Dosimetry of Static Small Photon Fields Jan Seuntjens McGill - - PowerPoint PPT Presentation

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
• f Health Research, the Natural Sciences and

Engineering Research Council, Canada through

• perating 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

• f 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

• ff-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

• f Si in photon beams and

volume averaging 2. Intrinsic energy dependence

• f 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 ratio of dose to kerma

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

, , , , , ,

msr ref msr msr msr msr msr

f f f f w Q Q D w Q Q Q Q Q

D M N k k 

msr clin msr clin msr msr clin clin

f f Q Q f Q w f Q w

D D

, , , ,

 

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

, , , Q Q Q w D

k N

Hypothetical reference field fref

micro MLC 10 cm x 10 cm

ref msr msr

f f Q Q

k

, ,

Radiosurgica l collimators

d = 1.8 cm

ref msr msr

f f Q Q

k

, ,

msr clin msr clin msr msr clin clin msr clin msr clin

f f Q Q f Q f Q f f Q Q

k M 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 Q w D f Q f Q w

N M D

, , ,

 

ref msr msr ref msr msr msr msr

f f Q Q f Q w D f Q f Q w

k N M D

, , , , ,

0 

 

ref msr msr ref ref msr msr msr msr

f f Q Q f Q Q f Q w D f Q f Q w

k k N M D

, , , , , ,

   

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)

) ( ) ( ) ( ) (

, ,

s d s d s TPR TPR        10 1 10 10

10 20 10 20

0.55 0.60 0.65 0.70 0.75 0.80 0.85 2 4 6 8 10 12 s / cm TPR 20,10(s) (b)

4 MV 10 MV 8 MV 6 MV 5 MV 25 MV 21 MV 18 MV 15 MV 12 MV

Ch 3. – Formalism / equations for beam quality in non-standard reference fields

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

                      

 

1 1 1 10

10 2 10 1 10 10 t s t s

e c e c s PDD PDD ) ( ) (         75 10 20 10 267 1 75 10 10 10

10 10 10 10 10

. ) ( , . ) ( . . ) ( ), ( ) ( PDD PDD PDD PDD PDD

x

(Palmans 2012 Med Phys 39:5513-9)

55 60 65 70 75 80 85 2 4 6 8 10 12 s / cm PDD 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

ref clin ref clin ref ref clin clin ref clin ref clin

f f Q Q f Q f Q f f Q Q

k M M

, , , ,

  

ref clin ref clin ref ref clin clin ref clin ref clin

f f Q Q f Q f Q f Q f Q f f Q Q

K IC M IC M M M

, , , ,

) ( ) ( (det) (det)

int int int int

   

) ( (det)

, , , , , ,

int int int det

IC k k K

ref ref clin clin ref clin ref clin

f f Q Q f f Q Q f f Q Q

 

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

ref msr ref msr

f f Q Q

k

, ,

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

correction factor with ref fint 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

correction factor with ref 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 msr clin msr clin msr clin msr clin

f Q rel f Q rel f Q f Q f Q f Q f f Q Q f f Q Q

M M M M M M k k   

msr clin msr clin msr msr clin clin msr msr msr msr clin clin clin clin msr msr clin clin msr msr clin clin msr clin msr clin

f f Q Q f Q f Q f Q f Q w f Q f Q w f Q f Q f Q w f Q w f f Q Q

k M M M D M D M M D D

, , , , , , , ,

             

clin clin msr msr msr msr clin clin msr clin msr clin

f Q f Q f Q w f Q w f f Q Q

M M D D k  

, , , ,

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 5 10 15 20 diameter / mm M / M60

A16 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 5 10 15 20 diameter / mm (M/M 60)2/(M/M 60)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 5 10 15 20 diameter / mm ratio of correction factors (MC or vol) PinPoint Diode 60008 Diode 60012 EDGE Alanine

) 2 ( ) 1 (

, , , ,

msr 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 PinPoint Diode 60008 Diode 60012 EDGE Alanine TLD EBT film Polymer gel

0.50 0.55 0.60 0.65 0.70 0.75 detector M clin / Mref (Mclin / Mref)* kclin,msr

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