dref(10 cm)
SSD/SAD = 100 cm
H2O
60Co
10 x 10 cm2
Definitive calibration in machine specificreference conditions
New reference
dosimetry protocol
0
00
0
0 ,,,
f
QQwD
f
Qw MND msr
msrmsr
msr
msr
f
QQwD
f
Qw MND ,,,
0
0
,
,
ff
QQmsr
msrk
dref(10 cm)
DSP = 85 cm
5 x 10 cm2
H2O
E.g.: Tomotherapy®
36
H2O
37
dref
SSD/SAD
H2O
CLIN
New dosimetry
techniques
dref
SSD/SAD
Machine
MSR
Output factors in nonstandard conditions
clin
clinclin
clin
clin
f
QQwD
f
Qw MND ,,,
msrclin
msrclin
ff
QQk,
,msr
msrmsr
msr
msr
f
QQwD
f
Qw MND ,,,
Machine
37
H2O
38
dref
SSD/SAD
H2O
CLIN
New dosimetry
techniques
dref
SSD/SAD
Machine
MSR
Output factors in nonstandard conditions
Machine
msrclin
msrclinmsr
msr
clin
clin
msr
msr
clin
clinmsrclin
msrclin
ff
QQf
Q
f
Q
f
Qw
f
Qwff
QQ kM
M
D
D,
,
,
,,
, 38
Output factors in nonstandard conditions
• Example by Alfonso et al. (2008)
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In summary
• New machines require new reference conditions: MSR fields
• Quality correction factors allow one to obtain calibration coefficient for a specific field and beam quality
• Small field output factor measurements require quality correction factors
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