Recent advances in dosimetry in reference conditions

for proton and light-ion beams

S. Vatnitskiya), P. Andreob)

and D.T.L. Jonesc)

a)

MedAustron, Wiener Neustadt, Austriab)

Medical Radiation Physics, Stockholm University -

Karolinska Institutet, Stockholm, Swedenc)

International Commission on Radiation Units the and Measurements, Bethesda, MD, USA

Presented to: IAEA Dosimetry SymposiumVienna, Austria November 8 –

12, 2010

about 30 treatment facilities are established 90,000

patients are treated with

all heavy-charged particles

another 20 light-ion centers are planned to be open in 5 years

treatment of large or deep seated

tumours

Main clinical applications of light ion beams

treatment of oculartumours

stereotactic

radiosurgery(cross-fire technique)

Consistent andharmonized

dosimetry guidelines

Accurate beam calibration

Perform planning of high-precision conformal therapy

Provide interchangeof clinical experience

and treatment protocols between facilities

Ensure exact delivery of prescribed dose

Provide standardizationof dosimetry in radiobiology

experiments

Absorbed dosedeterminationin reference

conditions for light ionbeams

Calorimeter

Thimble air-filled

ionizationchamber

Faraday Cup

Lack of national andinternational

dosimetry standards

Protocols/Code of Practice for proton and heavier ion beam dosimetry

ND,w

-

based formalism -

IAEA TRS-398

Dw (zref ) at any

user quality Q (photons, electrons, protons, heavier ions)

Dw,Q = MQ ND,w,Qo kQ,Qo

beam quality factor

calibration coefficient at Qo

corrected instrument reading at Q

Lack of standards => Qo = 60Co

( )( )

( )( ) 606060

,

,

w air airQ Q Q

Qairw air CoCoCo

s W pk

W ps=

Q dis wall cav celp p p p p=≈

1 for protons

≈

1 for heavier ions

≠

1 for 60Co

Stopping powers for proton beams

•

Basic proton stopping powers from ICRU 49

•

Calculation using MC code PETRA following Spencer-Attix cavity theory

•

Transport included secondary electrons and nuclear inelastic process

Ratio of stopping powers water/air for heavy ions calculated using the computer codes developed by Salamon (for C, Ne, Ar and He) and by Hiraoka and Bichsel (for C). Data for protons and He from ICRU 49.

0 5 10 15 20 25 301.120

1.125

1.130

1.135

1.140

protonsalpha particles

carbon ions

C (Salamon) Ne (Salamon) Ar (Salamon) He (Salamon) p (ICRU-49) He (ICRU-49) C (H and B)

wat

er/a

ir st

oppi

ng-p

ower

ratio

residual range (g cm-2)

1.13

A constant value of sw,air = 1.13 adopted in TRS 398 (ignores fragments)

Stopping powers for heavier ion beams

Geitner et al 2006

60

64

68

72

76

80

84

88

0 4 8 12 16 20

mean excitation energy of liquid water

I-water DRFI-water EXPI-water EST

I wat

er (e

V)

value #

ICRU 73 (2005)superseded

ICRU 37 (1984)ICRU 49 (1993)

ICRU (2009)tentative

0

50

100

150

200

250

300

350

400

26.0 26.5 27.0 27.5 28.0

12C 400 MeV/u on water SHIELD-HIT

Ene

rgy

depo

sitio

n (M

eV/c

m)

Depth (cm)

69 7775 79.7variation with I

wateris ~1 mm for 2 eV

5 mm

Transportable water calorimeters

ProtonsCalorimetryGy/MU

IonometryGy/MU

Difference%

Scattering 

Scanning 

9.087*10−3 

1.198*10−3 

9.118*10−3 

1.203*10−3 

0.34

0.42

Sarfehnia et al., 2010

CalorimetryGy/MU

IonometryGy/MU

Difference%

Protons  

182 MeV

C12

430 

MeV/u

2.95±0.04

2.77 ±

0.05

2.97±0.09

2.69 ±

0.08

+0.7

‐

3.0

Brede et al., 2006

McGill PTB

Graphite calorimetry protons at CCO carbon ions at NIRS

Palmans et al 2004

Sacama et al 2008

Values of w/e for protons and carbon ions deduced from comparison of ionization

chamber and calorimeter measurements

33

34

35

36

0 50 100 150 200

Proton energy / MeV

(wai

r/e) p

/ (J

C-1

)

Delacroix et al., 1997Palmans et al., 2004Palmans et al., 1996Hashemian et al., 2003Siebers et al., 1995Schulz et al., 1992Brede et al., 2006Medin et al., 2006

Sacama et al 2008ICRU 78

Proton beam

34.2 J C-1

35.7 J C-1

34.5 J C-1

TRS 398

Experimental and calculated ratios of proton perturbation factors in 75 MeV proton beam

0.995

1.000

1.005

1.010

1.015

1.020

0 5 10 15Chamber #

D w,N

E257

1/ Dw

,Ch

C-C&PTW30002

A150-Al&NE2581

PMMA-Al&PTW30001

Nylon66-Al

IC18

ExrT2

Data from Palmans et al 2001, and Palmans and Verhaegen, Montreal workshop 2001

080915 18

Monte Carlo calculated pwall

and pcel

for Farmer type chambers in proton beam

0.997

0.998

0.999

1.000

1.001

0 50 100 150 200 250

proton energy / MeV

pce

l

aluminium central electrode graphite central electrode

0.994

0.996

0.998

1.000

1.002

0 50 100 150 200 250

proton energy / MeV

pce

l,Al /

pce

l,C

Monte Carlo

Medin et al. 1995

Palmans et al. 2001 - ND,w-based

Palmans et al. 2001 - NK-based

1.000

1.002

1.004

1.006

0 50 100 150 200 250

proton energy / MeV

pw

all

0.998

0.999

1.000A150

graphite

1.000

1.002

1.004

1.006

1.008

0 50 100 150 200 250

proton energy / MeV

pw

all,A

150

/ pw

all,C

A150

Palmans et al. 2001 - ND,w based

Palmans et al. 2001 - NK based

average

Details see in the poster by Palmans et al IAEA-CN182-230, this symposium.

Experimental kQ values (Medin et al., 2006)

•

Passive beam deliveryNE 2571: 1.021 ±

0.7%

FC65-G: 1.021 ±

0.7%

•

Scanning beamNE 2571:1.032 ±

1.2%

These values can be compared with the tabulated theoretical values from IAEA TRS-398,

which are 1.039 ±

1.7% for both chamber types

(details in the next presentation by J. Medin)

recombination

initial(columnar)

general(volume)

-++

-

“intra-track” one single track

dose or dose-rate independent

“inter-tracks” multiple tracks

dose or dose-rate dependent

Recommendations for protons and heavier ions

Pulsed or pulse scanned proton beams

Two-voltage method

  2

2

12

2

11 ⎟⎟

⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛+=

MMa

MMaak os

VbMM //1/1 += ∞

Scanned lightIon beams

general recombination

  2

2

12

2

11 ⎟⎟

⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛+=

MMa

MMaak os

Reported discrepancies

Recommendation of TRS 398 and given equation for pulsed proton beams are valid if pulse duration is short compared to the ion transit time in the ionization chamber,

If pulse duration is long compared to the ion transit time in the ionization chamber, then the conditions in the chamber during the pulse will be similar to those of a continuous high intensity beam.

Palmans et al 2006Eye beam lineOver-estmate 2%

Lorin et al 2008continious scanningOver-estmate 1%

Jaekel et al 2002Raster scanningOver-estmate 1%

Proton beams –

ICRU 78

•

Cyclotrons (small pulses, high repetition, high dose per pulse)

•

Synchrotrons (Repetition < 0.5 Hz, Acceleration 0.5 –

1s

Effective pulse duration is long compared to ion collection time

of ionization chamber

continuous beam

( )( ) ( )LNLN

LNs MMVV

VVk//1/

2

2

−−

=

Example of proton scanned continuous beam

Cyclotron•

high dose per pulse (0.2 Gy)•

pulse length 400μs•

maximum transit time for the ionization chamber 152 μs (300 V) and 76 μs (600 V)

Lorin et al, 2008

Ion collection time of ionization chamber is shorter

compared

to pulse duration

Scanned continuous beam

( )( ) ( )LNLN

LNs MMVV

VVk//1/

2

2

−−

=

The user should verify the validity of recombination correctionsagainst independent method (Faraday cup, calorimeter, alanine)

Recommendations for scanned light ion beams

The user of a scanned light-ion beam delivery system can perform the measurements of the saturation effects at different voltages by placing the ionization chamber at a calibration depth.

If the data are well described by a linear fit, then the conditions of pulsed scanned beam are met.

If the data are well described by a quadratic fit, then the conditionsfor continuous radiation are met.

Jaekel et al 2002

Issues to be resolved in upcoming ICRU report on light-ion beams

•

TRS 398 may be adopted for light-ion beam dosimetry with beam-line specific adjustments

•

The currently recommended values of sw,air (and Wair ) for absolute dosimetry should be re-considered

•

Uncertainties in stopping powers, including those of the I-values for different tissues (5-10%), must be taken into account to re-estimate what “precision”

is

really achievable in clinical practice

•

ion chamber specific factors and perturbation effects

•

calculation of beam quality correction factors, for new ionization chambers and experimental verification of calculated values

•

calculation of stopping power ratios

•

determination of w-values

Future

improvements in proton and heavier ion beams dosimetry would be focused on:

Standard uncertainties in Dw

(TRS 398, ICRU 78)

u(ND,wSSDL) = 0.6 kQ calc

Co-60 gamma-rays

0.9High-energy photons

1.5

High-energy electrons

1.4-2.1

Proton beams

2.0-2.3Heavier ions 3.0-3.4

Uncertainties of reference dosimetry (adapted from Karger et al 2010)

Reference dosimetry method

Standard uncertainty (k=1), %

Protons Heavier ions

Ionization chamber 2.0 –

2.3 3.0 –

3.4

Water calorimetry 0.6

Graphite calorimetry 1.4

Faraday Cup dosimetry 1.5 2.3

Activation-based dosimetry

3.1 3.5

Implementation of ICRU Report 78IAEA TRS 398

ICRU Report xx provide a level of accuracycomparable to that

in calibration of photon and electron beams

harmonize clinical dosimetryat proton and heavier

ion beam facilities

Conclusion

Proton or light ion beam therapy centers in 2010

Protons

Light ionsThank you Thank you for your attention !for your attention !