MV Photon Dosimetry: Small Field Considerations · SSD/SAD = 100 cm H 2 O 60Co 10 x 10 cm2...

transcript

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

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

1. THE PHYSICS OF SMALL FIELDS

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.

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)

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

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

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

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

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

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

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

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)

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)

4. Volume averaging

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

• Some examples of this effect in nonstandard fields

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

I invite you to read…

• 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

• 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

water-equivalent

silicon (diam=0.6 mm)

2. THE IAEA-AAPM FORMALISM

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

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

Definitive calibration in standard referenceconditions

Qw MND refref f

Qw MND ,,,

dref(5 or 10 cm)

SSD/SAD = 80/100 cm

10 x 10 cm2

dref(10 cm)

SSD/SAD = 100 cm

10 x 10 cm2

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

Qw MND

Machine specific reference fieldStandard-lab field

msrmsr

Qw MND ,,,

clinclin

Qw MND ,,,

Clinical field

Definitive calibration

msrclin

Beam characterization or QA

The IAEA-AAPM formalism

• Remember the following rule:

Qw MND

Field 1 with beam quality 1

Qw MND

Field 2 with beam quality 2

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

The concept of the hypothetical field

• It is helpful to define a hypothetical field to use

Qw MND

Hypothetical reference fieldStandard-lab field

refref

Qw MND ,,,

QQrefk refmsr

Machine specific reference field

Definitive calibration

msrmsr

Qw MND ,,,

QQrefrefmsr

msrkkk 31

• Data available in TRS-398

Qw MND

Hypothetical reference fieldStandard-lab field

refref

Qw MND ,,,

QQrefk

• 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

refref

Qw MND ,,,

refmsr

msrmsr

Qw MND ,,,

Machine specific reference field

• 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

dref(10 cm)

SSD/SAD = 100 cm

10 x 10 cm2

Definitive calibration in machine specificreference conditions

New reference

dosimetry protocol

Qw MND msr

msrmsr

Qw MND ,,,

dref(10 cm)

DSP = 85 cm

5 x 10 cm2

E.g.: Tomotherapy®

SSD/SAD

New dosimetry

techniques

SSD/SAD

Machine

Output factors in nonstandard conditions

clinclin

Qw MND ,,,

msrclin

msrmsr

Qw MND ,,,

Machine

SSD/SAD

New dosimetry

techniques

SSD/SAD

Machine

msrclin

msrclinmsr

clinmsrclin

msrclin

• Example by Alfonso et al. (2008)

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

MV Photon Dosimetry: Small Field Considerations · SSD/SAD = 100 cm H 2 O 60Co 10 x 10 cm2...

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