NPL’s progress towards absorbed dose standards for proton beams

H. Palmans1, R. Thomas1, D. Shipley1, A. Kacperek2

1 National Physical Laboratory, Teddington, United Kingdom2 Clatterbridge Centre of Oncology, Wirral, United Kingdom

hugo.palmans@npl.co.uk

Presented at the Workshop on Absorbed Dose and Air Kerma Standards, Paris, 9-11 May 2007

Overview

• Proton therapy and to a lesser extent ion therapy are treatment modalities of increasing importance

• Dosimetry has not been as well established as in high-energy x-ray beams

• NPL’s activities in improving proton and ion dosimetry:SR project 2002-2004Graphite calorimetryInteraction dataAlanine dosimetryMonte Carlo simulation of perturbation correction factors

Topics discussed in this talk

• Calorimetry- Graphite calorimetry in CCO beam- Development of a primary standard level graphite calorimeter for light-

ions• Interaction/basic data

- (wair)p value- Stopping powers- Non-elastic nuclear interaction cross sections

• Correction factors for ionization chambers related to:- Recombination- Dose gradients- Secondary electrons- Non-elastic nuclear interactions

Why protons?

30 mm

Graphite calorimetry for protons CCO(Palmans et al 2004, Phys Med Biol 49:3737-49)

27.396

27.398

27.400

27.402

27.404

27.406

0 500 1000 1500 2000 2500 3000 3500 4000 4500

time (s)

T (º

C)

Graphite calorimetry for protons CCO(Palmans et al 2004, Phys Med Biol 49:3737-49)

Heat transfer: FE (Comsol)

depth (cm graphite)

0.950

0.970

0.990

1.010

1.030

1.050

0.0 0.5 1.0 1.5 2.0 2.5

k vol

,unm

odul

ated

0.995

0.996

0.997

0.998

0.999

1.000

1.001

1.002

1.003

1.004

1.005

kvol,modulated

Volume/gap effects: MC (McPTRAN.RZ)

-0.0010

0.0010

0.0030

0.0050

0.0070

0.0090

0.0110

0.0130

0.0150

0.0170

100.0 150.0 200.0 250.0 300.0 350.0 400.0

t (s)

ΔT

(K)

measurement

simulation

Graphite to water conversion 1: interaction cross sections(ICRU-49 and ICRU-63)

0.75

0.80

0.85

0.90

0.95

1.00

0 100 200 300

Energy (MeV)

[ σnu

cl/A

] w,g

(b)

1.10

1.11

1.12

1.13

1.14

1.15

1.E+00 1.E+01 1.E+02 1.E+03

Energy (MeV)

s w,g

(a)

Graphite to water conversion 2: dose conversion(ICRU-49 and ICRU-63)

0.985

0.990

0.995

1.000

1.005

1.010

1.015

1.020

1.025

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Water equivalent depth (cm)

Dw =

Dg x

con

vers

ion

stopping power ratio only

stopping power ratio and nuclearinteractions

0.5%

1%

Graphite to water conversion 3: fluence correction

0.990

1.000

1.010

1.020

1.030

1.040

1.050

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Water equivalent depth (cm)

Flue

nce

corr

ectio

n fa

ctor

GEANT4

MCNPX

McPTRAN.MEDIA

Experiment pp inphantom (prelimin)Experiment FC(prelimin)

Graphite calorimetry results CCO

0.96

0.97

0.98

0.99

1.00

1.01

1.02D

cal/D

ion

0.98

0.99

1.00

1.01

1.02

1.03

1.04

NE2561 (Co-60)NACP02 (Co-60)Markus (Co-60)NACP02 (e-19)Markus (e-19)

modulated beam

Jun-03

Jun-03

Jun-03

Jun-03

Jun-03

Jun-03

Jun-03

non-modulated beam

Uncertainty

New standard level graphite calorimeter for light-ion beams(cfr Mark Bailey / this workshop)

• Either calibrate ionization chambers or measure kQdata

• Large enough for scatter build-up

• Light enough to be portable

• Robustness, vacuum operation, core size, perturbation, alignment and beam monitoring considerations

Derivation of (wair)p from calorimeter measurements

ccairwcair

ppairwpair

psWpsw

⋅⋅⋅⋅

≈)()()()(

,

,

cwD

ppcalw

cwD

pwDQ N

MDNN

k ==,,

,,

,,

,,

ppairwcwD

ccairwcairpcalwpair

psN

psWDw

⋅⋅

⋅⋅⋅=

)(

)()()(

,,,

,,,

pM ⋅

Importance: new recommendation on proton dosimetry by ICRU/IAEA

33,0

34,0

35,0

36,0

37,0

0 100 200 300 400

E (MeV)

(wai

r) p(J

/C)

IAEA TRS398

New ICRU/IAEA – Jones 2006 (Rad Phys Chem 75:541-50)

Medin et al 2006 (Phys Med Biol 51:1503-21)

New ICRU/IAEA

Faraday cup measurements for range and attenuation measurements

Plates

Faraday cupMonitorchamber

Protonbeam

To elec-trometer

Guard

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

0.0 1.0 2.0 3.0 4.0

Graphite thickness (cm)

Cha

rge

(nC

)

Measured data points

Attenuation fit

Tangent at 50% range

Distal edge fit

Range results

Nuclear attenuation results

• Factor 2 to 3 higher than expected from ICRU 63 tables: not as yet understood.

• Hypothesis: wide angle secondary protons:

Plates

Faraday cup

Guard

Correction factors for ionization chambers: recombination(Palmans et al 2006, Phys Med Biol 51:903-17)

BEAM

PHANTOM

IC AIR CAVITIES

IC1 IC2BEAM

PHANTOM

IC AIR CAVITIES

IC1 IC2

0.0

0.1

0.2

0.3

0.4

0.0 0.2 0.4 0.6 0.8 1.0

Time (in 1/8 revolutions)

Dos

e ra

te (G

y s-1

)

surface

z = 23 mm z = 20 mmz = 10 mm

1.000

1.010

1.020

1.030

1.040

1.050

1.060

1.070

1.080

0.0 1.0 2.0 3.0IV or IV,eff (nA)

I V/I V

/n

(d)

0.998

1.000

1.002

1.004

1.006

1.008

1.010

1.012

1.014

1.016

1.018

Equati

on (1

)

Pulsed

(Boa

g)

Markus

1Mark

us 2

p ion

Perturbation correction factors for ionization chambers

protons

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

⋅water

air

S⋅=waterD airD ⋅ pcav ⋅ pwall ⋅ pcel ⋅ pdis

SA

• For high-energy x-rays: typical corrections of level 1% applied since 1970’s

• For protons: not applied in any recommendation

pdis: Monte Carlo - McPTRAN.CAVITY(Palmans 2006, Phys Med Biol 51:3483-501)

1

2

3

Geometry interrogation region

Protonbeam

E

dϕ /dE

Secondary electrons: EGSnrc + variance reduction techniques (Verhaegen&Palmans 2001 Med. Phys. 28:2088)

• D in cavities~~~~~~~~~~~~~~~~~• Histories resumed

Chamber completeNew depth

~~~~~~~~~~~~~~~~~• Variance reduction

Lateral range rejectionHistory splitting

~~~~~~~~~~~~~~~~~• PDDIC compared

with PDD in homogeneous water

pdis: analytical modelIntegrating the depth dose curve(Palmans 2006, Phys Med Biol 51:3483-501)

z

sleevewallairc.e.

z0

waterx

Rcel

Rcav

Rwall

Rsleeve

O

Proton

P QS T

U

I

pdis: comparison with experimentPDD’s(Palmans 2006, Phys Med Biol 51:3483-501)

Mobit et al. 2000 Med. Phys. 27:2780-2787

78 MeV protons

Jäkel et al. 2000 Phys. Med. Biol. 45:599-607

3 GeV 12C

0.0

0.5

1.0

1.5

2.0

2.5

0.0 20.0 40.0 60.0

depth (mm)

norm

alis

ed d

ose

(a.u

.)

AttixCapintec PR06Reconstructed

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

115.0 120.0 125.0 130.0

depth (mm)

rela

tive

dose

ReferencePTW-30006MarkusReconstructed

pcav,e: SA cavity theory

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

⋅=med

airairmed

SDD

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

⋅=med

airair

SD ⋅ cav,ep

med

air

med

airecav

SSp ⎟⎟⎠

⎞⎜⎜⎝

⎛ρ⎟⎟

⎠

⎞⎜⎜⎝

⎛ρ

=SA

,

protonsSA

δ-electrons

due to secondary electrons

pwall,e: SA cavity theory

δ-electrons SA

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

⋅=med

airairmed

SDD

protons

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

⋅med

wall

Swall

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

⋅=med

airair

SD ⋅ wall,epSA

med

air

wall

airwall,ep = S

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

SA

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

⋅med

wall

S S⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

SA

pcav,e & pwall,e: SA cavity theory for

FWT-IC18 type chamber

0.998

1.000

1.002

1.004

1.006

1.008

1.010

0.01 0.10 1.00 10.00 100.00Rres

p cav

,e.p

wal

l,e

PCAV*PWALL_P_TEP_E_TEE50 MeV

150 MeV250 MeV

FWT-IC18

pcav,e & pwall,e: Monte Carlo versus SA cavity theory (spher r = 0.25cm, Δ = 13.2 keV)

1.000

1.002

1.004

1.006

1.008

1.010

0 50 100 150 200 250 300

Eeff (MeV)

p cav

,e.p

wal

l,e

A150CPMMAwater

pcav,e & pwall,e: comparison with experiment

0.995

1.000

1.005

1.010

1.015

1.020

0 5 10 15

Chamber #

Dw

,NE2

571

/Dw

,Ch

C-C&PTW30002

A150-Al&NE2581

PMMA-Al&PTW30001

Nylon66-Al

IC18

ExrT2

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

0 20 40 60 80 100 120 140E (MeV)

φ E( cm-

2 MeV

-1 )

0.0E+00

2.0E-06

4.0E-06

6.0E-06

8.0E-06

1.0E-05

0 20 40 60 80 100 120 140E (MeV)

φ E( cm-

2 MeV

-1 )

p: α:

pwall,n: (simplistic) analytical model for slowing down spectra secondaryp & α (NE2571, 150 MeVp)

water bulk

“water” wallgraphite wall

due to secondaries from nonelastic nuclear interactions

pwall,n: secondary p & α perturbation (NE2571)

• BUT:

• ICRU 63 data (uC ~ 30-40%)

• Crude model

• → MC study needed

0.992

0.994

0.996

0.998

1.000

1.002

0 50 100 150 200 250 300E (MeV)

pert

urba

tion

fact

or

pp+α

Summary

• Graphite calorimetryMany operation characteristics, perturbation factors and heat

transfer phenomena are similar as for photonsPrimary standard level calorimeter is being builtConversion to dose to water is a serious issue

• Interaction/basic data:Substantial contribution to (wair)p valueRange and attenuation measurements

• Ionization chambersCorrections for recombination, gradients and secondary electronsFurther work: non-elastic nuclear interactions

Acknowledgments

• Simon Duane, Thomas Russell, David Shipley, Mark Bailey, Alan DuSautoy, Andrzej Kacperek (CCO), Frank Verhaegen, Jan Seuntjens and…