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Dose Measurements for a Radiotherapy Co-60 Machine

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Dose Measurements for a Radiotherapy Co-60 Machine Sudan Academy of Sciences Dose Measurements for a Radiotherapy Co-60 Machine By ALI MOHAMMED ELBASHIR ALI A^jD (JAijJl Atlt (WU Sudan Academy of Sciences Atomic Energy Council Dose Measurements for a Radiotherapy Co-60 Machine By ALI MOHAMMED ELBASHIR ALI B.Sc (Honours) A thesis submitted as a partial fulfillment for the requirements of M.Sc. Degree in Medical Physics Supervisor: Dr. Ibrahim Idris Soliman 2008 Sudan Academy of Sciences Atomic Energy Council Examination Committee Name Title Dr. Farouk Idris Habbani External examiner Mr. Mohaned Mohammed ] W^cademic Affairs Elhassen Representative Dr. Ibrahim Idris Supervisor Soliman 2008 I r frr f *^ P h ^ r o'xo/0( *~5 P vbJk yy "*""? i^r^r? |[TT17^ re> tr|jf> rr-e—rrj f> jap-^ jrc—T^ ;' ^% ^r—f jr-p \Fff> nrifv i^rilv FT* * * * xir*^ ir^^ \\vv^y * r*1' ip\* • jrwrerry rwCfe^P ' v^o |rsfppi ir^Crr^i n1"^1!* fO^^rff? 'JipTC' V—I yW ABSTRACT Measurements were conducted for the determination of the absorbed dose to water for high energy photon beam of the Co-60 machine at the Institute of Nuclear Medicine, Molecular Biology and Oncology (INMO), Jazeera State with the new dosimetry equipments. The aims of this study were: 1- To make quality control for the Co-60 machine (CIRUS-Cis Biointernational-made at 1998) and to understand the relevant dosimetry principles. 2- To verify the acquired data with published data such as PTW- FREIBURG (German) and TRS398 -protocols. 3- To determine the absorbed dose to the water using above two protocols. A reference water phantom was used for the dosimetry measurements. The total deviation in the determination of the dose to water during calculation of the beam was found to be < 2.5 % and this magnitude was within the accepted limits of recommendation of the IAEA protocol ^(2.5%). The discrepancies in the determination of the absorbed dose to water between the two protocols were 1.4%. Comparative measurements showed a deviation of less than 2% between our measurements and protocol DIN-6800-2, and TG,51 protocol. The results obtained showed that our measurements were within the accepted limit. II DEDICATION c/fr'kMil ofmujht/ie* JMohammed <Q/6a6h'i*. and to- mn mo/Aep KJVOO-P and for- mu wife Jffadeet' and fa* ma on/a one 6tite* a/tdfor a// my 6*etAexS and fnend6. Ill ACKNOWLEDGMENTS This investigation was -performed-within the framework of the TtRs -398 and (FItW-(F<^EI(BV(S^ protocols .In this research. I have 6een helped by Mr.JAwed (Daraj and my great thanks go for my supervisor (Dr. I6rahim Idris Soluman for supervising the research. My great thanks go for the HeadofMedical(Physics (Department ofUMMO who helped me to oBtain the data and to acquire the necessary and sufficient skills in setting the dosimetry equipment for required measurements. IV List of symbols A Area; field size; atomic mass number; activity 0 Reference activity BQ becquerel (SI unit of activity) C CEMA, converted energy per unit mass c speed of light C coulomb (SI unit of charge) °C degree Celsius (unit of Celsius temperature) C* Curie (unit of activity) d distance; D Absorbed dose D Dose rate Dw Dose-to-water Average energy transferred into kinetic energy of charged particles per interaction. W-Q Absorbed dose to water at the reference depth, ref, in a water phantom irradiated by a beam of quality Q. The subscript Q is omitted when the reference beam quality is 60Co / Source-surface distance (SSD) Gy Gray (SI unit of dose) HVL Half-value layer J Joule (SI unit of energy) IS Q.Qo Factor to correct for the difference between the response of an ionization chamber in the reference beam quality Q0 used for calibrating the chamber and in the actual user beam quality, Q. The subscript Q0 is omitted when the reference quality is ^Co gamma radiation. IT P0' Polarization correction factor s Ion recombination correction factor T-p Temperature and pressure correction factor K Kerma ? rt K KERMA Rate K coi Collision kerma rad radiative kerma LET Linear energy transfer m Mass M U monitor unit (unit of quantity MU) ™ Number of particles (photons); ionization chamber calibration coefficient Number of electrons per gram Avogadro's number V D.w.Qo Calibration factor in terms of absorbed dose to water for a dosimeter at a reference beam quality Q0. P Pressure Pa Pascal (SI unit of pressure) p eff Effective point of measurement P 0 Standard air pressure (101.325 kPa) PDD Percentage depth-dose. Q charge, point-of-interest in phantom; beam quality r radius; R roentgen (unit of exposure), Radiant energy Rad old unit of exposure and equal to 10"2 J.Kg"1 S linear stopping power; scatter function; cell surviving fraction S, Collimator scatter factor s p Phantom scatter factor s c-p Total scatter factor "" Mass stopping power SAD Source-axis distance SCD Source-chamber distance SSD Source-surface distance '1/2 Half-life T Temperature T° Standard air temperature (273.2K or 0°C) TMR Tissue-maximum ratio TPR Tissue-Phantom ratio. X Exposure 2 Depth in phantom; atomic number Maximum depth wa/i Water equivalent thickness of the phantom's entrance window P Beta particle ^ Gamma ray P Density a Cross section (p fluence ^ Energy fluence Q Fluence rate or flux density P Linear attenuation coefficient a^ Atomic attenuation coefficient <"^ Electronic attenuation coefficient VI CONTENTS Chapter 1 Introduction: 1.1 Preface 1 1.2 The aim of the study 2 1.3 The important of the study 2 1.4 The Hypothesis 2 1.5 Material and Method 3 1.5.1 Relative dose measurements 3 1.5.2 Absolute dose measurements 4 1.6 Quantities and Units 5 1.6.1 Quanitities 5 1.6.2 Interaction Coefficients 7 1.6.3 Dosimetric Quantities 11 1.7KERMA , 15 1.8 Energy fluence and kerma (photons) 15 1.9 Collision kerma and exposure 16 1.10 Absorbed dose and exposure 16 1.11 Charged particle equilibrium 18 1.12 Interaction of ionization radiation with matter 21 1.13 Beam quality 24 Chapter 2 Relative and Absolute dosimetry 2.1 Introduction 26 2.2 Fundamental Absorbed Dos-methods measurement 26 2.2.1 Calorimetry 27 2.2.2 Principles of absorbed does calibration 27 2.2.3 Chemcal Dosimetry 28 2.2.4 Ionization Dosimetry 29 2.2.5 Ionization measurements in water phantion 29 2.2.6 Ionization measurements in graphite phantion 30 2.2.7 Simplified Theory of TLD 30 2.3 PDD 31 2.4 Air temperature, pressure and humidity effects: KT,P 33 :'.-> 2.5 Chamber polarity effects: polarity correction factor Kpoi 33 ?f- 2.6 Chamber voltage effects: recombination correction factor Ks ; 3.4 2.7 Absorbed dose-to-water -. ........35 Chapter 3 Material and Methods: 3.1. Introduction 38 3.2. Equipment 38 3.2.1. Dosimetry of system 39 3.2.2. Phantoms 39 VII 3.2.2.1. Reference water phantom 39 3.3. Absolute measurement 39 3.3.1. Absorbed does to water measurment 39 3.3.1.1. Stability check and response characteristics 39 3.3.1.2. Absorbed does to water calculation 40 3.3.1.3. Determination of the polarity correction factor 41 3.3.1.4. Determination of the ion recombination factor 41 3.3.1.5. Determination of the temperature and pressure correction factor 42 3.4. Relative measurement 43 3.4.1. Beam quality Specification 43 Chapter 4 Result and Discussion: 4.1. Introduction 44 4.2. Absolute measurements 44 4.2.1 Determination of absorbed does to water 44 4.3. Results and discussion 45 4.4. Response characteristic of dosimetry system 47 Chapter 4 4.5. Conclusion 52 References 53 VIII CHAPTER ONE INTRODUCTION 1.1. Preface After a therapy machine is accepted and before it can be placed in clinical service, the physicist must acquire an extensive set of radiation measurements that characterize its performance and to confirm the beam characteristics against the machine manufacturer's specification. All acquired data is entered into the Radiotherapy Treatment Planning System (RTPS) for the purpose of the dose calculation. The Institute of Nuclear Medicine, Molecular Biology and Oncology (INMO), Medani is the one of radiotherapy centers in Sudan that continues to expand and recently instollted a Co -60 machine. Many dosimetry kits and tools were purchased by the INMO to improve the quality assurance. A water phantom was purchased to facilitate beam data acquiring. The water phantom permits the ion chamber to scan through the radiation field in two dimensions. There is also therapy dosimeter device (dose 1) for measuring dose and dose rate in radiation therapy. With these tools the central axis (CA) data can be acquired and determination of the^ absorbed dose to water under reference conditions can be measured. The collected data% verified by qualified medical physicists'and entered to the RTPS. In terms of dosimetry the CA data is referred to as relative dosimetry, while determination of the absorbed dose to water is referred to as absolute dosimetry. Many national and international protocols are published to guide the medical physicists to conduct the dosimetry. Examples of such protocols are American Association of Medical 1 Physicists (AAPM), International Atomic Energy Agency (IAEA). All these organizations now recommend using water as a reference media for the measurements. Moreover all of these protocols shift from exposure and kerma factors to absorbed dose to water. All these issues will be discussed in depth in the body of this study. 1.2. The aim of the study: 1- To acquire necessary and sufficient skills in setting the new dosimetry equipment.
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