ethz-metas-psi project: dosimetry for scanned proton therapy beams with water calorimetry

Bundesamt für Metrologie METAS

ETHZ-METAS-PSI Project:

Dosimetry forScanned Proton Therapy Beams with Water Calorimetry

Solange Gagnebin, Damian Twerenbold

Sektion Thermometrie und Ionisierende StrahlungBundesamt für Metrologie METAS3003 Bern-Wabern

Eros Pedroni, David Meer, Silvan Zenklusen, Christian BulaPaul Scherrer Institut, Proton Radiation Therapy Centre, 5232 Villingen

ETHZ-METAS-PSI Project 4 June 2009 © Copyright METAS 2009/Gaso 2

Why Dosimetry is so important?

harmful for the patient

†does not destroy tumors

healing of the patient

Dose

DOSE dose

†


ETHZ - METAS - PSI Project: Goal

Realizing a water calorimeter to determine directly

the dose in scanned proton therapy beam at PSI

Gantry 2.


Plan of the presentation

• Principles of Water Calorimetry

• Dosimetry with ionizing chambers

• Conclusion and Perspectives


Water Calorimetry: Basic Principles

very simple principle:

absorbed-dose-to-water [Gy = 1 J/kg] at position x

absorbed

W

ET

c radiation

: specific heat capacity of water (very well known, since 1930‘s)

Dose-to-water :

main advantage: main advantage: direct determination of dose-to-waterdirect determination of dose-to-water

kcTD WWW

Wc

1k


METAS Water Calorimeter for Protons

water calorimeter in water phantom sealed water vessel

thermistor

thermistor bead (metal oxide NTC)

250 m


Schematic scanned proton dose profile in the vessel of the water calorimeter


Proton Dosimetry : Results 2008

0.3

0.35

0.4

0.45

0.5

0.55

0.6E

x_456

NR

C_2_1

NR

C_2_2

NR

C_2_3

NR

C_2_4

NR

C_2_5

NR

C_2_6

NR

C_2_7

NR

C_2_8

NR

C_2_9

NR

C_2_10

NR

C_2

NR

C_6_1

NR

C_6_2

NR

C_6_3

NR

C_6_4

NR

C_6_5

NR

C_6_6

NR

C_6_7

NR

C_6_8

NR

C_6_9

NR

C_6_10

NR

C_6

do

se / G

y

Comparison: Ionization Chamber Exradin #456 and Water Calorimeter thermistors NRC_2 and NRC_6

ionizing chamberExradin #456

thermistor NRC_2 thermistor NRC_6

6.5 cm proton box centered atreference depth: 15 cm water

heat capacity water at 4 � C: Cw = 4206 J kg-1 K-1 statistical uncertainty

type A

relative standarduncertainty: 2%

ICRU-2007


Now Ideal

Measurements

10 about 250(1)

Statistical uncertainty

5 % < 1%

Problem: Water Calorimetry needs a lot of statistics

(1) J. Medin, C. K. Ross, N. V. Klassen, H. Palmans, E. Grussel, J.-E. Grindborg, Phys. Med. Biol. 51 (2006) 1503-1521.


Nevertheless: Dosimetry has to be SI unit traceable

Calibration factor of the ionizing chambers

Verification with indirect method

using ionizing chambers only.

IAEA-TRS-398 protocol:

]/[, pCGyN QW

Co60 radiationand water calorimeter

calculated correction factors kQ


]/[][ , pCGyNpCMUD QWW

Dosimetry: Ionizing chamber

Beam

MU: monitor units

beam qualities and ionizing chamber dependent on the calibration factor

water tank


QCoWQW kNN ,,

Determination of the calibration factor

Co-60 beam

measured

kQ factor determines the transition from Co-60 tothe user‘s beam quality


Experimental verification of factor

measured atMETAS Co-60with ionizing chambermeasured at PSI

proton beamwith ionizing chamber

,

WQ

W Co

Dk

MU N

Measurements in July 2009

Qk

1,,1

2,,2

2,

1,

CoW

CoW

Q

Q

NMU

NMU

k

k

For the same dose:

Journal of the ICRU, Vol. 7, No 2, Report 78, Oxford University Press, 2007.

DW drops when performingrelative measurements


Conclusion and Perspectives

First results show good agreement between calculated dose-to-water as determined by ionizing chamber and measured with water calorimeter

However, Water Calorimetry needs a lot of statistics

An alternative is to verify the calibration factors of ionizing chambers with an indirect method

=> Measurements at PSI Gantry 2

July 2009: Dosimetry with Ionizing Chambers to verify the correction factor kQ

ethz-metas-psi project: dosimetry for scanned proton therapy beams with water calorimetry

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