IAEA/AAPM Code of Practice for the

Dosimetry of Static Small Photon

Fields

Jan Seuntjens

McGill University

Montréal, Canada

Acknowledgements

• IAEA/AAPM small and composite field working group: Hugo Palmans (Chair), Rodolfo Alfonso, Pedro Andreo, Roberto Capote, Saiful Huq, Joanna Izewska, Jonas Johansson, Warren Kilby, T Rock Mackie, Ahmed Meghzifene, Karen Rosser, Jan Seuntjens, Wolfgang Ullrich

• Edmond Sterpin, Mania Aspradakis, Simon Duane, Hugo Palmans, Pedro Andreo for discussions on a variety of aspects related to this effort.

Disclosures

• My work is supported in part by the Canadian Institutes

of Health Research, the Natural Sciences and

Engineering Research Council, Canada through

operating grants and training grants.

• Sun Nuclear Corporation provided untied funding to

support the graphite probe calorimeter project.

• Some brand names of commercial products are

mentioned in this presentation. This does not represent

any endorsement of one product or manufacturer over

another

3

Learning Objectives

• Review the problems of small field dosimetry

and the solutions that have been identified

• Learn about the IAEA-AAPM recommendations

and data for small field dosimetry

Overview

• The problems in small-field dosimetry

• The IAEA dosimetry formalism

• Conclusions

What constitutes 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

Lateral charged particle loss broad photon field

volume volume

narrow photon field

A small field can be defined as a field with size smaller

than the “lateral range” of charged particles

is a measure of the degree of charged particle

equilibrium or transient equilibrium

Concept of rLCPE

Lateral charged particle loss

MC calculations, Seuntjens (2013)

Detector size relative to field size

• Small field conditions exist when one of the

edges of the sensitive volume of a detector is

less then a lateral charged particle equilibrium

range (rLCPE) away from the edge of the field

(Li et al. 1995 Med Phys 22, 1167-1170)

rLCPE (in cm) = 5.973•TPR20,10 – 2.688

Slide courtesy: H. Palmans

Source occlusion

Large field conditions Small field conditions

(Figure courtesy M.M. Aspradakis et al, IPEM Report 103)

Overlapping of beam penumbras

Das et al. 2008 Med Phys 35: 206-15

definition

of field

size is not

unique

Detector-related small field condition

Based on criterion 1, one could claim that the GammaKnife

18 or 14 mm diameter fields are not small (quasi point

source + electron equilibrium length about 6 mm).

Meltsner et al., Med

Phys 36:339 (2009)

Exradin A16 inner

diameter

Exradin A16 outer

diameter

Detector dependence of output factor

From Sanchez-Doblado et al. 2007 Phys Med 23:58-66

Detector issues in small field dosimetry

• Energy dependence of the response

• Perturbation effects

– Central electrode

– Wall effects

– Fact that cavity is different from water, fluence perturbation

– Volume averaging

• These effects depend somewhat on the beam spot size

Dosimetry protocol values (e.g., TG-51) of these factors are

applicable usually only in TCPE and only for the

conditions:

10 x 10 cm2; zref = 10 cm; SSD or SAD 100 cm

Detector issues in small field dosimetry

15

Stopping power ratio water to air

Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)

Very

small

effects!

0.5% effect

Andreo&Brahme PMB 8:839 (1986)

Role of different perturbation factors

080915

Crop et al., Phys Med Biol 54:2951 (2009)

PP31006 and PP31016

chambers

Magnitude of correction factors on and

off-axis

080915

Crop et al., Phys Med Biol 54:2951 (2009)

8 mm x 8 mm field, 10 cm depth (0.6 mm, 2 mm spot sizes)

Very large effects!

Very large effects! Relatively small effects!

Correction factors for ionization

chambers

Benmahklouf and Andreo (2013)

Diodes for small field dosimetry

Sauer and Wilbert 2007

Med Phys 34:1983-8

Shielded and unshielded diodes

Benmahklouf and Andreo (2013)

Benmahklouf and Andreo (2013) 22

Summary of issues leading to dosimetric

uncertainties in small fields

• Beam dependent issues

– Beam focal spot size

– Lateral disequilibrium

– How do we measure beam quality in practice?

• Detector effects

– There is no ideal detector

– Volume averaging and fluence perturbation effects

– Corrections depend on beam spot size

What are the single set of two largest contributors to correction

factors and their uncertainties for commercial air-filled ionization

chambers in small photon fields?

3%

17%

75%

5%

1% 1. The stopping power ratio and the central electrode

effect

2. The stopping power ratio and the chamber wall

effect

3. The fluence perturbation effect and the volume

averaging effect

4. The stopping power ratio and the volume averaging

effect

5. The ionization chamber wall effect and the stem

effect

• Correct answer: 3 The fluence perturbation effect and the volume

averaging effect

• Discussion: The field size dependence of stopping power ratios is

0.5% or less. For most ionization chambers the field size

dependence of wall corrections is limited to a few percent. The

volume averaging and fluence perturbation corrections are

potentially very large (on the order of 10-30% or more depending on

the situation)

• Reference:

– Crop et al (2009) Phys Med Biol 54 2951-2969

– Bouchard et al (2009) Med Phys 36 (10), 4654-4663

Which two competing effects lead to field size dependent correction factors of unshielded diode detectors?

1. Intrinsic energy dependence

of Si in photon beams and

volume averaging

2. Intrinsic energy dependence

of Si in photon beams and

perturbation effects

3. Polarity effect and

recombination

4. Polarity effect and

electrometer calibration

5. Recombination effect and

diode doping 1. 2. 3. 4. 5.

14%

78%

5%0%

3%

• Correct answer: 2 Intrinsic energy dependence of Si in photon

beams and electron fluence perturbation effects

• Discussion: Volume averaging is usually small in diodes because of

the small size of the sensitive volume. Diodes are not polarized by

an external bias, so there is no polarity effect. Recombination effects

and diode doping are not relevant in this context.

• References:

– Francescon et al 2011, Med Phys 38: 6513

– Benmakhlouf et al 2014, Med Phys 41: 041711

IAEA TECDOC small field dosimetry

• Code of Practice / working document

• Physics relevant to reference and relative dosimetry

• Formalism

• Instrumentation

• Practical implementation

– Machine-specific reference dosimetry

– Relative dosimetry

• Data

Ch. 2 - Physics of small fields

e.g. Small field conditions

LCPE source occlusion detector size

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5 2.0 2.5

beam radius / cm

rati

o o

f d

ose

to

ke

rma

Co-60

6 MV

10 MV

15 MV

Meltsner et al. 2009

Med Phys 36:339-50

Aspradakis et al 2010

IPEM Report 103

Seuntjens

MC

0 0

,

, , , , ,msr refmsr msr

msr msr msr

f ff f

w Q Q D w Q Q Q Q QD M N k kmsrclin

msrclin

msr

msr

clin

clin

ff

QQ

f

Qw

f

Qw DD,

,,, Machine specific

reference field fmsr Clinical field

fclin

Tomotherapy

5 cm x 20 cm

REFERENCE DOSIMETRY RELATIVE DOSIMETRY

GammaKnife

d = 1.6/1.8 cm

CyberKnife

6 cm

Ionization chamber

Broad beam reference field

fref

00 ,,, QQQwDkN

Hypothetical

reference field fref

micro MLC

10 cm x 10

cm

refmsr

msr

ff

QQk,

,

Radiosurgica

l collimators

d = 1.8 cm

refmsr

msr

ff

QQk,

,

msrclin

msrclinmsr

msr

clin

clinmsrclin

msrclin

ff

QQf

Q

f

Qff

QQ kM

M,

,

,

,

30

Reference Fields Small Fields

Ch3. – Formalism (Alfonso et al) / Dw in

machine specific reference (msr) fields

• Chamber calibrated specifically for the msr field

• Chamber calibrated for the conventional reference field and

generic correction factors are available

• Chamber calibrated for the conventional reference field and

generic correction factors not available

msr

msr

msr

msr

msr

msr

f

QwD

f

Q

f

Qw NMD ,,,

refmsr

msr

refmsr

msr

msr

msr

ff

QQ

f

QwD

f

Q

f

Qw kNMD,

,,,, 00

refmsr

msr

refrefmsr

msr

msr

msr

ff

QQ

f

QQ

f

QwD

f

Q

f

Qw kkNMD,

,,,,, 00

fref==10 x 10 cm2

Q0= 60Co

Equivalent square fields - msr

WFF beams:

BJR 25 - equivalent

field size is energy

independent

FFF beams:

equivalent field size is

energy dependent;

Tables are provided

for 6 MV and 10 MV

Ch 3. – Formalism / equations for

beam quality in non-standard

reference fields

for TPR20,10(10) = TPR20,10

(Palmans 2012 Med Phys 39:5513)

)(

)()()( ,,

sd

sdsTPRTPR

101

1010

1020

1020

0.55

0.60

0.65

0.70

0.75

0.80

0.85

2 4 6 8 10 12

s / cm

TP

R2

0,1

0(s

)

(b)4 MV

10 MV

8 MV

6 MV

5 MV

25 MV21 MV18 MV15 MV12 MV

Ch 3. – Formalism / equations for

beam quality in non-standard

reference fields for PDD10X(10) = %dd(10)X

11

1

1010

2

10

110

10

t

s

t

s

ec

ecsPDD

PDD

)(

)(

07510020102671

075101010

1010

1010

10.)(,.)(.

.)(),()(

PDDPDD

PDDPDDPDD x

(Palmans 2012 Med Phys 39:5513-9) 55

60

65

70

75

80

85

2 4 6 8 10 12

s / cmP

DD

10(s

)

4 MV

10 MV

8 MV

6 MV

5 MV

25 MV

21 MV

18 MV

15 MV

12 MV

(d)

(TG-51)

Note about volume averaging in FFF

beams

Pantelis et al. 2009 Med Phys 37:2369

Volume averaging in FFF beams

Ch3. – Formalism / determination of

field output factors

• Field output factor relative to reference field (ref stands here for a

conventional reference or msr field)

• Field output factor relative to reference field using intermediate field or ‘daisy chaining’ method

where

refclin

refclinref

ref

clin

clinrefclin

refclin

ff

QQf

Q

f

Qff

QQ kM

M,

,

,

,

refclin

refclinref

ref

clin

clinrefclin

refclin

ff

QQf

Q

f

Q

f

Q

f

Qff

QQ KICM

ICM

M

M ,,

,

,)(

)(

(det)

(det) intint

int

int

)((det),

,

,

,

,

,int

int

int

detICkkK ref

ref

clin

clin

refclin

refclin

ff

QQ

ff

QQ

ff

QQ

Ch 4 – Instrumentation

• Required equipment, detectors, phantoms for

msr dosimetry

• Required equipment, detectors, phantoms for

relative dosimetry

Ch 5 – Practical implementation msr

dosimetry

• Reference conditions for beam quality and msr

dosimetry

• Overall correction factors for ionization

chambers

• Correction for influence quantities

• Measurement in plastic phantoms and cross-

calibration

Ionization chambers,

recombination, polarity

LeRoy et al., PMB 56:5637-51 (2011)

Agostinelli et al., Med Phys 35:3293-301 (2008)

Note on the use of plastic phantoms

(Seuntjens et al 2005, Med. Phys. 32: 2945)

Ch 5 – Practical implementation msr

dosimetry / availability data

refmsr

refmsr

ff

QQk,

,

Francescon et al: Phys. Med. Biol. 57 (2012) 3741–3758

Correction factor data (cont’d)

Ch 6 – Practical implementation

relative dosimetry

• Required equipment, detectors, phantoms

• Measurements of profiles and field output factors

• Correction factors for determination of output

factors

Ch 6 – Practical implementation

relative dosimetry / correction

factors for OF

• Examples of different sources of correction

factors will be further discussed in the next

presentation (I. Das)

• IAEA-AAPM code of practice data tables is

based on a vetted set of correction factor from

the literature

• Uncertainty analysis has been performed

Field output factors – correction

factors - example

• PTW-60012 – unshielded diode

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

0.0 2.0 4.0 6.0 8.0 10.0

co

rre

ctio

n fa

cto

r w

ith

re

f fin

t

square field size / cm

Cranmer Sargison 2011 / Elekta

Cranmer Sargison 2011 / Varian

Krauss 2008

Schwedas 2007

Griesbach 2005

Ralston et al 2011 - cones

Ralston et al 2011 - microMLC

Fit (exp)

Fit (exp + MC)

Field output factors – correction

factors - diode

• IBA SFD – unshielded diode

0.950

0.960

0.970

0.980

0.990

1.000

1.010

1.020

1.030

1.040

1.050

0.0 2.0 4.0 6.0 8.0 10.0

co

rrection f

acto

r w

ith r

ef fint

square field size / cm

Azangwe 2014 Lechner 2013 WFF Lechner 2013 FFF Bassinet 2013 /Novalis+muMLC Cranmer Sargison 2011 / Elekta Cranmer Sargison 2011 / Varian Sauer 2007 Ralston et al 2011 - cones Ralston et al 2011 - microMLC Fit (exp) Fit (exp + MC)

Field output factors – correction

factors • PTW-31006 - Pinpoint

Uncertainty in correction factor

introduced due to field size definition

Cranmer Sargison et al Med. Phys. 38, 6592–6602 (2011) Benmakhlouf et al Med. Phys. 41, 041711 (2014)

Output factors – validation

methodology

)1(

)2(

)2(

)2(

)1(

)1(

)2(

)1(

,

,

,

,

,

,

clin

clin

clin

clin

msr

msr

clin

clin

clin

clin

msr

msr

msrclin

msrclin

msrclin

msrclin

f

Qrel

f

Qrel

f

Q

f

Q

f

Q

f

Q

ff

QQ

ff

QQ

M

M

M

M

M

M

k

k

msrclin

msrclinmsr

msr

clin

clin

msr

msr

msr

msr

clin

clin

clin

clin

msr

msr

clin

clin

msr

msr

clin

clinmsrclin

msrclin

ff

QQf

Q

f

Q

f

Q

f

Qw

f

Q

f

Qw

f

Q

f

Q

f

Qw

f

Qwff

QQ kM

M

MD

MD

M

M

D

D,

,

,

,

,

,,

,

clin

clin

msr

msr

msr

msr

clin

clinmsrclin

msrclin f

Q

f

Q

f

Qw

f

Qwff

QQM

M

D

Dk

,

,,

,Where:

Output factors – example CyberKnife

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 60012

EDGEAlanineTLDEBT 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

(M/M

60) 2

/(M

/M6

0) 1

PinPoint

Diode 60008

Diode 60012

EDGE

Alanine

TLD

EBT film

Polymer gel

0.950

1.000

1.050

1.100

1.150

1.200

1.250

1.300

0 5 10 15 20

diameter / mm

rati

o o

f co

rre

cti

on

fac

tors

(M

C o

r vo

l) PinPoint

Diode 60008

Diode 60012

EDGE

Alanine

) 2

( ) 1

( , ,

, , m

sr

clin

msr

clin

msr

clin

msr

clin

f f

Q

Q

f f

Q

Q

k

k

0.85

0.90

0.95

1.00

1.05

1.10

1.15

Diode 60008

Diode 60012

EDGE

TLD

EBT film

Polymer gel

A16

PinP

oint

Diode 60008

Diode 60012

EDGE

Alanine

TLD

EBT film

Polymer gel

0.50

0.55

0.60

0.65

0.70

0.75

detector

Mc

lin /

Mre

f

(M

clin

/ Mre

f )* k

clin

,ms

r

ExrA16 PinPoint SHD USD EDGE alanine TLD EBT GEL

Pantelis et al.

2010 Med Phys

37: 2369

Slide courtesy:

H. Palmans

For what purpose is the measurement of the

beam quality specifier required?

1. To specify the correction factors to be applied to the output ratios measured in small fields

2. To specify small field output factors

3. To specify the beam quality correction factor in the msr field

4. To ensure the beam is of adequate quality

5. To specify the absorbed dose calibration coefficient for a small field 1. 2. 3. 4. 5.

22%

6%

13%

1%

58%

• Correct answer: 3 To specify the kQmsr,Qref beam quality correction

factor in the msr field

• Discussion: In general, no beam quality measurement is performed

in small fields, only in msr fields.

• References:

– Palmans 2012 Med Phys 39: 5513

Conclusions

• Solutions to most small-field dosimetry problems

have been described and translated in

formalised procedures

• The IAEA CoP will be coming out in the very

near future – timeline < 6 months