Absolute Dosimetry for Proton Beams-Doktorandenseminar 11 Sept 2008

Bundesamt für Metrologie METAS

ETHZ-METAS-PSI Project:

Absolute Dosimetry for
Scanned Proton Therapy
Beams with Water Calorimetry
Solange Gagnebin, Damian Twerenbold
Sektion Thermometrie und Ionisierende Strahlung
Bundesamt für Metrologie METAS
Lindenweg 50, Bern-Wabern

Eros Pedroni, David Meer, Silvan Zenklusen, Christian Bula
Paul Scherrer Institut, The Proton Therapy Project

ETHZ - METAS - PSI Project: Goal

Realizing a primary standard for absolute
dosimetry in scanned proton therapy with a
water calorimeter.

2
ETHZ-METAS-PSI Project 10-11 September 2008 © Copyright METAS 2008/Gaso

Dosimetry of Clinical Proton Beams

Dosimetry is important: in cancer therapy, the dose
should be correct to a few percent, because:

• too high dose is harmful for the patient
• too low dose does not effectively destroy tumors

3

Dosimetry of Clinical Proton Beams
In clinical applications Ionization Chambers are used for the
determination of the absolute dose (stable, reliable).

However, Ionization Chambers have to be calibrated for
dose-to-water.

In current dosimetry protocol for protons (IAEA TRP 398) the
calibration factor Nw is obtained from Co60 radiation and
calculated correction factors.

Comparing Ionization Chambers with Water Calorimetry will
determine directly the calibration factor Nw

4

Water Calorimetry: Basic Principles
very simple principle:

Eabsorbed
∆T =
radiation
cW

cW : specific heat capacity of water
dose [Gy] at
(very well known, since 1930‘s)
position x
difficulties:
• very high pure water required
• chemical heat defect
• sensitivity chemical heat defect kHD:

1
= ∆T ⋅c ⋅
D
main advantage:
W W 1−k
W
direct determination of dose-to-water
dose-to- HD

5

Water Calorimetry at Gantry 2: Measurements 2008

at new proton
delivery system
Gantry 2

METAS water calorimeter

6

METAS Water Calorimeter as Primary
Standard for Protons
water calorimeter in water phantom sealed water vessel

thermistor

250 µm

thermistor bead (metal oxide NTC)

7

PSI-METAS Proton Dosimetry:
schematic scanned proton dose profile in
the vessel

8

Proton dose box with scanned proton beam

9

Proton dose box: thermistor response

Thermistor response of proton
dose deposition
(PSI Annual Report 2006)

10

Measuring dose with a water calorimeter

Wheatstone bridge

energy depostion
in water

temperature rise

resistance change
resistance-to-voltage
voltage change
factor has to be calibrated

11

Calibration of resistance-to-voltage
NRC-2

measured resistance-to-voltage
calibration factors of our two
thermistors

NRC-6
0.0451 /µV

0.0419 /µV

requirements:
• precise resistance step = 0.4001 Ω
• high voltage resolution = 50 nV

12

Measuring dose with Water Calorimeter
P1_PSI_Aug08_proton_K7.m

∆UNRC_2 = 3.1923 µV
P1_PSI_Aug08_2_diff_29.08.2008_16h01m34s.txt
figure 3 sensor_voltage_1_X 09-Sep-2008 16:09:24
0.5
R_Dekade_1 = 9652.79
R_Dekade_2 = 9205.6
0

∆R = ∆UNRC_2·0.0451 /µV = 0.1440
-0.5

∆T = TNRC_2(R0-∆R)-TNRC_2(R0) = 0.367 mK
-1

∆UNRC_2
sensor voltage 1 [uV]

-1.5

∆UNRC_6 Dw = ∆T·Cw = 1.545 Gy
-2

-2.5

-3

∆UNRC_6 = 3.3736 µV
-3.5

-4
3900 4000 4100 4200 4300 4400 4500 4600 4700 4800

∆R = ∆UNRC_6·0.0419
time [sec]

/µV = 0.1413
∆T = TNRC_6(R0-∆R)-TNRC_6(R0) = 0.376 mK
heat capacity of water:
Dw = ∆T·Cw = 1.593 Gy
Cw = 4206 J kg-1 K-1 = 4.206 Gy mK-1

13

Heat Capacity of Water at 4°
C

Cw = 4206 J kg-1 K-1 = 4.206 Gy mK-1

14

Results of Measurements August 2008

Comparison: Ionization Chamber Exradin #456 and
Water Calorimeter thermistors NRC_2 and NRC_6
0.6

6.5 cm proton box centered at
reference depth: 15 cm water
0.55
ionizing chamber
Exradin #456
0.5
dose / Gy

thermistor NRC_2 thermistor NRC_6
0.45

0.4

0.35 heat capacity water at 4°
C:
statistical uncertainty
-1 -1
Cw = 4206 J kg K
type A
0.3
Ex_456

NRC_2_1
NRC_2_2
NRC_2_3
NRC_2_4
NRC_2_5
NRC_2_6
NRC_2_7
NRC_2_8
NRC_2_9
NRC_2_10

NRC_2

NRC_6_1
NRC_6_2
NRC_6_3
NRC_6_4
NRC_6_5
NRC_6_6
NRC_6_7
NRC_6_8
NRC_6_9
NRC_6_10

NRC_6
15

Conclusion and Perspectives
Conclusion:

Water calorimeter can be used as a primary standard
for absolute dosimetry of scanned protons.
First results show good agreement between calculated
dose-to-water as determined by ionizing chamber and
measured with water calorimeter

Perspectives:

Systematic studies are required to determine the
influence of the heat defect in water calorimeter and
influence of proton energy and deposition depths.

16

Absolute Dosimetry for Proton Beams-Doktorandenseminar 11 Sept 2008

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