MV Photon Dosimetry: Small Field Considerations · SSD/SAD = 100 cm H 2 O 60Co 10 x 10 cm2 Definitive calibration in machine specific reference conditions New reference dosimetry

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

11

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

12

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


13

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


14

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)

16

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)

18

4. Volume averaging

• This effect can be described mathematically using the profile shape and detector geometry

• Some examples of this effect in nonstandard fields

19

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

20

I invite you to read…

21

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

22

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

23

2. THE IAEA-AAPM FORMALISM

24

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

25

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

27

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

28

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

30

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

QQ

ff

QQ

ff

QQrefrefmsr

msr

msr

msrkkk 31


• 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

32


• 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

34

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


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


• Example by Alfonso et al. (2008)

39

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

40

Documents

MV Photon Dosimetry: Small Field Considerations · SSD/SAD = 100 cm H 2 O 60Co 10 x 10 cm2 Definitive calibration in machine specific reference conditions New reference dosimetry