Radiotherapy users group meeting, NPL, 5 June 2007

An Overview of Radiotherapy Dosimetry at the NPL

Hugo PalmansNational Physical Laboratory, UK

hugo.palmans@npl.co.uk

Radiotherapy users group meeting, NPL, 5 June 2007 2/33

Overview

• High-energy photon and electron beams + 60Co:

→ Absorbed dose to water standards• Low- and medium-energy x-rays + 137Cs + 60Co:

→ Air kerma standards• Brachytherapy:

→ 192Ir: Air kerma standard + recent developments towards absorbed dose to water

• Opthalmic beta applicators:

→ Absorbed dose to water standard

Radiotherapy users group meeting, NPL, 5 June 2007 3/33

Absorbed dose standards: definition of absorbed dose

hν1

dm

T

T’

hν2hν3

hν4

e-

ε = hν1 – (hν2 + hν3 + T’)

Example

dmdD med

medε

=

Mean energy imparted to matter per unit of mass.Energy imparted = energy incident minus energy leaving + energy released in transformations

Radiotherapy users group meeting, NPL, 5 June 2007 4/33

Absorbed dose standards: calorimetry

radiation energy turns into heat

heat is tiny even though biological effect is lethal in radiotherapytiny, but measurable / our primary standards for absorbed dose are

calorimeters

Radiotherapy users group meeting, NPL, 5 June 2007 5/33

Absorbed dose standards: principlesof calorimetry

tDTct

Tc∂∂+∇=

∂∂ 2... αD=c·∆T

c(J·kg-1·K-1)

∆T/D(mK·Gy-1)

water 4180 0.24

graphite 710 1.41

α(m2·s-1)

1.44×10-7

0.80×10-4

Radiotherapy users group meeting, NPL, 5 June 2007 6/33

Absorbed dose standards: principlesof calorimetry

• thermistors to measure temperature differences

• ultimate sensitivity is around one µK

• signal is around one mK

• signal to noise is of the order 1000:1

⎟⎟⎠⎞

⎜⎜⎝⎛ −⋅

⋅= 0

11

0TTeRR

β

Radiotherapy users group meeting, NPL, 5 June 2007 7/33

Absorbed dose standards: the most direct method is water calorimetry (e.g. PTB, NRC, METAS,NMi, LSDG)

Cooling fluidAir4ºCBeam

Radiotherapy users group meeting, NPL, 5 June 2007 8/33

Absorbed dose standards: graphitecalorimetry

Beam

Dw DC

Radiotherapy users group meeting, NPL, 5 June 2007 9/33

Absorbed dose standards at NPL: primary standard for high-energy x-rays

Radiotherapy users group meeting, NPL, 5 June 2007 10/33

Absorbed dose standards at NPL: conversion procedure for high-energy photons

(1)w

C

en

C

w

C

w

C

wCw ⎟⎟

⎠

⎞⎜⎜⎝

⎛ρµ

⋅ββ⋅⋅⋅=

dd

QQDD

(2)w

C

en

C

w

C

wCw

2

⋅⎟⎟⎠

⎞⎜⎜⎝

⎛ρµ

⋅ββ⋅⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅= K

ffDD

Radiotherapy users group meeting, NPL, 5 June 2007 11/33

Absorbed dose standards at NPL: electron graphite calorimeter (electron calibration service)

Radiotherapy users group meeting, NPL, 5 June 2007 12/33

Absorbed dose standards at NPL: conversion procedure for high-energy electrons

Reference depth in water

cm 1.06.0 ,50 −⋅= ww RzTheoretical conversion of dose to graphite to dose to water

airg

airw

gref

wrefgrefDwrefD s

spp

NN,

,

,

,,,,, ⋅⋅=

Compare user and reference chambers at zw in water

guser

wrefwrefDwuserD M

MNN

,

,,,,, ⋅=

Range scaling to get depth in graphite

w

gwg R

Rzz

,50

,50⋅=

Calibrate chamber against graphite calorimeter, in graphite

gref

ggrefD M

DN

,,, =

(McEwen et al 1998, Phys Med Biol 43:2503-19)

Radiotherapy users group meeting, NPL, 5 June 2007 13/33

Absorbed dose standards at NPL: portable graphite calorimeter (off site measurements)

Radiotherapy users group meeting, NPL, 5 June 2007 14/33

Absorbed dose standards at NPL: Dose determination with ion chamber

psDD ⋅⋅= airw,airw

eW

mQD ⋅=

airair e

WV

Q ⋅⋅

=airρ

eW

dxdQ

A⋅⋅

⋅=

ρ1

eW

dVdQ

air

⋅⋅=ρ1

eW

VQDair

air ⋅⋅=ρ1

A

x

HV guard

c.e.

E.g. extrapolation chamber:

Radiotherapy users group meeting, NPL, 5 June 2007 15/33

Absorbed dose standards at NPL: alanine dosimetry

Radiotherapy users group meeting, NPL, 5 June 2007 16/33

Absorbed dose standards at NPL: alanine dosimetry

• Not a primary standard but plays a significant role:

Auditing and quality assuranceTomotherapy and potentially IMRT dosimetryOphtalmic beta applicators

• See presentation Simon Duane

Radiotherapy users group meeting, NPL, 5 June 2007 17/33

Absorbed dose standards at NPL –new directions: proton and ion beams

30 mm

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)

CCO experiment(Palmans et al 2004, Phys Med Biol 49:3737-49)

Radiotherapy users group meeting, NPL, 5 June 2007 18/33

Absorbed dose standards at NPL –new directions: proton and ion beams

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 –

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

New ICRU/IAEA

Importance: new recommendation on proton dosimetry by ICRU/IAEA (Jones 2006, Rad Phys Chem 75:541-50)

Radiotherapy users group meeting, NPL, 5 June 2007 19/33

Absorbed dose standards at NPL – new directions: proton and ion beams

New standard level graphite calorimeter for light-ion beams (cfr new photon electron calorimeter / Mark Bailey)

• 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

Radiotherapy users group meeting, NPL, 5 June 2007 20/33

Absorbed dose standards at NPL –new directions: 192Ir

• Calorimeter with ring-shaped core• Dimensions optimised by Monte Carlo simulations and heat transfer

simulations• Source self heating shielded from core by vacuum gaps• Operated in isothermal mode

Radiotherapy users group meeting, NPL, 5 June 2007 21/33

Absorbed dose standards at NPL –new directions: 192Ir, schematic drawing of prototype calorimeter(RZ-geometry)

R = 10 cm

Z =

14 c

mZ

R

1 mm vacuum gap

Absorber:Graphite ring, 2 mm x 5 mm, R = 2.5 cm

Radiotherapy users group meeting, NPL, 5 June 2007 22/33

Absorbed dose standards at NPL –new directions: the future

• More sensitive thermometry (low-temperature)

• Non-contacting thermometry (microwave resonators)

• Micro-bolometers for microdosimetry

Radiotherapy users group meeting, NPL, 5 June 2007 23/33

Air kerma standards: definition of air kerma

hν1

dm

T

T’

hν2hν3

hν4

e-

Etr = T

Example

dmdEK tr

air =

dEtr : sum of initial kinetic energies of all charged particles liberated by uncharged particles in a mass of air dm

Radiotherapy users group meeting, NPL, 5 June 2007 24/33

Air kerma standards: based on the measurement of exposure/ionization

• ionisation in air – easier to measure than heating in waterlimiting sensitivity is a few fA (10-15 amps)signal is hundreds of pA (10-10 amps)signal to noise is of the order 105:1

• free-air chamber – photons interact in air, electrons stopped in air• cavity chamber – photons, electrons interact in wall (e.g. graphite), not air• measured current corrected for :

- air density ρair(p,T)- humidity (NPL calibrations): 0.3% correction is included – nothing else needed if

RH at the time of measurement is between 20% and 70%- recombination (NPL calibrations)

NOTE: air kerma applies to readings without further correctionabsorbed dose applies to readings corrected to zero ion recombinationrecombination is generally significant only in pulsed beams with large instantaneous dose rates

Radiotherapy users group meeting, NPL, 5 June 2007 25/33

Air kerma standards at NPL: free-air chambers

• primary standard of exposure (we measure the ionisation per massof air), rather than of air kerma (we don’t measure the energy released)

• every electron track producing ionisation which will be collected should originate from a primary photon absorption or scattering event in air.

• no electron track can reach the collecting electrodes.

Radiotherapy users group meeting, NPL, 5 June 2007 26/33( )air

aireW

VMK /..ρ

=air

Air kerma standards at NPL: free-air chambers

escahs kkkkk .....( )airair

eWVMK /..ρ

=air

A.L

LA

Radiotherapy users group meeting, NPL, 5 June 2007 27/33

Air kerma standards at NPL: free-air chambers

• Improved correction factors by measurement (attenuation and photon scatter but also recombination) and Monte Carlo simulations (including fluorescence generated by argon)

• Will lead to improved uncertainty analysis

• (Contact Rebecca Nutbrown)

Radiotherapy users group meeting, NPL, 5 June 2007 28/33

Air kerma standards at NPL: cavity ionisation chambers (as primary standards)

• at energies much higher than 300 kV, the size of a free-air chamber becomes impractically large.

• use cavity standards instead, but ensure that volume is measurable

• apply corrections for

wall material (graphite instead of air)the fraction going into radiation lossesattenuation and scatter in the wall

Radiotherapy users group meeting, NPL, 5 June 2007 29/33

Air kerma standards at NPL: priniciplefor high-energy gamma beam

( ) attCcolCairenaircol kKK ⋅⋅ρµ= ,,,

aircolair g

KK

−= ,

1

cepC kD ⋅air

airC

SA

DS

⋅ ⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

,

air

air

eW

VQ ⋅ρ⋅

Radiotherapy users group meeting, NPL, 5 June 2007 30/33

Air kerma standards at NPL: priniciplefor high-energy gamma beam

( ) ( ) iCairenairC

SA

air

airair kS

geW

VQK ⋅ρµ⋅⎟⎟

⎠

⎞⎜⎜⎝

⎛ρ

⋅−

⋅ρ⋅

= ∏,,1

attcep kkk ⋅=∏ ionk⋅polk⋅sk⋅humk⋅stemk⋅sc k⋅

wallK

5 10 15 20graphite thickness (mm)

0.94

0.96

0.98

1.00

1.02

0

rela

tive

resp

onse katt.ksc x kcep=

kwall

Radiotherapy users group meeting, NPL, 5 June 2007 31/33

Air kerma standards at NPL: 60Co

• At present based on three 1.8cc chambers

• A set of variable volume chambers has been constructed to replace the aging existing standards

• (Contact Andy Shah)

Radiotherapy users group meeting, NPL, 5 June 2007 32/33

Air kerma standards at NPL: 192Ir (RAKR)

Chamber stem (carbon fibre)

Central conductor and electrode (carbon)

Spherical shell (carbon)

Insulator

Radiotherapy users group meeting, NPL, 5 June 2007 33/33

Air kerma standards at NPL: 192Ir (RAKR)

• Comparison with LNHB (equiv. within uncertainties)

• Monte Carlo calculated perturbation correction factors• Improved correction factors for ion recombination and polarity• Improved correction factor for attenuation and scatter• Improved uncertainty budget

• (Contact Thorsten Sander)