Faculty of Physcal Sciences. Department of Physics and Astronomy. Ugwoke Oluchi C. AROH, FABIAN ONYEMAECHI PG/M.Sc/08/49517

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1 i AROH, FABIAN ONYEMAECHI PG/M.Sc/08/49517 DETERMINATION OF BEAM QUALITY CORRECTION FACTORS FOR TWO IONIZATION CHAMBERS OF THE LINAC UNIT AT UNTH Department of Physics and Astronomy Faculty of Physcal Sciences Ugwoke Oluchi C. Digitally Signed by: Content manager s Name DN : CN = Webmaster s name O = University of Nigeria, Nsukka OU = Innovation Centre

2 ii 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, CERTIFICATION

3 iii 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

4 iv DEDICATED TO THE MEMORY OF MY FATHER

5 v 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.

6 vi ABSTRACT The purpose of this work was to determine values of the beam quality correction factors K Q 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 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 cm 3 ionization chamber using its 60 Co 60 absorbed dose to water calibration factor N CO. The dose to water at the reference depth of 5 cm D, W 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 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 Farmer reference ionisation chamber in the 6 MV and 15 MV photon beams were used to determine k Q 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 N gas 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 k q,e were then derived for each of the ionisation chambers at each of the electron energies. The measured values of K Q and K q.e shows that the average observed difference between the measured values and those published in the IAEA TRS-398 protocol was 0.2% for

7 vii the PTW cm 3 Farmer in the photon beams and 1.2% for the PTW 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.

8 viii TABLE OF CONTENTS CERTIFICATION...iii DEDICATION...iv ACKOWLEDGEMETS... v ABSTRACT...vi TABLE OF CONTENTS... viii CHAPTER ONE: INTRODUCTION 1.1 Introduction Objectives of the study Justification of the study Scope of the study Limitations of the study... 4 CHAPTER TWO: LITERATURE REVIEW 2.1 Literature review Ionization chamber dosimetry Electrometer Photon beam dosimetry Electron beam dosimetry Beam quality specification Photon beam quality specification Electron beam quality specification Theoretical expressions for the beam quality correction factors in high energy photons and electron beams Theoretical expression for kq (photon beams) Theoretical expression for kq,e (electron beams)

9 ix 2.7 Reference conditions of the irradiation geometry for absorbed dose measurements using an ionisation chamber inaphantom CHAPTER THREE: MATERIALS AND METHODS 3.1 Research design Study locations Instrumentations Ionisation chambers used in this study Electrometer used in this study The cross-calibration of ionisation chamber in photon and electron beams Cross-calibration of the N CO D W, for ionisation chambers in 60 Co beam Cross-calibration of the 60Co exposure calibration factor N x Cross-calibration of the N gas for plane-parallel chambers in electron beams The absorbed dose measurement in megavoltage photon beams The absorbed dose measurement in electron beams Determination of beam quality correction factors CHAPTER FOUR: RESULTS AND DISCUSSIONS 4.1 The results of the cross-calibration of the ionisation chambers Measurement results in 6 MV and 15 MV photon beams Measurement results in the electron beam qualities DISCUSSIONS CHAPTER FIVE: CONCLUSIONS AD RECOMMENDATIONS DEFINITIONS OF TECHNICAL TERMS,ACRONYMS AND SYMBOLS REFERECES

10 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 ( N CO ) (in G y/c) for the D, W 60 ionisation chamber in the 60 Co reference beam together with a theoretical beam quality conversion factor ( K Q 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 K Q and Kq,E at various beam qualities intrinsically takes into account the response of different ionisation chambers. In contrast, the calculated values of K Q ignore

11 2 chamber-to-chamber variations in response to energy within a given chamber type, and its uncertainty is therefore larger than for experimentally determined K Q 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 K Q 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 K Q 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

12 3 (iii) Compare the experimentally determined values of K Q and Kq,E with published data for the Farmer ionisation chamber. (iv) Derive K Q 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 K Q and K q, 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 N D,W with cylindrical and plane-parallel ionisation chambers (ii) Determination of beam quality correction factors K Q for PTW Semi-flex cylindrical ionisation chamber. (iii) ) Determination of beam quality correction factors K q,e for PTW plane-parallel ionisation chamber.

13 4 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.

14 5 CHAPTER TWO: LITERATURE REVIEW 2.1 Literature review IAEA,(2000) emphasized that directly measured values of beam qaulity correction factor K Q 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 k Q 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 K q.e, which are in good agreement with the data published in the IAEA protocols, with a deviation of % 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 k Q can be measured with a standard uncertainty of less than 0.3% (Achim & Ralf-Peter, 2007; IAEA, 2000; Saiful, 2001).

15 6 The (IAEA TRS-398,AAPM TG-51 ) protocols have established the k Q and k q,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, NE cc Farmer, NE cc, NE cc robust Farmer, NE cc NPL Sec. Std, PTW N cc Farmer, PTW N cc all Graphite, PTW N cc Graphite, PTW cc 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 K Q and K q,e for PTW Semi-flex and PTW Advanced Markus ionisation chambers 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).

16 7 FIG. 2.1 Basic design of a cylindrical Farmer type ionization chamber 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

17 8 Ionization chamber Fig. 2.2 Negative feedback, operational amplifier The operational amplifier has a high open-loop gain (> 10 4 ) and a high input impedence (> 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 ohm, the output voltage will be current times the resistance or 10 V. Assuming open-loop gain of 10 4, 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:,

18 9 D W 60CO = M N D, W K Q i, K (1) M is the charge measured under standard conditions of temperature,pressure and humidity. 60 N CO D W, is the absorbed dose to water calibration factor (in Gy/C) for the ionisation chamber in the Co reference beam. K Q is a chamber specific factor which corrects N CO to the user s D, W beam quality Q (different from the 60 Co beam). K i, 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 60 N CO D W,.K Q = ( N Q ) is of special interest and is the absorbed dose to water calibration factor (in Gy/C) of the DW ionisation chamber in the beam quality Q. The current accepted relative uncertainty of D w in equation (1) is of the order of 1.5% as determined by ionometric methods and the uncertainty in k Q 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; D q, E W M N 60 D, W K CO = (2) q, E

19 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, 60 pressure and relative humidity of the air in the ion chamber. N CO D W, is the absorbed dose to water calibration factor (in Gy/C) of the ionisation chamber in the reference 60Co beam.k q,e is a 60 beam quality conversion factor for electrons to convert N CO to D, W N q, E for an electron beam of D, W quality q,e. 2.5 Beam quality specification Among the difficulties of the k Q and kq,e concept is the need for a unique beam quality specification and the possible variation in the k Q 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 = PDD 20, (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

20 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 E o, the mean electron energy of the incident spectrum striking the phantom surface (Podgorsak, 2005). Eo is empirically derived from R 50, the depth at which the electron beam depth dose decreases to 50% of its maximum value. The reference depth d ref, for electron beam calibrations in water according to (IAEA, 2000) is expressed as; d ref (cm) = 0.6R 50 (cm) (cm) (4) The reference depth d ref is used clinically because it is known to significantly reduce machine to machine deviations in chamber calibration coefficients (Hugo et al., 2002) 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, TPR 20,10 is an insensitive quality specifier. For example a 1% change in TPR 20,10 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 TPR 20,10 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).

21 Electron beam quality specification The beam quality index for electron beams is the half-value depth (R 50 ) 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 R 50 7 g/cm 2 (E o 16 MeV) and at least 20 cm x 20 cm for R 50 >7 g/cm 2 (E o 16 MeV). The AAPM TG-51 recommends the field size to be greater than 20 cm x 20 cm for R 50 > 8.5 cm, i.e., E > 20 MeV, where E o 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 E 0 < 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 R 50 as the beam quality index is a simplification and a change from specifying beam quality in terms of mean electron energy (E o ) of the incident spectrum striking the phantom surface.one way of determining R 50 is to determine the 50% ionization, I 50 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

22 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) d w according to d W = d pl C pl (5) d pl is the depth in plastic phantom. C pl 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=M pl h pl (6) where M is the reading when the chamber is used with plastic and h pl 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 Theoretical expression for k Q (photon beams). The k Q factor can be calculated using two different methods. The first method applies the AAPM TG-51 formalism (Almond et al., 1999). K Q = P P wall wall P P repl repl L ρ L ρ W air W air Q Q0 (7) Where

23 14 P P P = (8) repl gr fl P gr 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. P gr depends on the inner radius of the cavity of the ionisation chamber (Ma & Nahum, 1995). The cavity correction P fl corrects for the perturbation of the electron fluence due to scattering differences between the air cavity andthe medium ( Saiful, 2001). P wall 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 P cell, would also need to be made for this lack of equivalence. L l W air 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): K Q = ( S ) ( ) ( W, air Q W P S ) ( air Q W ) Q P W, air 60CO air 60CO 60 CO (9) ( S W,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. W air is 33.7 J/C, the mean energy expended in air per ion pair formed. P Q 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 Theoretical expression for kq,e (electron beams). According to (Khan, 2010) the electron beam quality conversion factor kq,e is given as K P K K q, E = (10) q, E gr R50 ecal

24 15, P q E gr. 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 (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, 60 N CO D W q into, N ecal, the absorbed dose to water calibration factor in the electron beam of quality D, W q ecal, (Almond et.al., 1999) i.e. 60 K ecal N CO q = D, W N ecal D, W (11) q K R50 is the electron quality conversion factor used to convert, N ecal 60 into D, W N CO for any beam D, W quality q,e, i.e. q K R50 N ecal =, D, W N E D, q W (12) where R 50 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 E 0 < 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).

25 16 CHAPTER THREE: MATERIALS AND METHODS 3.1 Research Design This is an experimental study. The beam quality correction factors k Q and k q, 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 Farmer and PTW Semi-flex cylindrical chambers with a PTW T10001 Unidos Electrometer (PTW, Freiburg, Germany) to calibrate photon beams and a PTW 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.

26 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,

27 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 halflife of 60Co is 5.26 years. 60 Co machine head Ion chamber Retort stand Figure 3.2: The Theratron Co External Beam Therapy System accelerator installed at University College Hospital Ibadan Ionisation chambers used in this study (i) Farmer cylindrical ionisation chamber model/ serial No: TM (ii) Semiflex cylindrical ionisation chamber model/ serial No: TM

28 19 (iii) Advanced Markus plane-parallel ionisation chamber model/ serial No: TM 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 Cavity Cavity Cavity Wall Wall Central Water chamber type Volume length radius material thickness electrode proof (cm 3 ) (mm) (mm) (g cm -2 ) material PTW PMMA Aluminium Yes Farmer PTW Semi-flex PTW Advanced Markus PMMA +graphite CH 2 Polyethylen e Aluminium Yes Yes The PTW cylindrical chambers were of the type cm 3 and two PTW cm 3, and the PTW plane-parallel chamber was of the type Advanced Markus. A track record of PTW cm 3 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 cm 3 )

29 20 Calibration Date 60 N CO D W, Stated uncertainty Oct E+07 Gy/C 2.2% Sept E+07 Gy/C 1.1% The PTW cm 3 model was selected as a reference chamber for this work because of its Geometric equivalence to the PTW cm 3, 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.

30 21 Fig 3.3a the PTW (0.6cm3) Farmer ionisation chamber used in this study Fig 3.3b the PTW34045 Advanced Markus ionisation chamber used in this study

31 22 Fig 3.3c the PTW 31010(0.125cm 3 ) Semiflex ionisation chamber used in this study Electrometer used in this study.electrometer name : PTW Unidos Electrometer model/ serial No: T 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. 60 All the N CO, N D, W x and N k calibration factors for the different ionization chambers were independently cross-calibrated in the 60Co beam against the calibrated PTW farmer reference ionisation chamber. N gas 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.

32 Cross-calibration of the N CO D W, for ionisation chambers in 60 Co beam. Control pendant Water phantom tank Ionisation chamber 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

33 24 chamber ref, is given by 60 ( N CO ( M ) ) D, W Y = ( M ) ref Y 60 ( N CO ) D, W 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 Cross-calibration of the 60 Co exposure calibration factor N x. The setup as in Fig, 3.6 was used to obtain D w using the IAEA technical report series 398. The 60 Co exposure calibration factor N x for the PTW Farmer chamber was calculated using D W IAEATRS 398 (N x ) AAPMG-21 = ( MfAeq BSF ) AAPMTG 21 (14) Where D w is as given in equation (1); f is 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 20 0 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 = M raw.k TP.k pol.k elec.k s (15) t +τ where M raw the uncorrected reading, τ is is the timer error, k TP is temperature pressure correction factor, k elec is the electrometer calibration correction factor and k s is the recombination correction factor.

34 Cross-calibration of the N gas (Cavity-gas calibration factor) for plane-parallel chambers in electron beams. The plane-parallel chambers were cross-calibrated against the PTW cm 3 reference ionisation chamber whose replacement correction (P repl ) was at 15 MeV, the highest electron energy available at the department. The AAPM TG-21 formalism was use i.e. ( (N gas ) p-p MN ) gas Pion Prepl = p p ( MP ) ion cylin..... (16) where M is the response of the chamber in question at d max, p-p and cylin refer to the plane parallel and cylindrical chambers respectively. Where P ion is a correction factor for ion recombination losses 3.5 The absorbed dose measurement in megavoltage photon Chamber position Field size Water phantom FIG.3.7. Experimental set-up for the determination of the beam quality index Q i.e Tissue Phantom Ratio (TPR 20,10 ).

35 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.. LINAC head Chamber connecting cable Field size of 10cm 10cm FIG.3.8. Experimental set-up for the determination of the absorbed dose D w using perspex phantom 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

36 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 60 water calibration factor N CO D W,. 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 nc (with medium range settings, 12.0 na) for 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 N Q. The measurement of D, W absorbed dose to water requires a beam quality specifier TPR 20,10. The beam quality specifier TPR 20,10 for the two photon energies (6 MV and 15 MV) was and , 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

37 28 were first cross-calibrated for N gas 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) d ref /(cm) L l perspex air (MeV.cm 2 /g) R 50 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

38 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 crosscalibration 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. (k Q ) Y = M Q ( DW ) ref Q 60CO Y ( N D, W ) Y (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). ( q, E N ) D, W Y K q.e = 60CO ( N D, W ) Y The absorbed dose to water calibration factors in the electron beam ( q E N, D, W ) Y (18) 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). q E ( D W ) Y N,, = q, E ( Dwater ( atd max )) ref, ( D ( atd )) E water max q Y (19) Where ( (13). Co N 60 D, w ) Y in equation (18) is obtained from the result of the cross-calibration in equation

39 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 ± 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. 60 Table 4.1 shows the results of the measured N CO 60 farmer reference chamber. Also shown are the N CO laboratory for each chamber. D W, from the cross-calibration against the PTW D W, values obtained from the PTW standards Table 4.1: The absorbed dose to water calibration factors for the ionization chambers used in this study. Chamber Model 60 N CO D, W Gy/C (PTW Certificate) 60 Measured N CO Gy/C (crosscalibration) D, W Deviation (%) from (PTW Certificate) (± 2.2%) (± 2.2%) PTW cm E+07 Reference i.e (5.328E+07 - ( Sept.2011) ) PTW cm 3 PTW Advanced Markus 2.997E+08 (Oct. 2005) 1.394E+09 ( Sept.2011) 2.984E+08 ± 3.4% E+09 ± 3.4% 3

40 31 The 60 Co exposure calibration factor, N X for the PTW Farmer reference ionisation chamber was 5.408E+09 R/C. The air-kerma calibration factor N k 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 N X and N gas calibration factors for the ionization chambers used. Chamber N x / R/C N gas /Gy/C PTW cm E E+07 PTW cm E E+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 and 1.002, respectively.the cross-calibration to determine N gas of the plane-parallel chambers from N gas 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 at 15 MeV. The result of the N gas cross-calibration process was 1.19E+09 Gy/C for PTW Advanced Markus ionisation chamber. 4.2 Measurement results in 6 MV and 15 MV photon beams The k Q derived as a function of TPR20,10 for the various ionisation chambers are shown in Table 4.3. The k Q results obtained for the PTW cm 3 Farmer ionisation chamber compare well with the IAEA TRS-398 data.

41 32 Table 4.3: The measured k Q as a function of TPR 20,10 of the various ionisation chambers Chamber First experiment Second experiment 0.676(6MV) (15MV) (6MV) (15MV) PTW PTW The measured k Q 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 cm3 Farmer ionisation chamber are tabulated below: Table 4.4: The measured k Q as a function of TPR 20,10 of the various ionisation chambers used in this study and the published IAEA TRS-398 k Q values for the PTW cm 3 chamber. Nominal Energy/MV TPR 20,10 PTW cm 3 PTW cm 3 PTW cm 3 (IAEA TRS 398 ) 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.

42 33 Table 4.5. Summary of the doses in Gy per 100 monitor units at d ref using each of the ionization chambers. Nominal R 50 / cm PTW PTW PTW Energy 0.6cm cm 3 Advanced (MeV) Markus 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 cm3, as used for the absorbed dose determination in the electron beams. R 50 PTW PTW 23333(KHAN) PTW /cm 0.6cm 3 0.6cm cm