1. ETHZ-METAS-PSI Project: Dosimetry for Scanned Proton Therapy Beams with Water Calorimetry
Solange Gagnebin, Damian Twerenbold
Sektion Thermometrie und Ionisierende Strahlung Bundesamt für Metrologie METAS 3003 Bern-Wabern
Eros Pedroni, David Meer, Silvan Zenklusen, Christian Bula Paul Scherrer Institut, Proton Radiation Therapy Centre, 5232 Villingen

2. Why Dosimetry is so important?
DOSE
dose † †
harmful for the patient
does not destroy tumors
Dose
healing of the patient

3. ETHZ - METAS - PSI Project: Goal
Realizing a water calorimeter to determine directly the dose in scanned proton therapy beam at PSI Gantry 2.

4. Plan of the presentation
Principles of Water Calorimetry
Dosimetry with ionizing chambers
Conclusion and Perspectives

5. Water Calorimetry: Basic Principles
very simple principle:
radiation
: specific heat capacity of water (very well known, since 1930‘s)
absorbed-dose-to-water [Gy = 1 J/kg] at position x
Dose-to-water :
main advantage:
direct determination of dose-to-water

6. METAS Water Calorimeter for Protons
water calorimeter in water phantom
sealed water vessel
thermistor
250 mm
thermistor bead (metal oxide NTC)

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

8. Proton Dosimetry : Results 2008

9. Problem: Water Calorimetry needs a lot of statistics
Now
Ideal
Measurements 10
about 250(1)
Statistical uncertainty
5 %
< 1%
(1) J. Medin, C. K. Ross, N. V. Klassen, H. Palmans, E. Grussel, J.-E. Grindborg, Phys. Med. Biol. 51 (2006)

10. Nevertheless: Dosimetry has to be SI unit traceable
IAEA-TRS-398 protocol:
Calibration factor of the ionizing chambers
calculated correction factors kQ
Co60 radiation
and water calorimeter
Verification with indirect method using ionizing chambers only.

11. Dosimetry: Ionizing chamber
beam qualities and ionizing chamber dependent on the calibration factor
Beam
MU: monitor units
water tank

12. Determination of the calibration factor
kQ factor determines the transition from Co-60 to
the user‘s beam quality
measured
Co-60 beam

13. Experimental verification of factor
Measurements in July 2009
DW drops when performing
relative measurements
measured at
METAS Co-60
with ionizing chamber
measured at PSI
proton beam
with ionizing chamber
For the same dose:
Journal of the ICRU, Vol. 7, No 2, Report 78, Oxford University Press, 2007.

14. 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