small and composite field dosimetry: the problems and ... · pdf filesmall and composite field...

International Dosimetry Symposium, IAEA, Vienna 9-12 Nov 2010

Small and composite field dosimetry: the problems and recent progress

H. Palmans

National Physical Laboratory, Teddington, UK

[email protected]

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2/25International Dosimetry Symposium, IAEA, Vienna 9-12 Nov 2010

Acknowledgements

• IAEA organisers• IAEA/AAPM small and composite field working

group (Rodolfo Alfonso, Pedro Andreo, Roberto Capote, Saiful Huq, Joanna Izewska, Jonas Johansson, Per Kjäll, Warren Kilby, Rock Mackie, Ahmed Meghzifene, Karen Rosser, Jan Seuntjens, Ken Shortt, Wolfgang Ullrich, Stan Vatnitsky)

• IPEM small field working party (Mania Aspradakis, John Byrne, John Conway, Simon Duane, Karen Rosser and Jim Warrington)

• DIN small field subcommittee (Gregor Bruggmoser, Dietrich Harder, Harald Feist, Simon Foschepoth, Ralph-Peter Kapsch, Martin Janich, Edmund Schüle, Oliver Dohm, Kai Henkel, Otto Sauer, Björn Poppe, Harald Treuer and Bernhard Rhein)

• Many others…

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A general thought

• The ideal dosimeter is the patient• Maybe even more ideal is a phantom which

is an identical copy of the patient• Then we can make ranking:

– Anthropomorphic phantoms– Simple phantoms + anatomical features– Simple phantom + patient-like outer dimensions– 30 cm x 30 cm x 30 cm water phantom

QA Ref Dosimetry

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Overview

• Problems and proposed solutions of small field dosimetry

• Problems and proposed solutions of composite field dosimetry

• Dosimetry formalism• Availability of data

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• Loss of lateral charged particle equilibrium

• Partial occlusion of the primary source

• Detector size too largeLi et al. 1995 Med Phys 221167-70

What is a small field – three criteria

From Meltsner et al. 2009 Med Phys 36:339-50

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Small field size – apparent field widening

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

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

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Beam quality

• Problem: no reference conditions• Solutions:

– Sauer 2009 Med Phys 36:4168-72: TPR20,10(smaller field)

– AAPM TG-148 (Langen et al. 2010 Med Phys 37:4817-53): “dd(10)x[HT-ref]”

– Duane 2010: DAP ratio (poster 221, also: Djouguela 2006 Z Med Phys 16:217 & Sanchez-Doblado 2007 Phys Med 23:58-66)

DAP20,10 – Elekta SL25 6MV

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Detector response – ionization chambers

• sw,air:Eklund and Ahnesjö 2008, Phys Med Biol 53:4231-47

Crop et al. 2009 Phys Med Biol 54:2951-69

• pQ:

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Detector response - diodes

Sauer and Wilbert 2007 Med Phys 34:1983-8

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IMRT field dosimetry

From Roberto Capote, IAEA

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Composite field dosimetry

Output measurementref field

Small field OF and profiles

TPS: clinical delivery sequence

Dose calculation in patient

Dose calculation in verification phantom

Verification

TPS: pcsr delivery sequence

Dose calculation in pcsr phantom

Output measurementpcsr field

1 2 3

Treatment delivery

Output measurementref field

Small field OF and profiles

TPS: clinical delivery sequence

Dose calculation in patient

Dose calculation in verification phantom

Verification

TPS: pcsr delivery sequence

Dose calculation in pcsr phantom

Output measurementpcsr field

1 2 3

Treatment delivery

20º 60º

100º

140º180º220º

260º

300º 340º

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Criteria for pcsr fields

• Dose homogeneous over volume surrounding ion chamber

• Simplicity (ref conditions) versus clinically relevant (as close as possible to clinical plan) (Poster 321: Öhrman et al)

• Based on homogeneity index (Chung, last talk of this session)

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Dose at a point (cfr. ICRU Report 83)

• Two other concepts:– For full CPE:

– Dose to ion chamber calculation

Bouchard et al. 2009Med Phys 36:4654-63

ref

refpcsr

pcsr fgr

ffQQ P

k 1,, =

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Ion recombination in dynamic fieldsRecombination in an IMRT sequence (Helical

TomoTherapy)

phantom

cavity

Fan beam

( )[ ]

( )V

t L

Lpulseair

t L

Lpulseair

pulsed

V

satVion e

dxdttxD

dxdttxD

VB

VA

QQep γγ −

+

−

+

−− ⋅

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

⋅++≈=⋅

∫ ∫

∫ ∫

0

2

2

0

2

2

2

1 /

/,

/

/,**

,

,

0.00

0.02

0.04

0.06

0.08

-4 -2 0 2 4

chamber offset / cm

∫Dai

rdx/

L / e

V g-1

0.000

0.001

0.002

0.003

0.004

∫( Dair ) 2dx/∫ D

air dx / eV g-1

Model + MC

0.00

0.02

0.04

0.06

0.08

-4 -2 0 2 4

chamber offset / cm

∫Dai

rdx/

L / e

V g-1

0.000

0.001

0.002

0.003

0.004

∫( Dair ) 2dx/∫ D

air dx / eV g-1

Model + experiment

0.00

0.01

0.02

0.03

-4 -2 0 2 4

chamber offset / cm

∫Dai

rdx/

L / e

V g-1

0.000

0.001

0.002

0.003 ∫ (Dair ) 2dx/∫ D

air dx / eV g-1

0.00

0.01

0.02

0.03

-4 -2 0 2 4

chamber offset / cm

∫Dai

rdx/

L / e

V g-1

0.000

0.001

0.002

0.003 ∫ (Dair ) 2dx/∫ D

air dx / eV g-1

V1/V2 = 250/100V1/V2 = 250/50V1/V2 = 100/50

Palmans et al. 2010 Med. Phys. 37:2876

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Formalism proposed in Alfonso et al 2008, Med Phys 35:5179

• Two related routes for Dw in non-conventional reference conditions, both requiring extension of concept of reference field and modified reference conditions

• Small static field dosimetry: intermediate machine-specific-reference field (msr)

• Composite field dosimetry: plan-class specific reference field (pcsr) – A pcsr field can be a 3-D irradiated volume or a 4-D

delivery sequence. – The pcsr should be as close as possible to a class of

clinical plans of interest, and provide a uniform dose over a region exceeding the dimensions of a reference detector

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0 0

,, , , , ,

msr refmsr msr

msr msr msr

f ff fwQ Q D wQ Q Q Q QD M N k k= msrclin

msrclin

msr

msr

clin

clin

ffQQ

fQw

fQw DD ,

,,, Ω=Machine specific

reference field fmsr Clinicalfclin

Tomotherapy5cm x 20cm

REFERENCE DOSIMETRY RELATIVE DOSIMETRY

GammaKnife∅ 1.6/1.8 cm

CyberKnife6 cm

1

≡ Ionizationchamber

Broad beamreference field

fref

00 ,,, QQQwD kNHypothetical

reference field fref

BrainLABmicro MLC10cmx10cm

refmsr

msr

ffQQk ,,

Radiosurgicalcollimators∅ 1.8 cm

refmsr

msr

ffQQk ,,

msrclin

msrclinmsr

msr

clin

clinmsrclin

msrclin

ffQQf

Q

fQff

QQ kMM ,

,,, ⋅=Ω

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refpcsr

pcsr

ffQQk,,

refpcsr

pcsr

pcsr

pcsr

pcsr

pcsr

ffQQQQQwD

fQ

fQw kkNMD ,

,,,,, 00= pcsrclin

pcsrclin

pcsr

pcsr

clin

clin

ffQQ

fQw

fQw DD ,

,,, Ω=Broad beam

reference field fref

Plan-classspecific reference

fieldfpcsr

00 ,,, QQQwD kN

Clinicalfclin

msrpcsr

msrpcsr

ffQQk ,,

Hypotheticalreference field e.g. 9-field prostate

pcsr

(e.g. IMRT Linac)

refmsr

msr

ffQQk ,,

fmsr

(e.g. Tomotherapy5cm x 20cm)

REFERENCE DOSIMETRY RELATIVE DOSIMETRY

20º 60º

100º

140º180º220º

260º

300º 340º

2

≡ Ionizationchamber

pcsrclin

pcsrclinpcsr

pcsr

clin

clinpcsrclin

pcsrclin

ffQQf

Q

fQff

QQ kM

M ,,

,, ⋅=Ω

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A few data on refmsr

refmsr

ffQQk ,

,

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Examples – msr/pcsr: TomoTherapy

Sterpin et al (2009)

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Field output factors – Monte Carlo calculated correction factors CyberKnife

Francescon et al 2008 Med Phys 35:504-13

msr

clin

msr

clin

ff

QQk

, ,

msrclin

msrclin

ffQQ,,Ω

collimator mm 60

,,

,,

,,

,,

=

⋅=

+⋅=

msr

MMk

bak

msrclin

msrclin

msrclin

msrclin

msrclin

msrclin

msrclin

msrclin

ffQQ

clin

msrffQQ

ffQQ

ffQQ

Ω

Ω

Different FWHM primary source

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Field output factors:

)1()2(

)2()2(

)1()1(

)2()1(

,

,,,

,,

clin

clin

clin

clin

msr

msr

clin

clin

clin

clin

msr

msr

msrclin

msrclin

msrclin

msrclin

fQrel

fQrel

fQ

fQ

fQ

fQ

ffQQ

ffQQ

MM

MM

MM

kk

=⋅=

msrclin

msrclinmsr

msr

clin

clin

msr

msr

msr

msr

clin

clin

clin

clin

msr

msr

clin

clin

msr

msr

clin

clinmsrclin

msrclin

ffQQf

Q

fQ

fQ

fQw

fQ

fQw

fQ

fQ

fQw

fQwff

QQ kMM

MDMD

MM

DD ,

,,

,

,

,,, ⋅=

⎥⎥⎦

⎤

⎢⎢⎣

⎡⋅==Ω

clin

clin

msr

msr

msr

msr

clin

clinmsrclin

msrclin fQ

fQ

fQw

fQwff

QQ MM

DD

k ⋅=,

,,,

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Field output factors - CyberKnife: Pantelis et al. 2010 Med Phys 37:2369

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 60012EDGEAlanineTLDEBT f ilmPolymer 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/M

60) 1

PinPointDiode 60008Diode 60012EDGEAlanineTLDEBT 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

ratio

of c

orre

ctio

n fa

ctor

s (M

C o

r vol

) PinPoint

Diode 60008

Diode 60012

EDGE

Alanine

)2()1(

, ,, ,

msr

clin

msr

clin

msr

clin

msr

clin

ff

QQ

ff

QQ

kk

0.85

0.90

0.95

1.00

1.05

1.10

1.15

Diode 60008

Diode 60012

EDGE

TLD

EBT film

Polymer gel

16 nPoi

nt

ode

6000

8

ode

6001

2

DG

E

anin

e

LD BT fi

lm

olym

er g

el

0.50

0.55

0.60

0.65

0.70

0.75

detector

Mcl

in /

Mre

f

(Mclin / M

ref )* kclin,m

sr

ExrA16 PinPoint SHD USD EDGE alanine TLD EBT GEL

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Plan class specific reference fieldsDynamic IMRT H&N – Chung et al. 2010 Med. Phys. 37:2404-13

VMAT – Rosser and Bedford 2009 Phys. Med. Biol. 54:7045-7061

TomoTherapy – Bailat et al. 2009Med. Phys. 36:3891-3896

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0.97

0.98

0.99

1.00

1.01

1.02

1.03

pcsr field

kpc

sr,re

f

Bailat et al. 2009

Rosser and Bedford 2009

Chung et al. 2010

pcsr field correction factors

0.97

0.98

0.99

1.00

1.01

1.02

1.03

pcsr field

kpc

sr,re

f

Bailat et al. 2009

Rosser and Bedford 2009

Chung et al. 2010

k=2

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Conclusions

• Small field dosimetry: proposed solutions for definition, field size, beam quality, msr field, correction factors,…

• Composite field dosimetry: continue exploration of ideas, proven for TomoTherapy (AAPM TG-148) to other modalities; step-and-shoot and dynamic IMRT, RapidArc, GammaKnife, …

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small and composite field dosimetry: the problems and ... · pdf filesmall and composite field...

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