faculty o physcal sciences
TRANSCRIPT
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Digitally Signed by: Content manager’s Name
DN : CN = Webmaster’s name
O = University of Nigeria, Nsukka
OU = Innovation Centre
Ugwoke Oluchi C.
Faculty of Physcal Sciences
Department of Physics and Astronomy
DETERMINATION OF BEAM QUALITY CORRECTION FACTORS
FOR TWO IONIZATION CHAMBERS OF THE LINAC UNIT AT UNTH
AROH, FABIAN ONYEMAECHI
PG/M.Sc/08/49517
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A RESEARCH PROJECT PRESENTED TO THE DEPARTMENT OF PHYSIC AND
ASTRONOMY, FACULTY OF PHYSICAL SCIENCES, UNIVERSITY OF NIGERIA
NSUKKA, IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
AWARD OF THE DEGREE OF MASTER OF SCIENCE IN MEDICAL PHYSICS
BY
AROH, FABIAN ONYEMAECHI
PG/M.Sc/08/49517
PROJECT TOPIC:
DETERMINATION OF BEAM QUALITY CORRECTION FACTORS FOR TWO
IONIZATION CHAMBERS OF THE LINAC UNIT AT UNTH
PROJECT SUPERVISORS:
PROF. C. M. I. OKOYE
DEPARTMENT OF PHYSICS AND ASTRONOMY
UNIVERSITY OF NIGERIA NSUKKA
PROF. K. K. AGWU
DEPARTMENT OF MEDICAL RADIOGRAPHY
UNIVERSITY OF NIGERIA NSUKKA
MAY, 2013.
CERTIFICATION
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Aroh Fabian Onyemaechi, a postgraduate student in the Department of Physics and Astronomy,
University of Nigeria, Nsukka, with Registration Number PG/M.Sc/08/49517 has satisfactorily
completed the requirements for the course and research work for the award of the Master of
Science (M.Sc) Degree in Medical Physics.The work embodied in this project report is original
and has not been submitted for any diploma or degree of this or any other University.
...................................... ....................................
Project Supervisor Signature/Date
............................................. ......................................
H.O.D Physics & Astronomy Signature/Date
........................................... ........................................
External Examiner Signature/Date
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DEDICATED
TO
THE
MEMORY
OF
MY
FATHER
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ACKOWLEDGEMETS
I greatly appreciate the Radiotherapy Centers University of Nigeria Teaching Hospital Enugu
and University College Hospital Ibadan for the use of their facilities.I am grateful for the
sacrifice of my supervisors Professors C.M.I Okoye and K.K Agwu, for their endless
encouragement and guidance throughout the entire training. I hereby also acknowledge Prof.
K.K. Agwu’s expertise, enthusiasm and skill in the field of medical Physics that has made me
what I am in the field of Medical Physics. I greatly value the support of Prof. (Mrs) R.U. Osuji
the head department of Physics And Astronomy University of Nigeria Nsukka. I would like to
appreciate the roles of Drs’J.K Audu and T.A Ige both of Medical Physics department National
Hospital Abuja towards research in the clinical medical physics community in Nigeria. I am
eternally thankful to Prof. F.I Obioha who introduced me to the field of medical physics for his
advice and encouragement to pursue a career in medical physics. I am extremely indebted to
Professor Ado Vans Rursberg of Pretoria Acedamic Hospital South Africa for his assistance
during training and execution of this research both in South Africa and here in Nigeria. I am
grateful too for the immeasurable efforts of Dr. K.C Nwankwo and my colleagues Sylverster
K.K, Ojiogu J.U, Chikezie A.C towards successful completion of this research. Lawretta
Amaka wife who has been so good in calming my nerves whenever my spirit gave way. She has
been a wonderful, and a lovely companion. Finally I would like to thank my lovely mother Mrs
M.O Aroh and my brothers and sisters for their love,support and encouragement .
To my GOD, I owe glorification.
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ABSTRACT
The purpose of this work was to determine values of the beam quality correction factors KQ in
clinical high-energy photon and electron beams for two ionization chambers in use at University
of Nigeria Teaching Hospital Enugu using a reference Farmer ionization chamber PTW 30013.
The dose at a point in the phantom were measured with ionization chambers at the center of the
sensitive volume. The centers of the chambers were aligned with the isocentre of the treatment
machine. The dose was compared to the PTW 30013 0.6 cm3 ionization chamber using its
60Co
absorbed dose to water calibration factor NCO
WD
60
,. The dose to water at the reference depth of 5 cm
was calculated using IAEA TRS 398 protocol. The chambers and the water phantom were
allowed to equilibrate with the ambient air temperature. Dose readings were taken for 100
monitor units. Throughout the study, the absolute value of the polarising voltages was
maintained at +400V,-400 or +200. The readings were corrected for the standard environmental
conditions of temperature and pressure, ion recombination and polarity. The cross calibrated
absorbed dose to water calibration factor for cylindrical chamber and the absorbed dose to water
due to the PTW 30013 Farmer reference ionisation chamber in the 6 MV and 15 MV photon
beams were used to determine kQ for ionisation chamber at the respective photon energies. The
plane-parallel and the cylindrical ionisation chambers were then cross-calibrated for cavity-gas
calibration factor Ngas in the 15 MeV electron beam. The absorbed dose to water in the electron
beams was then calculated from first principles using the AAPM TG-21 worksheets for the two
chambers. The kq,E were then derived for each of the ionisation chambers at each of the electron
energies. The measured values of KQ and Kq.E shows that the average observed difference
between the measured values and those published in the IAEA TRS-398 protocol was 0.2% for
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the PTW 30013 0.6 cm3 Farmer in the photon beams and 1.2% for the PTW 34045 Advanced
Markus ionisation chamber in the electron beams.
In conclusion beam quality correction factors for ionisation chambers can be determined
experimentally or confirmed in an end-user’s beam quality.
viii
TABLE OF CONTENTS
CERTIFICATION.......................................................................................................................iii
DEDICATION..............................................................................................................................iv
ACKOWLEDGEMETS ................................................................................................. v
ABSTRACT...................................................................................................................................vi
TABLE OF CONTENTS............................................................................................. viii
CHAPTER ONE: INTRODUCTION
1.1 Introduction ………………………………………..………………………….................. 1
1.2 Objectives of the study…………………………………..……………….….….............. 2
1.3 Justification of the study................................................................................................... 3
1.4 Scope of the study ...................................................................................................... 3
1.5 Limitations of the study........................................................................................ 4
CHAPTER TWO: LITERATURE REVIEW
2.1 Literature review ...................................................................................................................... 5
2.2.1 Ionization chamber dosimetry............................................................................................... 6
2.2.2 Electrometer...........................................................................................................................7
2.3 Photon beam dosimetry…………………………………….…………………............. 8
2.4 Electron beam dosimetry…………………………………………………….................. 9
2.5 Beam quality specification………………………… ………………......................... .. 10
2.5.1 Photon beam quality specification………………………………...........….…........... 11
2.5.2 Electron beam quality specification…………………………….….............…......... 12
2.6 Theoretical expressions for the beam quality correction factors in high energy photons and
electron beams……………….………………………................................... 13
2.6.1 Theoretical expression for kQ (photon beams)..……..............…………….…..... 13
2.6.2 Theoretical expression for kq,E (electron beams)……..…...........……………..... 14
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2.7 Reference conditions of the irradiation geometry for absorbed dose measurements using an
ionisation chamber inaphantom………………………………….….……………. 15
CHAPTER THREE: MATERIALS AND METHODS
3.1 Research design........................................................................................................... 16
3.2 Study locations................................................................ …………………….... ... 16
3.3 Instrumentations......................................................................................................................16
3.3.1 Ionisation chambers used in this study................................................................................19
3.3.2 Electrometer used in this study............................................................................................22
3.4 The cross-calibration of ionisation chamber in photon and electron beams ……… 23
3.4.1 Cross-calibration of the NCO
WD
60
, for ionisation chambers in
60Co beam………...... 23
3.4.2 Cross-calibration of the 60Co exposure calibration factor Nx……………................ 25
3.4.3 Cross-calibration of the Ngas for plane-parallel chambers in electron beams....... 25
3.5 The absorbed dose measurement in megavoltage photon beams ……………..…. 26
3.6 The absorbed dose measurement in electron beams……………………………..…. 28
3.7 Determination of beam quality correction factors………………….………..…......... 29
CHAPTER FOUR: RESULTS AND DISCUSSIONS
4.1 The results of the cross-calibration of the ionisation chambers....…………………. 31
4.2 Measurement results in 6 MV and 15 MV photon beams………………………..…. 32
4.3 Measurement results in the electron beam qualities ……………………….………… 33
4.4 DISCUSSIONS .............................................................................................................. 36
CHAPTER FIVE: CONCLUSIONS AD RECOMMENDATIONS …………….……….....38
DEFINITIONS OF TECHNICAL TERMS,ACRONYMS AND SYMBOLS...................... 39
REFERECES……………………………………………………………………...…………...42
1
CHAPTER ONE: INTRODUCTION
1.1 Introduction
The development of new techniques in external radiotherapy has led to an increase in the
complexity of the procedures used. Treatment delivery to the patient therefore involves many
steps, parameters, and factors. As a result, more complex quality controls are required during the
radiotherapy process to ensure that each step has as low an uncertainty as possible. The
International Atomic Energy Agency (IAEA) and the American Association of Physicists in
Medicine (AAPM) are among the various organisations that have published dosimetry protocols
and Codes of Practice for the calibration of radiotherapy beams (Andreo & Saiful, 2001).
Currently an ionisation chamber, calibrated in terms of the absorbed dose to water in a 60
Co
gamma ray beam, is used to determine the dose in a medium. The rationale of this trend is to deal
directly with absorbed dose to water, a quantity which relates closely to radiobiological effects in
humans and is therefore of interest in the clinical practice (IAEA, 2000). The dosimetry
procedure uses the absorbed dose to water calibration factor ( NCO
WD
60
,) (in G y/C) for the
ionisation chamber in the 60
Co reference beam’ together with a theoretical beam quality
conversion factor ( KQ for photons or Kq,E for electrons) for the determination of absorbed dose
to water in other high-energy beams excluding neutrons (IAEA, 2000; Saiful, 2001). The
absorbed dose in a 60
Co gamma ray beam is therefore an international reference standard, which
provides global uniformity in radiotherapy dosimetry.
It is important that dose is measuered accurately and precisely as possible in order to deliver the
prescribed dose to a point or a given volume of interest (AAPM, 1983). The experimental
determination of KQ and Kq,E at various beam qualities intrinsically takes into account the
response of different ionisation chambers. In contrast, the calculated values of KQ ignore
2
chamber-to-chamber variations in response to energy within a given chamber type, and its
uncertainty is therefore larger than for experimentally determined KQ values. Direct calibration,
in terms of absorbed dose to water at each beam quality, reduces the total uncertainty of
absorbed dose determination in the user’s beam by 1 to 1.5% (Hubert, Hugo & Wim 1999).
There are two ionization chambers in use at University of Nigeria Teaching Hospital Enugu, the
Physikalisch Technische Werkstätten (PTW) Semi-flex cylindrical and PTW Advanced Markus
plane-parallel ionisation chambers. These ionization chambers have no published data of beam
quality correction factor KQ for absorbed dose to water in high photon and electron energies.
Consequently, this research seek to determined accurately in a clinical set up the beam quality
correction factors of these ionization chambers at different high energy photon and electron
beams.
Many reviewers ( Hugo et al 2002; Podgorsak, 2005; Rogers, 1990) recommend that the beam quality
correction factors for megavoltage radiotherapy beams should be measured directly in the user’s beam
for each ionisation chamber.
1.2 Objectives of the study
The general objective of this study is to experimentally determine the beam quality correction
factors KQ and Kq,E for two ionisation chambers within high-energy photon and electron beams
range used at the University of Nigeria Teaching Hospital Enugu. The specific aims of the
study are ;
(i) Cross-calibrate the Semi-flex and Advanced Markus ionisation chambers against the
calibrated Farmer reference ionisation chamber.
(ii) Determine the absorbed dose-to-water for various clinically useful photon and electron
energies using IAEA Technical Report Series 398 with Farmer reference ionisation chamber
3
(iii) Compare the experimentally determined values of KQ and Kq,E with published data for the
Farmer ionisation chamber.
(iv) Derive KQ and Kq,E for the Semi-flex and Advanced Markus models of ionisation chambers
for on-site clinical application.
1.3 Justification
With the beam quality correction factors KQ and Kq,E for Semi-flex cylindrical and Advanced
Markus plane-parallel ionisation chambers respectively that we have established, the chambers
can be used for routine dosimetry at the hospital and elsewhere with very minimal
uncertainties. The result of our research is being used to calibrate the Linear accelerator to give 1
Gray per 100 monitor units at any particular point in time during clinical applications.Without
the results of this research the ionisation chambers under study is clinically not very useful
because they cannot be applied for absorbed dose to water determination in high photon and
electron energies.
1.4 Scope
The areas covered by this research work include;
(i) Measurement of absorbed dose to water ND,W with cylindrical and plane-parallel ionisation
chambers
(ii) Determination of beam quality correction factors KQ for PTW 31010 Semi-flex cylindrical
ionisation chamber.
(iii) ) Determination of beam quality correction factors Kq,E for PTW 34045 plane-parallel
ionisation chamber.
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1.5 Limitations of this study
A computer software programme such as Monte Carlo simulation code [ EGSnrc] should have
used to test the validity of the statistical uncertainties of our results however, this is not presently
available.
5
CHAPTER TWO: LITERATURE REVIEW
2.1 Literature review
IAEA,(2000) emphasized that directly measured values of beam qaulity correction factor KQ for
an individual chamber within a given chamber type are the preferred choice.
Pablo Castro et al (2008), proposed that the typical uncertainty in the determination of absorbed
dose to water during beam calibration is approximately 1.3% for photon beams and 1.5% for
electron beams.
Zakaria GA, Schuette W. (2007) established that the beam quality index for electrons is a
function of half-value depth R 50 and practical range R p in water.
Seuntjen.J.P et al (2000), observed that a system making use of absorbed-dose calibration and
calculated beam correction factor kQ values, is more accurate than a system based on air-kerma
calibration in combination with calculated compound conversion factor.
Using the perturbation factor for the different elecron energies and dose for the reference beam
quality 60
Co (K.Zink and J.Wulff; 1988) calculated the beam quality correction factors Kq.E,
which are in good agreement with the data published in the IAEA protocols, with a deviation of
0.5 - 0.8 % for lower electron energies.
Gonzalez-Castano.D.M et al (1999), proposed that the beam quality correction factors can be
generated both by measurements and by the Monte Carlo simulations with an uncertainty at least
comparable to that given in current dosimetry protocols.
Many reviewers (Hugo et al., 2002; Podgorsak, 2005; Rogers, 1990) recommend that the beam
quality correction factors for megavoltage radiotherapy beams are measured directly in the user’s
beam for each ionisation chamber. Often these factors are calculated theoretically from data
available in different protocols. It is known that kQ can be measured with a standard uncertainty
of less than 0.3% (Achim & Ralf-Peter, 2007; IAEA, 2000; Saiful, 2001).
6
The (IAEA TRS-398,AAPM TG-51 ) protocols have established the kQ and kq,E the beam
quality correction factors for high photon and electron energies respectively for various
ionisation chambers such as listed below;
Capintec PR-50/PR-05P, Capintec PR-06C/G 0.6cc Farmer, Extradin A12 Farmer,
NE2505/3,3A 0.6cc Farmer, NE2571 0.6cc Farmer, NE2577 0.2cc, NE2581 0.6cc robust
Farmer, NE 2611 0.3cc NPL Sec. Std, PTW N30001 0.6cc Farmer, PTW N30002 0.6cc all
Graphite, PTW N30004 0.6cc Graphite, PTW 31003 0.3cc waterproof, PTW 30006/30013
Farmer, Wellhofer IC-10/IC-5.
It is obvious from the list above that there is no documented data with regards to the KQ and Kq,E
for PTW 31010 Semi-flex and PTW 34045 Advanced Markus ionisation chambers.
2.2.1 IONIZATION CHAMBER DOSIMETRY
Ionization chambers are used in radiotherapy and in diagnostic radiology for the determination of
radiation dose. An ionization chamber is basically a gas filled cavity surrounded by a conductive
outer wall and having a central collecting electrode (see Fig.2.1) .The wall and the collecting
electrode are separated with a high quality insulator to reduce the leakage current when a
polarizing voltage is applied to the chamber. A guard electrode is usually provided in the
chamber to further reduce chamber leakage. The guard electrode intercepts the leakage current
and allows it to flow to ground directly, bypassing the collecting electrode. The guard electrode
ensures improved field uniformity in the active or sensitive volume of the chamber (for better
charge collection).
7
FIG. 2.1 Basic design of a cylindrical Farmer type ionization chamber.
2.2.2 ELECTROMETER
Since the ionization current or charge to be measured is very small, special electrometer circuits have
been designed to measure it accurately. The most commonly used electrometers use negative-feedback
operational amplifiers. Figure 2.2 schematically shows simplified circuits that are used to measure
ionization in the integrate mode, rate mode, and direct-reading dosimeter mode. The operational
amplifier is designated as a triangle with two input points. The negative terminal is called the inverting
terminal and the positive one as the non inverting position. This terminology implies that a negative
voltage applied to the inverting terminal will give a positive amplified voltage and a positive voltage
applied to the non inverting terminal will give a positive amplified voltage. A negative-feedback
connection is provided, which contains either a capacitor or a resistor
8
Fig. 2.2 Negative feedback, operational amplifier
The operational amplifier has a high open-loop gain (> 104) and a high input impedence (> 10
12 ohm).
Because of this, the output voltage is dictated by the feedback element, independent of the open-loop
gain, and the potential between the positive and negative inputs of the amplifier (called the error
voltage) is maintained very low (< 100 mV). For example, if the ionization current is 10-9
A and the
resistor in the feedback circuit of Fig. 2.2 is 1010
ohm, the output voltage will be current times the
resistance or 10 V. Assuming open-loop gain of 104, the error voltage between the input terminals of the
amplifier will be 10-3
V or 1 mV. This leads to a very stable operation, and the voltage across the
feedback element can be accurately measured with the closed-loop gain of almost unity.
2.3 Photon beam dosimetry
According to IAEA TRS-398 (2000), the absorbed dose to water Dw, at a reference depth (point
of measurement) in a photon beam of quality Q ,and in the absence of chamber is directly
determined from:,
Ionization chamber
9
NMDCO
WDW
60
,= K Q ∏ ,iK
(1)
M is the charge measured under standard conditions of temperature,pressure and humidity.
NCO
WD
60
,is the absorbed dose to water calibration factor (in Gy/C) for the ionisation chamber in
the 60
Co reference beam. KQ is a chamber specific factor which corrects NCO
WD
60
,to the user’s
beam quality Q (different from the 60
Co beam). ∏ ,iK is the product of the factors to correct for
non-reference conditions in the setup and incomplete ion collection efficiency of the ionisation
chamber ( Rogers, 1990). Factors ki represent a correction for the effect of i-th influence
quantity. Such correction factors may have to be applied as the calibration coefficient refers,
strictly speaking, only to reference conditions. By definition, the value of ki is unity when
influence quantity i, assumes its reference value (Rogers, 1990). The product NCO
WD
60
,.KQ =
( NQ
DW) is of special interest and is the absorbed dose to water calibration factor (in Gy/C) of the
ionisation chamber in the beam quality Q. The current accepted relative uncertainty of Dw in
equation (1) is of the order of 1.5% as determined by ionometric methods and the uncertainty in
kQ is 1% (Achim & Ralf-Peter, 2007).
2.4 Electron beam dosimetry
According to AAPM TG-51(Almond et al., 1999), the absorbed dose to water in an electron
beam of quality q,E is given by;
KNMD EqWD
Eq
W
CO
,
60
,
,=
(2)
10
M is the reading of the dosimeter with the point of measurement of the chamber positioned at the
reference depth under reference conditions and corrected for ion recombination, polarity effect,
electrometer correction factor and the standard environmental conditions of temperature,
pressure and relative humidity of the air in the ion chamber. NCO
WD
60
, is the absorbed dose to
water calibration factor (in Gy/C) of the ionisation chamber in the reference 60Co beam.Kq,E is a
beam quality conversion factor for electrons to convert NCO
WD
60
,to N
Eq
WD
,
, for an electron beam of
quality q,E.
2.5 Beam quality specification
Among the difficulties of the kQ and kq,E concept is the need for a unique beam quality
specification and the possible variation in the kQ and kq,E values for different chambers of the
same type (Hubert, Hugo & Wim 1999). The AAPM TG-21 (AAPM, 1983) protocol specifies
photon beam energy in terms of the energy of the electron beam as it strikes the target (the
nominal accelerating potential) which is related to the “ionisation ratio”. The ionisation ratio is
defined as the ratio of the ionisation charge or dose measured at twenty (20) cm depth in water to
that measured at ten (10) cm depth for a constant source to detector distance in a 10 cm x 10 cm
field at the plane of the chamber. The ionisation ratio is the same as the TPR 20,10 expression used
by the IAEA TRS-398 (IAEA, 2000) dosimetry protocol. The ionization ratio or TPR 20,10 is a
measure of the effective beam attenuation coefficient through 10 cm of water. TPR 20,10 is
empirically related to the percentage depth dose, through (Khan 2010)
TPR 20,10 = 1.2661PDD20,10 - 0.0595 (3)
where PDD 20,10 is the ratio of percentage depth doses at 20 cm and 10 cm depths for a field size
of 10 cm x 10 cm field size defined at the water phantom surface with a source to surface
distance of 100 cm ( IAEA, 2000; Podgorsak, 2005).When linear accelerator electron beams
11
strike a phantom or a patient surface at the nominal SSD, a spectrum results from the energy
spread. This is caused by interactions within the air and with the linear accelerator components
like the collimators, scattering foil, monitor chamber and applicator. The electron beam is
therefore degraded and contaminated. The quality of clinical electron beams has been specified
as Eo, the mean electron energy of the incident spectrum striking the phantom surface
(Podgorsak, 2005). Eo is empirically derived from R50, the depth at which the electron beam
depth dose decreases to 50% of its maximum value. The reference depth dref, for electron beam
calibrations in water according to (IAEA, 2000) is expressed as;
dref (cm) = 0.6R50 (cm) - 0.1 (cm) (4)
The reference depth dref is used clinically because it is known to significantly reduce machine to
machine deviations in chamber calibration coefficients (Hugo et al., 2002).
2.5.1 Photon beam quality specification
The use of ionisation ratios for the determination of photon beam quality indices provides an
acceptable accuracy owing to the slow variation with depth of water/air stopping power ratios
(Podgorsak, 2005) and the assumed constancy of ionisation chamber perturbation factors beyond
the depth of maximum dose. For high-energy beams, TPR20,10 is an insensitive quality specifier.
For example a 1% change in TPR20,10 for values near 0.8 leads to a 3 MV change in the nominal
accelerating potential (near 20 MV) and a 0.4% change in the water to air stopping-power ratio.
In contrast, for values of TPR20,10 near 0.7 a 1% change corresponds to a 0.1% change in
stopping-power ratio and only 0.5 MV change in the nominal accelerating potential (Rogers,
1990).
12
2.5.2 Electron beam quality specification
The beam quality index for electron beams is the half-value depth (R50) in water. This is the
depth in water at which the electron beam depth dose decreases to 50% of its maximum value,
measured with a constant SSD of 100 cm and a reference field size at the phantom surface.
Different protocols recommend different field sizes for different mean incident electron energies.
According to IAEA TRS 398, the field sizes should be at least 10 cm x10 cm for R50 ≤ 7 g/cm2
(Eo ≤ 16 MeV) and at least 20 cm x 20 cm for R50 >7 g/cm2 (Eo ≥ 16 MeV). The AAPM TG-51
recommends the field size to be greater than 20 cm x 20 cm for R50 > 8.5 cm, i.e., E > 20 MeV,
where Eo and E is the mean energy of an electron beam at the phantom surface and at any depth,
respectively. Nitschke (1998) recommends a field size of at least l2 cm x l2 cm for E0 < 15 MeV
or 20 cm x 20 cm for E0 ≥ 15 MeV. A plane parallel chamber is recommended for E0 ≤ 10 MeV
(AAPM, 1983; IAEA, 1987; AAPM, 1991, IAEA, 2000) and for all relative dose measurements.
The use of R50 as the beam quality index is a simplification and a change from specifying beam
quality in terms of mean electron energy (Eo) of the incident spectrum striking the phantom
surface.One way of determining R50 is to determine the 50% ionization, I50 in a water phantom at
an SSD of 100 cm from the relative depth-ionization curve. For cylindrical chambers, there is a
need to correct for gradient effects by shifting the relative depth-ionization curve upstream by
0.5 rcav, the radius of the air cavity in a chamber in question. For plane-parallelchambers no
shift is needed, as the effective point of measurement is at the inside surface of the front
electrode which is at the point of interest. All the readings must be corrected for ion
recombination and polarity (IAEA, 2000; Khan, 2010). As an alternative the percentage depth
dose curve can be determined directly using a good quality diode detector. This requires test
comparisons with an ionisation chamber in order to establish whether the diode is suitable for
13
depth dose measurements or not (Almond etal., 1999). If a plastic phantom is used for measuring
dose, the values of the depths are scaled to water equivalent depths (IAEA, 1987; Nitschke,
1998) dw according to
dW = dplCpl (5)
dpl is the depth in plastic phantom. Cpl is the plastic to water depth scaling factor and the reading
in plastic is scaled to the equivalent reading in water according to (Khan 2010)
M=Mplhpl (6)
where M is the reading when the chamber is used with plastic and hpl is a material dependent
fluence scaling factor to correct for the differences in electron fluence in plastic compared with
that in water at the equivalent depth. The plastic material should be conductive. However,
insulating materials can be used provided the problems resulting from charge storage are
considered. The effect of charge storage can be minimized by using sheets not exceeding 2 cm in
thickness (IAEA, 2000).
2.6 Theoretical expressions for the beam quality correction factors in high energy photon
and electron beams.
2.6.1 Theoretical expression for kQ (photon beams).
The kQ factor can be calculated using two different methods. The first method applies the AAPM
TG-51 formalism (Almond et al., 1999).
0Q
W
air
replwall
Q
W
air
replwall
Q
L
L
PP
PP
K
=
ρ
ρ (7)
Where
14
PPP flgrrepl= (8)
Pgr accounts for the fact that the cavity introduced by a cylindrical chamber with its centre at the
reference depth, samples the electron fluence at a point which is closer to the radiation source
than the reference depth. Pgr depends on the inner radius of the cavity of the ionisation chamber
(Ma & Nahum, 1995). The cavity correction Pfl corrects for the perturbation of the electron
fluence due to scattering differences between the air cavity andthe medium ( Saiful, 2001). Pwall
in equation (7) accounts for the differences in the photon mass energy-absorption coefficients
and the electron stopping powers of the chamber wall material and the medium. If the central
electrode of a cylindrical ionisation chamber is not air equivalent, a correction Pcell, would also
need to be made for this lack of equivalence.
W
air
L
l
is the mean restricted collision mass
stopping power of water to air (AAPM, 1983).The second method uses the IAEA TRS-398
formalism (IAEA, 2000; ARPANSA, 2001; Achim & Ralf-Peter, 2007):
KQ = ( ) ( )
( ) ( ) PWSPWS
COCOCO airairW
QQairQairW
606060,
, (9)
( SW,air)Q is the Spencer-Attix water to air stopping-power ratio for beam quality Q, which is the
ratio of the mean restricted mass stopping powers of water to air, averaged over a complete
spectra. Wair is 33.7 J/C, the mean energy expended in air per ion pair formed. PQ is the
perturbation factor (includes the displacement effect) taking into account the deviations from the
ideal Bragg-Gray conditions when real ionisation chambers are used.
2.6.2 Theoretical expression for kq,E (electron beams).
According to (Khan, 2010) the electron beam quality conversion factor kq,E is given as
KKPK ecalR
Eq
grEq 50
,
,=
(10)
15
.,
PEq
grcorrects for the gradient effects at the reference depth when a Cylindrical chamber is used
in an electron beam, and depends on the ionisation gradient at the point of measurement (Kubo,
Kent & Krithivas, 1986). K ecal is the photon to electron conversion factor defined for a given
chamber model and is used to convert the absorbed dose to water calibration factor at 60
Co,
NCO
WD
60
,into N
ecalq
WD,, the absorbed dose to water calibration factor in the electron beam of quality
qecal, (Almond et.al., 1999) i.e.
Kecal NCO
WD
60
,= N
ecalq
WD, (11)
KR50 is the electron quality conversion factor used to convert , Necalq
WD, into N
CO
WD
60
, for any beam
quality q,E, i.e.
KR50 Necalq
WD, = N
Eq
WD
,
, (12)
where R50 is usually fixed at 7.5 g cm-2
for nominal energies of 3 MeV to 50 MeV and with field
sizes ≥ 10 cm x 10 cm (Almond et.al., 1999).
2.7 Reference conditions of the irradiation geometry for absorbed dose measurements
using an ionisation chamber in a phantom.
A water phantom is the reference medium for the absorbed dose measurements. For absolute
dose measurements in electron beams with E0 < 10 MeV and for relative dose measurements, a
plastic phantom may be used but depths and ranges must be converted to the water equivalent.
There should be a margin of at least 5 cm on all sides of the largest field size used at
measurement depth, and beyond the maximum depth of measurement. The chamber is always
used with its effective point of measurement at the reference depth.The effective point of
measurement for a plane parallel chamber is the inside surface of the front electrode (IAEA,
2000).
16
CHAPTER THREE: MATERIALS AND METHODS
3.1 Research Design
This is an experimental study. The beam quality correction factors kQ and kq,E for Semi-flex
cylindrical and Advanced Markus plane-parallel ionisation chambers respectively will be
determined by calibrating it against reference standard .
3.2 Study locations
This research work was carried out at the University of Nigeria Teaching Hospital(UNTH)
Enugu, Enugu State and University College Hospital Ibadan, Oyo State both in Nigeria.
3.3 Instrumentations
Our study with Linac was carried out using two beam modalities in the energy range common to
radiotherapy: photons with nominal energies of 6 MV and 15 MV, and electrons with nominal
energies of 4,6, 8,10, 12,and 15MeV produced by the linear accelerator, Elekta Precise. Linac
House Fleming Way, Crawley United Kingdom in 2005 (see Fig.3.1). The 60
Co beam used in
this study is produced by a Theratron-780 External Beam Therapy System MDS Nordion
manufactured in Canada in 1951 (Fig. 3.2). We used a PTW 30013 Farmer and PTW 31010
Semi-flex cylindrical chambers with a PTW T10001 Unidos Electrometer (PTW, Freiburg,
Germany) to calibrate photon beams and a PTW 34045 Advanced Markus parallel-plate chamber
with the PTW T10001 Unidos Electrometer to calibrate electron beams. The ionization
chambers and the electrometer were together calibrated for absorbed dose to water in 60
Co beam
quality by the manufacturer, which is a secondary standard dosimetry laboratory (SSDL) and is
traceable to the PTB (Physikalisch-Technische-Bundesanstalt) primary standard dosimetry
laboratory (PSDL) in Germany.
17
.
Figure 3.1: An Elekta PRECISE linear accelerator installed at University of Nigeria Teaching Hospital Enugu
The 60
Co beam used in this study is produced by a Theratron-780 External Beam Therapy
System (Figure 3.2). This model is an 80 cm SAD unit. The therapy used is a sealed capsule. The
head of the machine is shielded with lead. A pneumatic air system controls the source drawer,
18
which drives the source from the fully shielded position to the fully exposed position. The source
drawer is a cavity of approximately 2.8 cm diameter by 12 cm long, held in place with an end
plug and securing clip. The machine is equipped with a display monitor, to display beam
parameters, primary and secondary timers and system messages. The control panel allows for
treatment control and monitoring. The source is a metallic isotope of 60Co, sealed in two
stainless steel capsules of approximately 1.5 cm in diameter and 3 cm long. The 60Co nuclei
decay to 60Ni with emission of gamma rays of energies of 1.17 MeV and 1.33 MeV. The half-
life of 60Co is 5.26 years.
Figure 3.2: The Theratron-780 60
Co External Beam Therapy System accelerator installed at University College
Hospital Ibadan.
3.3.1 Ionisation chambers used in this study
(i) Farmer cylindrical ionisation chamber model/ serial No: TM30013-1612
(ii) Semiflex cylindrical ionisation chamber model/ serial No: TM31010-1350
Ion chamber
Retort stand
60Co machine
head
19
(iii) Advanced Markus plane-parallel ionisation chamber model/ serial No: TM34045-0343
All the three chambers were manufactured by PTW-FREIBURG, Germany in 2005
Other specifications of the above mentioned ionisation chambers are shown on the table 3.1
Table 3.1: The characteristics of the different ionisation chambers types used in this study.
Ionisation
chamber type
Cavity
Volume
(cm3)
Cavity
length
(mm)
Cavity
radius
(mm)
Wall
material
Wall
thickness
(g cm-2
)
Central
electrode
material
Water
proof
PTW 30013
Farmer
0.6 23 3.0505 PMMA 0.057 Aluminium
Yes
PTW
31010
Semi-flex
0.125 0.325 0.36 PMMA
+graphite
0.055+0.015 Aluminium
Yes
PTW 34045
Advanced Markus
0.02 2.50 CH2
Polyethylen
e
0.003 Yes
The PTW cylindrical chambers were of the type 30013 0.6 cm3 and two PTW 31010 0.125 cm
3,
and the PTW plane-parallel chamber was of the type 34045 Advanced Markus. A track record of
PTW 30013 0.6cm3 reference ionisation chamber absorbed dose to water calibration factors over
the years is shown in table 3.2.
Table 3.2 : The calibration factor history of reference ionisation chamber (PTW 30013 0.6cm3)
20
Calibration Date NCO
WD
60
, Stated uncertainty
Oct. 2005 5.334E+07 Gy/C 2.2%
Sept. 2011 5.328E+07 Gy/C 1.1%
The PTW 30013 0.6 cm3 model was selected as a reference chamber for this work because of its
Geometric equivalence to the PTW 23333 0.6 cm3, its proven stability, and because it was
representative of a series of over three ionisation chambers used for the daily calibration of the
teletherapy machines. The Advanced Markus is marketed as a perturbation-free version of the
Markus chamber. The plane-parallel chambers have nominal useful ranges of energies of 2 MeV
to 45 MeV. The nominal useful range for the cylindrical chambers is from 60
Co to 50 MV for
photons and from 10 to 45 MeV for electrons. The Advanced Markus exceptionally covers a
useful range of 66 keV to 50 MeV for electron beams. The description of the wall, build up caps
and the various dimensions for the four ionisation chambers are shown in Table 3.1. Figure 3.3
shows the ionisation chambers used for this study. The measurement volumes of all the above
chambers are vented, fully guarded and suitable for use in solid state phantoms
.
21
Fig 3.3a the PTW 30013 (0.6cm3) Farmer ionisation chamber used in this study
Fig 3.3b the PTW34045 Advanced Markus ionisation chamber used in this study
22
Fig 3.3c the PTW 31010(0.125cm3) Semiflex ionisation chamber used in this study
3.3.2 Electrometer used in this study
.Electrometer name : PTW Unidos
Electrometer model/ serial No: T10001-1147
Manufacturer: PTW-FREIBURG, Germany in 2005
The electrometer used was a PTW Unidos T10001 (see Figure 3.5) capable of positive and
negative polarity settings over a range of 0 to 400 V in intervals of 50 V. For in air dosimetric
methods, a retort stand was used to hold the chamber firmly at the measurement point.
Figure 3.4: The PTW T10001 Unidos Electrometer
3.4 The cross-calibration of ionisation chamber in photon and electron beams.
All the NCO
WD
60
,, Nx and Nk calibration factors for the different ionization chambers were
independently cross-calibrated in the 60Co beam against the calibrated PTW farmer reference
ionisation chamber. Ngas for the plane parallel chambers was derived from the cross-calibration at
15 MeV against the reference ionisation chamber. The recommendations of the AAPM TG-21
and IAEA TRS-398 protocols were followed for the cross-calibration procedures.
23
3.4.1 Cross-calibration of the NCO
WD
60
, for ionisation chambers in
60Co beam.
Fig 3.6 Experimental set up for dose measurements with ionization chambers
As shown on the figure above,the ionization chamber is placed at a reference depth of 10cm in a
water phantom.The reference point of the semi-flex cylindrical chamber is on the central axis at
the centre of the cavity volume.For Advanced markus plane-parallel chamber, on the inner
surface of the window at its centre.The source to chamber distance is 80cm and a field size of 10
X 10cm is used. The chamber is connected to an electrometer and the charge reading for 100MU was
recorded.With the above setup,the ionization charmbers under study were cross-calibrated
against a calibrated reference Farmer cylindrical chamber in a 60
Co beam. The absorbed dose to
water calibration factors for any ionisation chamber Y, under test against a reference ionisation
Ionisation
chamber
Water
phantom tank
Control
pendant
24
chamber ref, is given by
( NCO
WD
60
,)Y =
Y
ref
M
M
)(
)(( N
CO
WD
60
,)ref (13)
Where (M)ref and (M)Y are the electrometer readings for an ionisation chamber in the 60
Co beam
for the reference and the chamber under test, respectively, corrected for the influence quantities.
3.4.2 Cross-calibration of the 60
Co exposure calibration factor Nx.
The setup as in Fig, 3.6 was used to obtain Dw using the IAEA technical report series 398.
The 60
Co exposure calibration factor Nx for the PTW Farmer chamber was calculated using
(Nx)AAPMG-21= ( )
21
398
−
−⟩⟨
AAPMTGeq
IAEATRSW
BSFMfA
D (14)
Where Dw is as given in equation (1); f is 0.967 cGy/R, the dose to water per roentgen of
exposure; Aeq is 0.989, a factor that accounts for attenuation and scattering in a small mass of
water of 0.5 cm radius at the reference depth; BSF is the 0.5 cm depth tissue air ratio; and M
(nC) is the electrometer reading for 10 cm x 10 cm field size, normalized to 200 C temperature
and a pressure of one standard atmosphere and corrected for timer errors in accordance with the
IAEA TRS-398 formalism i.e
M = τ+t
M raw .kTP .kpol .kelec .k s (15)
where M raw the uncorrected reading, τ is is the timer error, kTP is temperature pressure
correction factor, kelec is the electrometer calibration correction factor and ks is the
recombination correction factor.
25
3.4.3 Cross-calibration of the Ngas (Cavity-gas calibration factor) for plane-parallel
chambers in electron beams.
The plane-parallel chambers were cross-calibrated against the PTW 30013 0.6 cm3 reference
ionisation chamber whose replacement correction (Prepl) was 0.994 at 15 MeV, the highest
electron energy available at the department. The AAPM TG-21 formalism was use i.e.
(Ngas )p-p
= ( )
( ) pp
ion
cylin
repliongas
MP
PPMN
−...........................................……………….…. (16)
where M is the response of the chamber in question at dmax, p-p and cylin refer to the plane
parallel and cylindrical chambers respectively. Where Pion is a correction factor for ion
recombination losses
3.5 The absorbed dose measurement in megavoltage photon
FIG.3.7. Experimental set-up for the determination of the beam quality index Q i.e Tissue Phantom
Ratio (TPR20,10).
Chamber position
Water phantom
Field size
26
As shown in the figure above, the source-to-axis distance (SAD) is kept constant at 100 cm and
measurements are made with 10 cm and 20 cm of water over the chamber. The field size at the position
of the reference point of the chamber is 10 cm × 10 cm. The ratio of ionization readings at depth 20cm
and 10 cm for 100MU were obtained for two photon energies i.e 6 and 15 MV.
.
FIG.3.8. Experimental set-up for the determination of the absorbed dose Dw using perspex phantom
The charge readings at a point in the perspex phantom were measured with ionization chambers
with the center of the sensitive volume placed at 5cm depth(see Fig. 3.8) , the water equivalent
reference depth as used for calibration of the ionisation chambers in the 60
Co beams i.e. 5 cm of
Perspex phantom
Chamber
connecting
cable
LINAC head
Field size of 10cm ×
10cm
27
water. The centers of the chambers were aligned with the isocentre of the treatment machine.
The dose was referenced to the PTW Farmer ionisation chamber using its 60
Co absorbed dose to
water calibration factor NCO
WD
60
,. The dose to water at the reference depth with the chamber
removed was calculated using equation (1). The chambers and the perspex phantom were
allowed to equilibrate with the ambient air temperature. With the PTW Farmer reference
chamber connected to the electrometer and the machine in the beam off mode, the leakage at the
positive polarity of the electrometer was -0.023 nC (with medium range settings, 12.0 nA) for
732.0 seconds. Charge readings were taken for 100 monitor units. The measurements were
repeated three times at each polarity of each ionization chamber. The mean value of the readings
was then calculated. Throughout the study, the absolute value of the polarising voltages was
maintained at either +400V, -400V or +200 V (+200 V was used in determining the ion
recombination correction factor ). The readings were corrected for the standard environmental
conditions of temperature and pressure, ion recombination and polarity effects but the humidity
corrections were not considered because it is within 20% to 80% . The resultant corrected charge
reading and the known absorbed dose rate to the water under reference conditions were used to
derive the calibration factor for each cylindrical ionization chamber NQ
WD,. The measurement of
absorbed dose to water requires a beam quality specifier TPR20,10. The beam quality specifier
TPR20,10 for the two photon energies (6 MV and 15 MV) was 0.6770 and 0.7630, respectively.
3.6 The absorbed dose measurement in electron beams.
The charge readings for 100 monitor units in a perspex phantom were measured with the centre
of the sensitive volume of the ionization chambers placed at the depth of maximum dose, at a
constant source to surface distance of 100 cm, in a 10 cm x 10 cm field size. The chambers and
the perspex phantom were allowed to equilibrate with the ambient air temperature. The chambers
28
were first cross-calibrated for Ngas against the cylindrical reference ionisation chamber at 15
MeV using equation (16). The measurements were repeated three times at each polarity of the
ionization chamber. The mean value of the readings was then calculated.Throughout the study,
the absolute value of the polarising voltages was maintained at either +400V, -400V or +200 V.
The readings were corrected for the standard environmental conditions of temperature and
pressure, ion recombination and polarity effects. Humidity corrections were not considered.
Equation (2) was used for the determination of absorbed dose to water. Table 3.4 shows the
beam characteristics used for the measurement and calculation process.
Table 3.4: The beam characteristics for the clinical electron beams and the mean restricted collision mass stopping
power of perspex to air used in this study.
Energy
(MeV)
R50
/(cm)
Eo
/(MeV)
dref
/(cm)
perspex
air
L
l
(MeV.cm2/g)
4 1.633 3.805 0.880 1.088
6 2.429 5.660 1.357 1.074
8 3.268 7.614 1.861 1.062
10 3.993 9.304 2.296 1.053
12 4.750 11.068 2.750 1.046
15 6.041 14.076 3.525 1.035
R50 is extracted from the commissioning data at University of Nigeria Teaching Hospital Enugu Elekta Precise
Linear accelerator.
3.7 Determination of beam quality correction factors
29
The photon beam quality correction factors were determined according to equation (1) in which the
dose measured by PTW Farmer ionisation chamber was used as the reference dose. The corrected
average measured charge readings and the absorbed dose to water calibration factor from the cross-
calibration process in the 60
Co were used for calculation calculation according to (Hubert, Hugo & Wim,
1999, Achim & Ralf-Peter, 2007,) as shown in equation (17) i.e.
(kQ)Y = ( )
( )Y
CO
WD
Q
Y
ref
Q
W
NM
D
60
,
(17)
Where Q denotes the quality of the beam in which the chambers named ‘ref’ and ‘Y’ were used
for beam quality correction measurements. The electron beam quality correction factors (Kq,E)
were determined as the ratio of the absorbed dose to water calibration factors in the electron
beam and the reference 60
Co beam for that particular chamber Y according to (Hubert, Hugo &
Wim, 1999, Achim & Ralf-Peter, 2007).
Kq.E = ( )( )
Y
CO
WD
Y
Eq
WD
N
N
60
,
,
,
(18)
The absorbed dose to water calibration factors in the electron beam ( )Y
Eq
WDN,
, is determined as the
ratio of the absorbed dose to water measured by the PTW farmer reference ionisation chamber to
the absorbed dose to water measured by the chamber Y under test (Hubert, Hugo & Wim, 1999).
( )
Y
Eq
WDN,
, = ( )( )
( )( ) Eq
Ywater
Eq
refwater
atdD
atdD
,
max
,
max (19)
Where ( )Y
Co
wDN60
, in equation (18) is obtained from the result of the cross-calibration in equation
(13).
30
CHAPTER FOUR: RESULTS AND DISCUSSIONS
4.1 The results of the cross-calibration of the ionisation chambers.
The experiment with each ionisation chamber was repeated on three occasions and a mean value
then calculated. The maximum deviation observed between any three measurements taken with
all ionisation chambers was ± 0.009 nC. As expected the 60
Co energy does not change and so
any deviations would thus be attributed to the dosimetric apparatus’ drift (Kaumba 2010). It was
observed that the dosimetric apparatus showed no significant drift during the time of the study.
Table 4.1 shows the results of the measured NCO
WD
60
,from the cross-calibration against the PTW
farmer reference chamber. Also shown are the NCO
WD
60
, values obtained from the PTW standards
laboratory for each chamber.
Table 4.1: The absorbed dose to water calibration factors for the ionization chambers used in this study.
Chamber
Model
NCO
WD
60
, Gy/C
(PTW Certificate)
(± 2.2%)
Measured N
CO
WD
60
,Gy/C (cross-
calibration)
Deviation (%) from (PTW
Certificate)
(± 2.2%)
PTW 30013 0.6 cm3 5.328E+07
( Sept.2011)
Reference i.e (5.328E+07
)
-
PTW 31010
0.125cm3
2.997E+08
(Oct. 2005)
2.984E+08 ± 3.4% 0.4
PTW 34045
Advanced
Markus
1.394E+09
( Sept.2011)
1.352E+09 ± 3.4% 3
31
The 60
Co exposure calibration factor, NX for the PTW Farmer reference ionisation chamber was
5.408E+09 R/C. The air-kerma calibration factor Nk for PTW Farmer was 4.754E+07 Gy/C. This
calibration factor was then used in the cross-calibration of other ionisation chamber in air and is
shown in Table 4.2.
Table 4.2: The results of NX and Ngas calibration factors for the ionization chambers used.
Chamber Nx/ R/C Ngas/Gy/C
PTW 30013 0.6 cm3 5.473E+09 4.614E+07
PTW 31010 0.125cm3 3.124E+10 2.625E+08
The in-air measurements were taken for 0.5 minute irradiations in a 60
Co beam, at 80 cm source
to chamber distance in a 10 cm x 10 cm field size, with the 60
Co build-up cap and using the
T10001 electrometer. The polarity correction factor and recombination correction factor for the
reference ionisation chamber was 0.999 and 1.002, respectively.The cross-calibration to
determine Ngas of the plane-parallel chambers from Ngas of the PTW Farmer ionisation chamber
was done at 15 MeV, the highest electron energy available in phantom. The replacement
correction factor for PTW Farmer reference ionisation chamber is 0.994 at 15 MeV. The result of
the Ngas cross-calibration process was 1.19E+09 Gy/C for PTW 34045 Advanced Markus
ionisation chamber.
4.2 Measurement results in 6 MV and 15 MV photon beams
The kQ derived as a function of TPR20,10 for the various ionisation chambers are shown in Table
4.3. The kQ results obtained for the PTW 30013 0.6 cm3 Farmer ionisation chamber compare
well with the IAEA TRS-398 data.
32
Table 4.3: The measured kQ as a function of TPR20,10 of the various ionisation chambers
Chamber First experiment Second experiment
0.676(6MV) 0.7630(15MV) 0.6775(6MV) 0.7569(15MV)
PTW 30013 0.991 0.974 0.9918 0.9732
PTW31010 0.9952 0.9721 0.9960 0.9730
The measured kQ values as a function of TPR20,10 (the tissue-phantom ratio in water at depths of
20 cm and 10 cm, for a field size of 10 cm x 10 cm and a constant source-chamber distance of
100 cm) for the different ionisation chambers and the published IAEA TRS-398 kQ values for
the PTW 30013 0.6 cm3 Farmer ionisation chamber are tabulated below:
Table 4.4: The measured kQ as a function of TPR20,10 of the various ionisation chambers used in this study and the
published IAEA TRS-398 kQ values for the PTW 30013 0.6 cm3 chamber.
Nominal
Energy/MV
TPR20,10 PTW 31010
0.125cm3
PTW 30013 0.6cm3 PTW 30013 0.6cm
3
(IAEA TRS 398 )
6 0.677 0.996 0.991 0.990
15 0.763 0.973 0.974 0.975
4.3 Measurement results in the electron beam qualities.
For the electron beams, the doses were measured with the reference point of each of the
chambers at the reference depth in a perspex phantom using a 10 cm x 10 cm applicator and an
SSD of 100 cm. The measured electron doses are as summarized in Table 4.5.
33
Table 4.5. Summary of the doses in Gy per 100 monitor units at dref using each of the ionization chambers.
Nominal
Energy
(MeV)
R50/ cm PTW 30013
0.6cm3
PTW 31010
0.125cm3
PTW 34045
Advanced
Markus
4 1.633 0.970 0.996 0.986
6 2.429 0.976 0.996 0.984
8 3.268 0.966 0.981 0.968
10 3.993 0.968 0.980 0.970
12 4.750 0.965 0.958 0.962
15 6.041 0.954 0.950 0.951
Table 4.6: The replacement correction factors for the cylindrical ionisation chambers at each electron beam quality
and the replacement correction factors published by Khan (1991) for the PTW 23333 0.6 cm3, as used for the
absorbed dose determination in the electron beams.
R50
/cm
PTW 30013
0.6cm3
PTW 23333(KHAN)
0.6cm3
PTW 31010
0.125cm3
1.633 0.956 0.958 0.959
2.429 0.959 0.960 0.962
3.268 0.961 0.964 0.966
3.993 0.966 0.969 0.969
4.750 0.969 0.974 0.972
6.041 0.982 0.982 0.978
34
The measured doses were used to derive the absorbed dose to water calibration factors for the
electron beams. These absorbed dose to water calibration factors NEq
WD
,
, (shown in Table 4.7)
were in turn used to determine the kq,E for each of the ionisation chambers at each electron
energy. The kq,E obtained as a function of R50 for the cylindrical chambers and for the parallel
plate chambers are shown in Table 4.8
Table 4.7: The calculated NEq
WD
,
, x 10
7 Gy/C at each electron energy for the various ionisation chambers.
R50
/ cm
PTW 30013
0.6 cm3 Reference
chamber
PTW 31010
0.125 cm3
Chamber 1
PTW 34045
Advanced Markus
Chamber 2
1.633 4.79 26.7 129
2.429 4.73 26.5 128
3.268 4.67 26.3 126
3.993 4.60 26.2 125
4.750 4.55 26.1 123
6 .041 4.53 25.9 121
Table 4.8: The results of the kq,E values determined as a function of R50 for the various ionisation chambers.
R/50
(cm)
PTW 30013
PTW 31010
PTW 34045
1.633 0.899 0.895 0.954
35
2.429 0.889 0.888 0.947
3.268 0.877 0.881 0.935
3.993 0.863 0.878 0.923
4.750 0.854 0.875 0.912
6.041 0.850 0.868 0.896
4.4 DISCUSSIONS
Cross-calibrations of NCO
WD
60
, and Nx for the PTW 31010 0.125 cm
3 ionisation chamber and the
34045 Advanced Markus ionisation chamber against the PTW Farmer reference ionisation
chamber were performed. The values of quality correction factors kQ obtained for the PTW
30013 0.6 cm3 Farmer ionisation chamber compare well with the IAEA TRS-398 data. Overall,
the average deviation of the measured doses with all the chambers from the dose measured with
the PTW 30013 0.6cm3 Farmer ionization chamber was 0.8%. The PTW 34045 Advanced
Markus has a smaller volume compared to either the PTW 30013 0.6cm3 Farmer ionization
chamber or the PTW 23343 Markus. The PTW 34045 Advanced Markus therefore perturbs the
water medium less and the electron fluence may be taken to be closer to unity. Furthermore the
PTW 34045 Advanced Markus has a better spatial resolution than the PTW 30013 0.6cm3
Farmer ionization chamber . Since the results of the PTW 34045 Advanced Markus do not
compare well with the results of the PTW 30013 0.6cm3 Farmer ionization chamber , it could be
confirmed that cylindrical chambers should not be used to measure the dose to water in electron
beams of Eo ≤ 10 MeV (AAPM, 1983; IAEA, 1987; AAPM, 1991; IAEA, 2000). Cross-
calibrations of NCO
WD
60
, and Nx for the PTW 31010 0.125 cm
3 ionisation chamber and the PTW
34045 Advanced Markus ionisation chamber against the PTW 30013 0.6 cm3 reference
36
ionisation chamber were performed. The cross-calibration factors compare well with those on
their respective chamber certificates. These cross-calibration factors have been obtained using
the existing international dosimetry protocols, they are therefore traceable to standard dosimetry
laboratories and they can be applied in the routine and periodical quality assurance programmes
of University of Nigeria Teaching Hospital (UNTH) Enugu radiation clinics, with some
confidence.
The beam quality correction factors for the PTW 30013 0.6 cm3 ionisation chamber in photon
beams with TPR20,10 of 0.677 and 0.763 were determined with an accuracy of 0.2%, compared
to the IAEA TRS-398 published values.The beam quality correction factors for the PTW 34045
Advanced Markus ionisation chamber in a range of electron beam qualities of R50 of 1.633 cm
to 6.041 cm (3.80 MeV≤ Eo ≤14.08 MeV) were determined with an accuracy of 1.2%, compared
to the IAEA TRS-398 published values.
Since the uncertainties are systematically low and not significant, this study establishes that any
of the ionization chamber types used in this study could be used as reference chambers for
clinical dosimetry. Although different centers may have different beam designs and measuring
methods, the KQ values for the chambers used in this study can be applied to other beams of the
same beam quality.
The overall deviation of 5% in the results of the PTW 34045 Advanced Markus from the results
of the PTW 30013 0.6 cm3 confirms that cylindrical chambers should not be used to measure the
dose to water in electron beams of Eo ≤ 9 MeV. Cylindrical chambers, however, can be used for
less precise daily quality control checks of electron beams of Eo ≤ 9 MeV where compliance to a
range of dose or dose rates only is to be confirmed. The beam quality correction factors KQ and
Kq,E for the PTW 31010 0.125 cm3 and PTW34045 Advanced Markus models of ionisation
37
chambers for which no published data exist, were determined with reasonable accuracy. The
electron beam quality correction factors were determined at a dose-rate of 400 MU/ min.
38
CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS
This work demonstrates clearly the ability to determine beam quality correction factors in a
clinical setting. Cross-calibrations were performed of the absorbed dose to water calibration
factors of the Advanced Markus ionisation chambers (M’ule, 2008). The Kq,E values determined
for the Advanced Markus ionisation chamber will provide improved accuracy in dosimetry with
this chamber since the error previously introduced by using published or extrapolated Kq,E
values for the Markus (old version of Advanced Markus) ionisation chamber is now eliminated.
The semiflex ionisation chamber can also be used for absolute dosimetry since the beam quality
correction factors are now determined for the beam qualities available at the University of
Nigeria Teaching Hospital Enugu.
Although the results of this study are clinically used with some confidence , a Monte Carlo
simulation [EGSnrc] could be performed to test the validity of their statistical uncertainties.
39
DEFINITIONS OF TECHNICAL TERMS, ACRONYMS AND SYMBOLS.
(AAPM) American Association of Physicists in Medicine.
(ARPANSA) Australian Radiation Protection and Nuclear Safety Agency.
(TG ) Task Group.
(AAPM TG-21) A protocol of the AAPM TG-21 for the determination of absorbed dose from
high-energy photon and electron beams.
(IAEA) International Atomic Energy Agency.
(TRS) Technical Report Series.
(IAEA TRS-398 ) An international code of practice for the absorbed dose determination in
external beam radiotherapy, published by the IAEA on its own behalf, and on behalf of the
World Health Organisation (WHO), the Pan Ameraican Health Organisation (PAHO) and the
European Society of Therapeautic Radiology and Oncology (ESTRO).
(NAP) Nominal accelerating potential.
(qecal) An arbitrary electron beam quality taken as R50= 7.5 cm. It is introduced to simplify
the factors needed in electron beam dosimetry in IAEA TRS-398
(TPR20,10 ) The ratio of doses on the beam central axis at depths of 20 cm and 10 cm in a
water phantom, obtained with a constant source-chamber distance of 100 cm and a field size of
10 cm x 10 cm at the plane of the chamber.
NCO
WD
60
,The absorbed dose to water calibration factor (in Gy/C) for the ionisation chamber in the
reference 60Co beam.
NEq
WD
,
,The absorbed dose to water calibration factor (in Gy/C) for the ionisation chamber in an
electron beam of quality q,E.
40
(KQ ) A chamber specific factor which corrects the absorbed dose to water calibration factor in a
60Co beam to another photon beam of quality Q.
(Kq,E) is a beam quality conversion factor for electrons to convert NCO
WD
60
,to N
Eq
WD
,
, for an
electron beam of quality q,E. In this report, the notation kq,E is adopted for electron beam
qualities to distinguish it from kQ for photon beam qualities.
(Kecal ) The photon to electron conversion factor defined for a given chamber model that converts
the absorbed dose to water calibration factor at 60Co to the absorbed dose to water calibration
factor in the electron beam of quality qecal.
(KR50) The electron quality conversion factor used to convert , N
ecalq
WD,, into the absorbed dose to
water calibration factor NEq
WD
,
, for any electron beam of quality q,E.
( Dw )The absorbed dose to water for a particular set up and monitor units.
(Eo) The mean electron energy of the incident spectrum striking the phantom surface.
(MU) The number of monitor units or time for which a given irradiation is performed.
(Pcav ) Factor that corrects the response of an ionisation chamber for effects related to the air
cavity ; predominately the in-scattering of electrons that makes the electron fluence inside a
cavity different from that in the absence of the cavity.
(Pgr ) Corrects for the gradient effects at the reference depth when a cylindrical chamber is used
in an electron beam, and depends on the ionisation gradient at the point of measurement.
(SSDL) Secondary Standards Dosimetry Laboratory.
(BIPM) Bureau International de Poids et Mesure, Paris.
(PTB) Physikalisch-Technische Budesanstalt.
(PTW) Physikalisch Technische Werkstätten
41
(E) The mean energy of an electron beam at any depth.
(MV) Megavoltage.
(Q) The beam quality in the user’s photon or electron beam for which clinical reference
dosimetry is performed. For photon beams it is in terms TPR20,10 and for electron beams, in
terms of R50. However, in this report Q is used exclusively for photon beams and q,E is for
electron beams.
(SAD) Source-axis distance.
(SSD) Source-surface distance.
(TPR) Tissue phantom ratio.
42
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