Experimental determination of absorbed dose to water in a scanned proton beam using a water ...

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Experimental determination of
absorbed dose to water in a
scanned proton beam using a
water calorimeter and an ionization
chamber
Doctoral Thesis
Author(s):
Gagnebin, Solange Estelle
Publication date:
2010
Permanent link:
https://doi.org/10.3929/ethz-a-006186920
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In Copyright - Non-Commercial Use Permitted
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Diss. ETH No. 19139
Experimental determination of absorbed dose
to water in a scanned proton beam using a
water calorimeter and an ionization chamber
A dissertation submitted to the
SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH
for the degree of
DOCTOR OF SCIENCES
presented by
Solange Estelle GAGNEBIN
MSc Physics University of Neuchâtel
born June 24, 1982
citizen of Tramelan (BE), Switzerland
accepted on the recommendation of
Prof. Dr. G. Dissertori, examiner
Prof. Dr. J.-L. Vuilleumier, co-examiner
PD Dr. D. Twerenbold, co-examiner
Zurich
2010
Abstract
In the clinical environment, the dose delivered by the radiotherapy
installation is controlled regularly in order to prevent damage to the
healthy patient tissues. On the other hand, a correct dose has to
be delivered in order to destroy the tumor in an optimal way. The
reference physical quantity for the energy absorbed in tissue is the
absorbed dose to water. This quantity is routinely measured with
ionization chambers. However, ionization chambers have to be cal-
ibrated in order to convert the measured electrical charge into ab-
sorbed dose to water. The currently used protocols demand that
these conversion factors have to be traceable to a primary standard
of absorbed dose to water. The preferred primary standard is a wa-
ter calorimeter, which determines the dose directly by measuring
the temperature increase in water. This thesis presents experimen-
tal results of the water calorimeter developed by the Federal Office
of Metrology (METAS) exposed to the scanned proton beams at the
Paul Scherrer Institute (PSI). Ionization chamber measurements are
compared with the direct determination of absorbed dose to water
from the water calorimeter. The agreement of 3.2% of the dose
values measured by the two techniques are within their respective
statistical uncertainties, and confirm the possibility to use a water
calorimeter as primary standard for all types of existing proton ther-
apy systems.
In addition, different types of ionization chambers have been ex-
posed to identical proton doses in order to compare their chamber
specific correction factors k . The measurements confirm the rec-
Q
ommended k values as proposed by the protocols.
Q
Résumé
Dans l’environnement hospitalier, la dose délivrée par les installa-
tions de radiothérapie est contrôlée régulièrement pour prévenir
d’éventuels dommages dans les tissus sains des patients. D’autre
part, une dose correcte doit être délivrée pour détruire les tumeurs
de façon optimale. La grandeur physique de référence pour l’énergie
absorbée dans les tissus est la dose absorbée dans l’eau. Cette
grandeur est mesurée systématiquement avec des chambres ion-
isantes. Cependant, les chambres ionisantes doivent être calibrées
pourconvertirlamesuredechargeélectriqueendoseabsorbéedans
l’eau. Les protocoles actuellement utilisés exigent que les facteurs
de conversion soient reliés à un étalon primaire pour la dose ab-
sorbéedansl’eau. L’étalonprimairedepréférenceestuncalorimètre
àeauquidétermineladosedirectementenmesurantl’augmentation
de la température dans l’eau. Cette thèse présente les résultats ex-
périmentaux obtenus avec le calorimètre à eau, développé à l’Office
Fédéral de Métrologie (METAS), exposé au faisceau de protons scan-
nés de l’Institut Paul Scherrer (PSI). Les mesures d’une chambre ion-
isante sont comparées avec la détermination directe de la dose ab-
sorbée dans l’eau provenant du calorimètre à eau. Les valeurs de
la dose mesurée par les deux techniques sont en accord dans les
3.2% selon leurs incertitudes respectives, ce qui confirme la possi-
bilité d’utiliser le calorimètre à eau comme étalon primaire pour tous
les types de systèmes de protonthérapie existants.
Deplus,différentstypesdechambresionisantesontétéexposées
à une dose identique dans le faisceau de protons pour comparer leur
facteur k spécifique à chaque chambre. Les mesures confirment les
Q
valeurs k recommandées par les protocoles.
Q
Metaphor
Figure 1: Metaphor for the absorbed dose to water in a scanned proton
beam using a water calorimeter.
"A picture is worth a thousand words". This section presents an
analogy with paintings that illustrates the heart of the project.
In the upper left corner of the figure, a Pop art painting (Alex Katz-
1970-"Vincent with Open Mouth") shows a large homogeneous col-
ored area. In the lower left corner a Pointillism painting (Paul Signac-
1892-"Femmes au puits") is shown with a lot of points of different
colors.
Lookingatthesquareareaofgrass, theareainthePopartstyleis
homogeneously green and in the Pointillism style the grass is a com-
binationofblue, yellowandgreenspots. Decreasingthedistancebe-
tweenthepaintingandtheeye,theappearanceofthegrasschanges
in the Pointillism style, but not in the Pop art style. Thus in the Pop
art style, the grass is always a homogeneous green area, but in the
Pointillism style the eye can zoom into the grass and sees only blue,
yellow or green spots. Zooming out the eye, one sees green grass
as in the Pop art style. Important to the painter is that the observer
sees green grass in the painting.
Now that the figure has been described, the link between the
ii
paintings and the absorbed dose to water in a scanned proton beam
using a water calorimeter can be explained.
The Pop art represents the traditional passive scattering method
for proton therapy, where the protons are diffused in an homoge-
neous way. The Pointillism represents the PSI spot scanning tech-
nique with deposition of protons by spots. The grass square is the
irradiation area in the water. The colors (homogeneous green, or the
blue, yellow or green spots) represent the temperature increase ow-
ing to the deposited dose into water. The temperature increase is
not the same for all the spots (different colors). The eye, which sees
the colors, is the water calorimeter. The water calorimeter is like a
thermometer, which detects the temperature increase.
In the traditional passive scattering method for proton therapy
(Pop Art), the water calorimeter is able to measure correctly the to-
tal temperature increase (green grass), in other words the total ab-
sorbed dose.
Will in the PSI spot scanning technique the water calorimeter also
be able to measure the total temperature increase (green grass)?
Or will only a part of the temperature increase be measured (the
blue, yellow or green spots)? The aim of the thesis is to investigate
whether the water calorimeter is able to measure the total absorbed
dose to water in a scanned proton beam (green grass). If this is the
case, thewatercalorimetercanbeusedasprimarystandardforboth
proton deposition methods (passive scattering or spot scanning).
Acknowledgments
I first express my gratitude towards my Ph.D. supervisor, Prof. Dr.
Günther Dissertori for his support and his valuable comments over
the years.
I wish to thank Prof. Dr. Ralph Eichler, my previous Ph.D. su-
pervisor, for having encouraged the proton dosimetry project at the
beginning when he was the PSI director and accepted to be my first
Ph.D. supervisor.
I wish to thank Prof. Dr. Jean-Luc Vuilleumier for having accepted
to be my Ph.D. co-supervisor.
I’d like to thank in particular Dr. Damian Twerenbold, head of
METAS section Thermometry and Ionising Radiation and co-supervi-
sor. His engagement, support, attention to detail, hard work, were
an example for me during my Ph.D.
I’dliketothankthePSIteamengagedinthisproject: ErosPedroni,
David Meer, Adolf Coray, Silvan Zenklusen, Christian Bula, Terence
Böhringer, Martin Grossmann and Christian Hilbes for beam time,
technical support, and explanations during the measurements with
the PSI scanned proton delivery system.
I’d like to thank the new director of METAS Dr. Christian Bock,
division head Dr. Philippe Richard, previous director Dr. Wolfgang
Schwitz and previous division head Dr. Bruno Vaucher for having
supported the proton dosimetry project over the last five years.
I also thank some of my METAS colleagues: Dr. Sandor Vörös,
Carel Meyer, Reto Schafer, Dr. Bénédicte Boillat, Dr. Anton Steiner,
Johanna Saner, Alexander Tschudin, Gilles Zwahlen, Olivier Brunsch-
wig of the section Thermometry and Ionising Radiation who have
shared with me their know-how about dosimetry, chemistry and cal-
ibration. Dr. Samuel Wunderli for his valuable explanation about the
chemistry of the glass surface and Dr. Alain Küng for his advice how
to position with precision the tip of the thermistors. The team of the
section Electricity, in particular Dr. Frédéric Overney and Dr. Blaise
Jeanneret for their know-how in Wheatstone bridge technique. And
finally the technical services team for having manufactured quickly
all the mechanical pieces I needed, the electronics services team for
having fabricated, among others, the electronic boxes for the water
calorimeter and the informatics services team for the good job done
as helpdesk.
Finally, I thank my family for their understanding and for giving
me confidence during my Ph.D.
Contents
1 Introduction 1
2 Physics of proton therapy 3
2.1 Proton interactions with matter . . . . . . . . . . . . . . . . . 3
2.1.1 Stopping power for electron interactions . . . . . . 5
2.2 Bragg Peak and Spread Out Bragg Peak . . . . . . . . . . . 9
3 Realization of proton therapy 12
3.1 PSI proton beam: proton source and accelerator . . . . . . 12
3.2 Traditional passive scattering method for proton therapy 13
3.3 PSI spot scanning technique . . . . . . . . . . . . . . . . . . . 13
3.3.1 Gantry 1: the first Gantry at PSI . . . . . . . . . . . . 15
3.3.2 Gantry 2: the new generation. . . . . . . . . . . . . . 15
3.4 Dosimetry in proton therapy . . . . . . . . . . . . . . . . . . . 18
4 Ionization Chamber for Dosimetry 19
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2 Definition of relevant quantities for cavity theory . . . . . 20
4.3 Bragg-Gray theory (cavity theory) . . . . . . . . . . . . . . . 21
4.4 Stochastic and nonstochastic quantities . . . . . . . . . . . 24
4.5 The Charged-Particle Equilibrium (CPE) . . . . . . . . . . . . 25
4.6 The Spencer-Attix theory . . . . . . . . . . . . . . . . . . . . . 26
4.7 The absorbed dose to water with an ionization chamber 28
4.7.1 The dosimeter reading M . . . . . . . . . . . . . . . . 29
Q
4.7.2 The calibration factor N . . . . . . . . . . . . . . 30
D,,Q0
4.7.3 The correction factor for the radiation quality of
the beam k . . . . . . . . . . . . . . . . . . . . . . . 31
Q,Q0
4.7.4 Relation between the absorbed dose to air D
r
and the absorbed dose to water D . . . . . . . . . . 33

4.8 Summary of the ionization chamber theory . . . . . . . . . 36
4.9 Measuring absorbed dose with Ionization Chambers . . . 38
4.9.1 Ionization chambers . . . . . . . . . . . . . . . . . . . . 39
4.9.2 Electrometer . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.9.3 Barometer and thermometer . . . . . . . . . . . . . . 42
4.9.4 Phantoms and sleeves for the chambers. . . . . . . 42
4.10Dose measurements with the Ionometric Dosimeter . . . 43
4.10.1Dosimeter calibration with 60Co . . . . . . . . . . . . 43
4.10.2Proton Measurement Procedure . . . . . . . . . . . . 45
5 Results of Proton irradiation of Ionization Chamber 47
5.1 Dose measurement with the Ionometric Dosimeter . . . . 47
5.2 Experimental verification of the correction factor k
Q,Q0
for protons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
References v
6 Water Calorimetry as Primary standard 56
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
6.2 The absorbed dose to water in a water calorimeter . . . . 56
6.3 The METAS Sealed Water Calorimeter . . . . . . . . . . . . . 59
6.3.1 The Water calorimeter and electronics . . . . . . . . 59
6.3.2 Glass vessel . . . . . . . . . . . . . . . . . . . . . . . . . 63
6.3.3 Thermistor . . . . . . . . . . . . . . . . . . . . . . . . . . 64
6.4 Water calorimeter: preparation and measurement pro-
cedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.4.1 Water calorimeter preparation . . . . . . . . . . . . . 72
6.4.2 Measurement procedure . . . . . . . . . . . . . . . . . 75
7 Results of Proton irradiation of Water Calorimeter 77
7.1 Determining the absorbed dose from the lock-in ampli-
fier voltage change . . . . . . . . . . . . . . . . . . . . . . . . . 77
7.2 Dose deposition of a scanned proton beam . . . . . . . . . 79
7.3 Comparingthedosemeasurementswiththewatercalorime-
ter and ionization chamber. . . . . . . . . . . . . . . . . . . . 81
7.4 Uncertainties on the dose measurement . . . . . . . . . . . 82
7.5 Improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
8 Conclusion 84
9 References 85
Introduction 1
1 Introduction
35’450newcasesofcancerarerecordedannuallyinSwitzerland(an-
nual mean value between 2003 and 2006 [1]). More than half of the
patientssufferingfromcanceraretreatedwithradiationtherapydur-
ing their illness. Radiation therapy relies on the destructive effects
of ionizing radiation on tumors. Different types of ionizing particles
(photons, electrons, protons, heavy ions, neutrons) can be used, de-
pending on the nature, the localization and size of the tumor. Pro-
tons in the energy range between 70 and 230 MeV are increasingly
used for radiation therapy of cancer [2], because protons deposit
the maximum of the dose inside the tumor and minimize the effect
on the surrounding healthy tissue. Compared to the clinical accel-
erators for photons and electrons, which are commercially available
and more than 6’000 units are installed world wide [3], proton and
heavy ion facilities still operate in an often more experimental envi-
ronment. Nevertheless, since 1954 more than 70’000 patients have
been treated with charged particle beams [4].
All clinical radiation therapy units have to be regularly controlled
toensurethecorrectenergyabsorbedinthepatienttissueaccording
to the treatment plan. In radiotherapy the reference value for the
energy absorbed in a tissue is the absorbed dose to water D ex-
W
pressed in the SI unit Gy = 1 Jole/kg. How to obtain this reference
quantity is described in the protocol of the International Atomic En-
ergy Agency (IAEA) Technical Report Serie (TRS) 398 [5]. This proto-
col recommends to calibrate the ionization chambers used in proton
therapy in a 60Co reference beam (photon energy 1 MeV), together
with a calculated ionization chamber specific conversion coefficient
k for the specific proton beam quality. However, the Interna-
Q,Q0
tional Commission on Radiation Units and Measurements (ICRU) has
recommendedintheirReportnr.78of2007[2], that"whenavailable,
calorimeters should be used as primary standards or, alternatively,
to confirm the proton calibration coefficient of the reference ioniza-
tion chamber". Primary standards for absorbed dose to water can be
realized using a water calorimeter, which is the most direct method
(for a review of other calorimeters see [2]).
Water calorimetry is an established primary standard for deter-
mining the absorbed dose to water for external high energy photon
beams [6, 7] and have already been used in scattering proton beams
[8, 9]. In a water calorimeter, the absorbed dose is determined by
measuring the temperature increase which is the product of the en-
ergy absorbed and the specific heat of water.
The aim of this thesis was to measure and compare the absorbed
dose with a water calorimeter and different ionization chambers for
scanned proton beams. To achieve this goal, the primary standard
of the Federal Office of Metrology METAS water calorimeter for ex-