FABRICATION OF A REUSEABLE PLASTIC
BASED X-RAY DOSIMETER FOR
RADIOTHERAPY
By
IQBAL TARIQ
Thesis submitted in fulfilment of the requirements
for the degree of
Doctor of Philosophy
JANUARY 2016
FABRICATION OF A REUSEABLE PLASTIC BASED X-RAY DOSIMETER FOR RADIOTHERAPY IQBAL TARIQ PhD, Medical Physics. Department of Nuclear Medicine and radiation therapy, School of Medical Sciences, Universiti Sains Malaysia, Health campus,16150 Kelantan , Malaysia. Introduction
Deposition of radiation energy within the interacting material is termed as dose, whose
unit is Gray (Gy), defined as joule of deposited energy per unit mass(kg) (Khan,
2010).Dosimetry is the measurement or estimation of dose, ideally at a defined point
within the interacting material, but it could also be performed for a plane of 2D or for
a volume of 3D.The goal of radiation therapy is to deliver uniform and homogeneous
prescribed dose to treatment volume but at the same time, save the adjacent healthy
tissues from radiation exposure or keep the doses to un-avoidable healthy tissues, well
below their radiation tolerance (Khan, 2010). The treatment plans are made with the patients
radiographic images X-rays, CT scan and MRI. Due to current treatment complexity
raising the risk for target miss or unnecessary irradiation of healthy tissues, pre-treatment
dosimetric verification of each treatment plan is highly recommended.
Now a days a variety of radiation dosimeters are used serving different purposes. Examples
of the commonl used dosimeters are,Ionization chambers, Silver halide Radiographic films,
Radiochromic films, Thermoluminescence dosimeters, Optically stimulated dosimeters,
Semi conductor detectors. Frick dosimeter, Gel dosimetry, Plastic dosimeters and some
Natural and synthetic materials.
Objectives The aim of the study is to fabricate a reusable tissue equivalent plastic composite dosimeter
with acceptable accuracies, that could serve the routine dosimetric requirements of a
radiation therapy centre.First, this work seeked to identify the most suitable material to be
used as the ingredient in the plastic composite mix. Secondly, this work seeked to determine
whether the sensitivity of any of these materials can be improved by synthetic introduction
of crystal defects. Thirdly, methods of sample preparations were to be evaluated. Finally,
these samples would be evaluated for their radiation dosimetric properties using X-ray
photon beam from a linear accelerator. They would be evaluated mainly for radiation
sensitivity, repeatability, fading, and dose linearity.
Methods and Materials To prepare the samples in which radiation sensitive material grains were uniformly
embedded within the body of the plastic, different techniques were tried and finally N1(pure
sodium chloride) samples were made with Haake polydrive internal mixing and hot press
technique. Samples prepared by, spraying plastic solution with grains of material, with
solution casting of plastic and material grains and b using hot plate with solution casting
and to melt plastic grains was found unsuitable for further experiments.
Results The final sample of N1 and PMMA composite in 40/60 ratio (wt/wt) has shown
good sensitivity for 3 Gy of 6 MV photons with very linear dose response up to 10 Gy.
From 10 Gy to 15 Gy the sensitivity of the plastic dosimeter was found to be
decreased. For the next observed dose of 20 Gy, the dosimeter appeared to
regain its sensitivity. The doses were delivered for alternate 4 days post optical bleaching of
18 hours. The area of dip in the percentage transmission curve, corresponding the absorbed
dose was found to have an average value of 96.4 arbitrary area units with standard
deviation of 4.85 %. The peak values of the dips were found to be between 460 nm to 472
nm. The fading in response for 5 Gy of 6 MV photons was found to be 16.93% in one week
and 48.07% in one month. Dosimetric evaluation of the materials were conducted with the
study of their optical properties pre and post irradiation of 2 Gy of 6 MV photons, using
Shimadzu UV 1800 Spectrophotometer.
Conclusions Zeff of the fabricated N1 PMMA composite dosimeter is 10.51.Linear response from 3Gy to
10Gy,and then from 15Gy till the observed dose of 20Gy was observed. The fading in
response for 5 Gy of 6 MV photons was found to be 16.93% in one week and 48.07% in one
month. Dose repeatibility for 5Gy of 6 MV photons was found to have standard deviation of
4.85 %.
Dr. Ahmad Lutfi Yuosoff Supervisor
Professor Dr. Ahmad Zakaria Co Suprvisor
Professor IR.Dr Mariatti Binti Jaafar Co Supervisor
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Dedication
I would like to dedicate my thesis to my beloved parents; Noor Jehan Begum and
Muhammad Ashab uddin (may Allah rest their souls in peace) whom deep love for
me and encouragements was always my source of inspiration and strength, to my
loving and extremely caring wife; Nausheen Fatima, who always stand with me in
facing any kind of trouble and with her utmost efforts either physical or emotional,
made situations favourable for me.
I also want to dedicate my work to my elder brother; Ajaz Tariq to me who is the
sincerest friend and reflection of my father, my loving sisters; Seema Tabassum and
Shabana Anjum; their families, and my sincere in laws, who always remained
concerned and pray for me and my family.
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Acknowledgements
All praises is for Allah subhan wa tala and slawat wassalam to His prophet
Mohammad (salalah wa sallam). Beyond any doubt, what I have achieved is only
due to the blessings of Allah sbhan wa talah whose unmatched kindness made my
dreams true.I would like to express my utmost gratitude to my main
supervisor, Dr. Ahmad Lutfi Yusoff , whose sincere concern, Knowledgeable
advises and kind guidance, from the very first step of my research till the end of my
thesis writing, made my work complete and presentable . I am also thank full to my
co supervisor Professor Ahmad Zakaria and Professor Mariatti Jaafer for their
advises, help and kind concern .
My deepest thanks go to Mr. Nik Ruzman, Mr. Redwan and all the radiation
therapy staff of the Department of Nuclear medicine and Radiation therapy, U.S.M,
for their kind concern and help in irradiation of samples and related works. I also
want to thankful to the staff of CRL (central research lab) of U.S.M, my friend Zaid
and other friends of USM Engineering Campus, the staff of USM Engineering
campus (staff of plastics and rubber section), for their guidance, kind help and
cooperation, regarding the use of equipments and related matters. I acknowledge my
mother Noor Jehan Begum (late) and father Muhammad Ashabuddin (late) (may
Allah rest their souls in peace), deep love and their encouragements to achieve my
goals and their advises and guidance which give me insight in the philosophy of life.
Their sweet memories are my most worthy treasure.
I must mentioned the support and help of my beloved wife Nausheen Fatima who
always stands with me against all odds and making things and situations favourable
for me, with her utmost sincerity, devotion and love. The cooperation of my kids,
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(Umar, Ali, Amna and Ahmed) is something which I would like to acknowledge
with love. I am proud to be father of such enduring, and loving kids. I am deeply
thank full to my brother Ajaz Tariq, my sisters SeemaTabassum, Shabana Anjum
and their families, my brother in law Zia Siddiqui and my in laws, for encouraging
me and my family, and to pray for all of us.
.
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List of Contents DEDICATION ....................................................................................................................................... II
ACKNOWLEDGEMENTS ................................................................................................................. III
............................................................................................................................................................ ..IV
LIST OF CONTENTS ........................................................................................................................... V
LIST OF FIGURES ............................................................................................................................ VIII
LIST OF TABLES ............................................................................................................................. XV
LIST OF ABBREVIATIONS ........................................................................................................... XVI
ABSTRAK ....................................................................................................................................... XVII
ABSTRACT ........................................................................................................................................ XX
CHAPTER 1 .......................................................................................................................................... 1
INTRODUCTION.................................................................................................................................. 1
1.1 BACKGROUND ....................................................................................................................... 1 1.2 STATEMENT OF THE PROBLEM ............................................................................................... 2 1.3 PURPOSE OF THE STUDY ........................................................................................................ 3 1.5 SCOPE OF THE STUDY............................................................................................................. 5
CHAPTER 2 .......................................................................................................................................... 6
LITERATURE REVIEW ...................................................................................................................... 6
2.1 RADIATION DOSE AND DOSIMETRY ........................................................................................ 6 2.2 MEDICAL DOSIMETRY ............................................................................................................ 7 2.3 IMPORTANCE OF DOSIMETRY FOR RADIATION THERAPY ........................................................ 7
2.3.1 Treatment planning, treatment planning systems and dosimetry ..................................... 8 2.3.2 Requirement of validating treatment plan ........................................................................ 9
2.4 OVER VIEW OF THE DOSIMETERS ......................................................................................... 10 2.4.1 Ionization chambers ....................................................................................................... 10 2.4.2 Film dosimetry ................................................................................................................ 11
2.4.2.1. Silver halide Radiographic films .......................................................................................... 11 2.4.2.2. Radiochromic films.............................................................................................................. 12
2.4.3. Luminescence dosimetry ............................................................................................ 13 2.4.3.1. Thermoluminescence dosimetry (TLD) .............................................................................. 13 2.4.3.2. Optically stimulated dosimeters .......................................................................................... 15
2.4.3.2.a. CW- OSL ........................................................................................................................ 16 2.4.3.2.b. LM-OSL ........................................................................................................................ 16 2.4.3.2.c. Pulsed OSL .................................................................................................................... 17 2.4.3.3. OSD VERSUS TLD ...................................................................................................... 17
2.4.4. Semi conductor detectors ........................................................................................... 18 2.4.4.a. Diodes ................................................................................................................................. 18 2.4.4.b. PN diodes ............................................................................................................................. 18 2.4.4.c. MOSFET ............................................................................................................................ 20 2.4.4.d. Diamond detector ................................................................................................................ 20
2.4.5. Chemical dosimetry .................................................................................................. 21 2.4.5.a. Frick dosimeter ................................................................................................................... 21
2.4.6. Gel dosimetry ............................................................................................................. 22 2.4.6.a. Fricke gel dosimetry ............................................................................................................ 22 2.4.6.b. Polymer gel dosimetry ....................................................................................................... 23
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2.4.7 Plastic dosimeters.......................................................................................................... 25 2.4.8. Natural and synthetic materials ................................................................................ 27
CHAPTER 3 ........................................................................................................................................ 29
MATERIALS AND METHODS ......................................................................................................... 29
3.1. DOSIMETRIC MATERIALS .................................................................................................... 30 3.1.1. Beryllium Oxide (BeO).............................................................................................. 30 3.1.2. Sodium Chloride (NaCl) ............................................................................................ 31 3.1.3. Potash Alum (K2SO4.24H2O) .................................................................................... 32 3.1.4. Bamboo salt .............................................................................................................. 33 3.1.5. Aluminium Oxide (Al2O3) .......................................................................................... 34
3.2. HOST MATERIALS ............................................................................................................... 34 3.2.1. UVette vial's plastic .................................................................................................. 35 3.2.2. Poly methyl metaacrylate (PMMA) ........................................................................... 35 3.2.3. Shredded branded plastic .......................................................................................... 36 3.2.4. Schematic representation of sample preparation ...................................................... 37
3.3. METHODS ........................................................................................................................... 38 3.4. SAMPLES ............................................................................................................................. 40
3.4.1. Physical form of the sample material ........................................................................ 40 3.4.2. Methods used for the fabrication of plastic sample .................................................. 41
3.4.2.1. Spray technique ................................................................................................................. 41 3.4.2.1.a. Quantification of spray gun parameters ......................................................................... 43 3.4.2.1.b. Characterisation of the spray gun ................................................................................. 45 3.4.2.1.c. Preparation of plastic solution of desired viscosity ........................................................ 45
3.4.2.2. Solution cast technique ........................................................................................................ 46 3.4.2.2.a. Preparation of branded shredded plastic solution ........................................................... 47 3.4.2.2.b. Cast for branded plastic solution and formation of thin films ......................................... 47
3.4.2.3. HOT PLATE TECHNIQUE ............................................................................................. 48 3.4.2.3.a. Branded plastic solution on hot plate .............................................................................. 48 3.4.2.3.b. Melting PMMA grains on hot plate ............................................................................... 49 3.4.2.3.c. Moulded plastic film/thin cut out on hot plate ............................................................... 49
3.4.2.4. Plastic container ................................................................................................................... 51 3.4.2.4.a. Bored branded plastic sheet .......................................................................................... 52 3.4.2.4.b. Uvette vial used for spectrophotometry ......................................................................... 52 3.4.2.4.c. Transparent plastic vials................................................................................................ 53
3.4.2.5 Preparation of plastic cut outs with melt casting ..................................................................... 53 3.4.2.5.1. Preparation of PMMA grains and materials................................................................... 54 3.4.2.5.2. Preparation of samples with melt blending with Haake poly drive ................................ 55 internal mixer and hot press machines ............................................................................................... 55
3.5 EVALUATIONS FOR THE DOSIMETRIC PROPERTIES OF THE SAMPLES .................................... 61 3.5.1. Plastic Containers ..................................................................................................... 61
3.5.1.a. Materials in Raw form ........................................................................................................ 61 3.5.1.b. Anhydrous materials ............................................................................................................ 62 3.5.1.c. Branded plastic sheets bored container ............................................................................... 64 3.5.1.d. Uvette vial ........................................................................................................................... 64 3.5.1.e. Plastic sheet embedding material grains ............................................................................. 65
CHAPTER 4 ........................................................................................................................................ 75
RESULTS AND DISCUSSION .......................................................................................................... 75
4.1. STUDY OF THE OPTICAL PROPERTIES OF PLASTICS USED AS CONTAINERS AND HOST ............ 75 4.2. PLASTIC CONTAINERS .......................................................................................................... 75
4.2.a. Bored branded plastic branded plastic sheet containers .......................................... 75
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4.2.b. Branded plastic vials and host material ..................................................................... 75 4.3. THIN PLASTIC FILM FABRICATION ........................................................................................ 79
4.3.a. Spray technique .......................................................................................................... 79 4.3.b. Solution cast technique .............................................................................................. 86
4.3.b.1. Branded plastic solution casts .............................................................................................. 86 4.3.b.2. Sandwiched layered film .................................................................................................... 89 4.3.b.3. Techniques with hot plate ................................................................................................... 90 4.3.b.4. Branded plastic solution on hot plate .................................................................................. 90
4.3.c. Melting PMMA grains on hot plate ............................................................................... 91 4.3.c.1. Moulded plastic film/thin cut out on hot plate .................................................................... 92
4.3.d. Preparation of plastic cut outs with melt casting ...................................................... 93 4.4. STUDY OF THE FACTORS OF A SAMPLE AFFECTING ITS OPTICAL PROPERTIES PRE AND ........ 96
post irradiation.................................................................................................................. 96
4.4.a. Optical transparency of irradiated plastics of vial and PMMA ............................... 96 4.4.B. EFFECT OF GRAIN SIZE ..................................................................................................... 98 4.5. BASIC DOSIMETRIC SENSITIVITY EVALUATION OF MATERIALS FOR 6 MV PHOTONS .......... 102
4.5.a. Materials in plastic vial with oil .............................................................................. 102 4.5.a.1. Aluminium oxide (Al2O3 ) ................................................................................................ 102 4.5.a.2. Beryllium oxide (BeO) ..................................................................................................... 103 4.5.a.3. Potash alum ....................................................................................................................... 104 4.5.a.4. Ammonium chloride (Noshadar) ....................................................................................... 105 4.5.a.5 Bamboo salt ......................................................................................................................... 105 4.5.a.6. N1 ..................................................................................................................................... 107 4.5.a.7. N2 ..................................................................................................................................... 108 4.5.a.8 N3 ..................................................................................................................................... 108 4.5.a.9. N4 ..................................................................................................................................... 109 4.5.a.10. N5 ..................................................................................................................................... 109 4.5.a.11. N6 ...................................................................................................................................... 110 4.5.a.12. N7 ...................................................................................................................................... 111 4.5.a.13. N8 ..................................................................................................................................... 111
4.5.b. Material embedded in PMMA ................................................................................. 112 4.5.b.1. Aluminium oxide (Al2O3) ................................................................................................ 112 4.5.b.2. Beryllium oxide (BeO) ..................................................................................................... 113 4.5.b.3 N1 .......................................................................................................................................... 113 4.5.b.4. N8 .......................................................................................................................................... 114
4.6 STUDY OF ENHANCEMENT IN 6 MV PHOTONS SENSITIVITY POST TREATMENT WITH .......... 115 9 MEV ELECTRON OF DIFFERENT MATERIALS ................................................................................ 115
4.6.a. UVette vial .............................................................................................................. 115 4.6.b. Transparent vial with optically clear oil ................................................................. 115
4.6.b.1. Aluminium oxide (Al2O3) ................................................................................................ 116 4.6.b.2. Beryllium oxide (BeO) ...................................................................................................... 117
4.6.b.3. Ammonium chloride (Noshadar) ............................................................................. 119 4.6.b.4. BAMBOO SALT .............................................................................................................. 120 4.6.b.5. Potash Alum ...................................................................................................................... 121 4.6.b.6. N8 ..................................................................................................................................... 122
4.7. N1 (STUDY OF ENHANCEMENT IN X6 SENSITIVITY DUE TO DIFFERENT E9 DOSES) ................. 123 4.7.a. Study of enhancement in X6 sensitivity of N1 due to 1600 Gy of 9 MeV ....................... 124 electrons .................................................................................................................................... 124 4.7.b. Study of enhancement in X6 sensitivity of N1 due to different doses of .................. 125 9 MeV electrons ......................................................................................................................... 125 4.7.c. Study of enhancement in X6 sensitivity due to 400 Gy /Day E9 doses ....................... 127
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( plastic vial & oil) .................................................................................................................... 127 4.7.d. Estimating suitable high dose E9 for optimum 6 MV photon .................................. 131 sensitivity ................................................................................................................................... 131 4.7.e. Single dose of 800 Gy of E9 ....................................................................................... 131 4.7.g. Dosimetric evaluation of N1 sample by fractioned dose of 400 Gy/day to ............. 135 cumulative dose of 800 Gy of 9 MeV electrons ........................................................................ 135 4.7.h. Comparison of graphs of N1 sample (final) for different doses of 6 MV ................. 141 photons post bleaching from 800 Gy 9 MeV electrons fractionated ....................................... 141 treatment .................................................................................................................................... 141
4.8 N1 FRESH PLASTIC CUT OUTS (UN IRRADIATED TO E9) .............................................. 145
4.9 RESPONSE LINEARITY VERSUS DOSE ............................................................................... 150
4.10 DOSE REPEATABILITY ........................................................................................................ 154 4.11 FADING .............................................................................................................................. 155 4.12 COMPARISON OF FABRICATED PMMA N1COMPOSITE DOSIMETER WITH........................... 155 PRESAGE...................................................................................................................................... 155
CHAPTER 5 ...................................................................................................................................... 157
CONCLUSIONS AND RECOMMENDATIONS ............................................................................ 157
5.1 CONCLUSIONS ................................................................................................................... 157 5.2 RECOMMENDATIONS FOR FUTURE RESEARCH .................................................................... 160
REFFERENCES ................................................................................................................................ 161
APPENDICES
ix
List of Figures
Figure 2.1. Model for thermo luminescence ...................................................................................... 14 Figure 3.1: Raw Potash Alum ............................................................................................................ 33 Figure 3.2: Shredded UVette vial plastic ........................................................................................ 35 Figure 3.3: PMMA grains of 2 to 3 mm in dimension ...................................................................... 36 Figure 3.4: Shredded branded plastic .............................................................................................. 36 Figure: 3.5 Materials for the proposed plastic composite radiation dosimeter ................................. 37 Figure 3.6: Shimadzu UV 1800 Spectrophotometer .......................................................................... 38 Figure 3.7: Cary eclipse florescence spectrophotometer for florescence study ................................... 39 Figure 3.8: Optical bleaching of samples with two bulbs of 100 watts................................................ 39 Figure 3.9: Spray gun ........................................................................................................................ 42 Figure 3.10: External mix .............................................................................................................. .....42 Figure 3.11: Internal mix.....................................................................................................................42 Figure 3.12 Mini Spray gun K-3A .................................................................................................... 43 Figure 3.13 Casts made by ordinary glass slides for plastic solution ................................................. 47 Figure 3.14 : Local brand plastic solution heated on hot plate .......................................................... 48 Figure: 3.15: PMMA grains melted on hot plate at 190 oC ................................................................ 49 Figure 3.16: Mould fabrication with glass slides for plastic solutions ............................................... 50 Figure 3.17: Mini 3D oven fabricated with glass slides and heated over hot plate .................... 50 Figure 3.18:Uvette special plastic vials for spectroscopic .................................................................. 51 studies for wavelength ranges 220 nm to 1600 nm .............................................................................. 51 Figure 3.19: Plastic sheet board containers ...................................................................................... 52 Figure 3.20: Transparent plastic vial ................................................................................................ 53 Figure 3.21: Planetary Mono Mill grinder used for grinding samples .............................................. 55 Figure 3.22: Haake poly drive internal mixer .................................................................................... 56 Figure 3.23: Hot press machine for moulding sample sheets by using hot and cold press .............. 57 Figure 3.24: Mechanical sieve for having different grain sizes of material ...................................... 59 Figure 3.25: Summarized steps of sample preparation ....................................................................... 60 Figure 3.26: Materials in transparent plastic vial kept in light sleeves ...................................... 62 Figure 3.27 : Plastic samples of N1 in light tight sleeves \ ............................................................. 65 Figure 3.28: Labelled plastic samples .............................................................................................. 65 Figure 3.29: Labelled plastic samples of N1 ...................................................................................... 66 Figure 4.1: Optical transparencies of, transparent empty vial and with oil, material free ................. 76 PMMA cut out empty UVette vial, nothing in spectrophotometer ........................................................ 76 Figure 4.2 : Ratio of Optical Transparencies of empty Uvette vial to Empty ..................................... 77 plastic vial (Un-normalised curves) ..................................................................................................... 77 Figure 4.3: Ratio of Optical transparencies of Plastic vial with clear oil to ..................................... 78 same empty plastic vial (unnormalised curves) .................................................................................. 78 Figure 4.4: Ratio of Optical transparencies of material free PMMA ................................................. 79 clear plastic cut out (2 mm) to empty plastic vial (un normalised curves) .......................................... 79 Figure 4.5: Dried sprayed plastic solution with solute to solvent ....................................................... 82 proportion 1.967 % .............................................................................................................................. 82 Figure 4.6: Dried sprayed plastic solution with solute to solvent proportion 3.622 % ....................... 83 Figure 4.7: Dried sprayed plastic solution with solute to solvent proportion 1.967 %.......................83 Figure 4.8: Dried sprayed plastic solution with solute to solvent proportion 3.622 % .................... .83 Figure 4.9: Dried sprayed plastic solution with h solute to solvent proportion 1.967 %......................83 Figure 4.10: Dried sprayed plastic solution with solute to solvent proportion 3.622 %.................... .83 Figure 4.11: Dried sprayed plastic solution with solute .................................................................... 84 to solvent proportion 4.89 % .............................................................................................................. 84 Figure 4.12: Dried sprayed plastic solution ....................................................................................... 85 with solute to solvent proportion 2.849 %........................................................................................... 85 Figure 4.13: Dried sprayed plastic solution ........................................................................................ 85 with solute to solvent proportion 2.849 %........................................................................................... 85 Figure 4.14: Branded plastic solution containing N1 grains, dried over glass .................................. 86 slide at room temperature ................................................................................................................... 86
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Figure 4:15: Branded plastic solution containing N1 grains ............................................................. 87 dried in petty dish at room temperature ............................................................................................... 87 Figure 4.16: Branded plastic solution containing N1 grains dried in hot air drier ............................. 88 chamber at 50 0C for 24 hours ............................................................................................................. 88 Figure 4.17: Branded plastic solution containing N1 grains dried in hot air ..................................... 88 drier chamber at 700C for 24 hour ...................................................................................................... 88 Figure 4.18: Layered plastic film made by solution cast method ......................................................... 89 Figure 4.19 : Branded plastic solution dried over hot plate ............................................................... 90 Figure 4.20 : Melted PMMA grains over glass slide using hot plate ................................................... 91 Figure 4.21: Solid plastic cut out embedding N1 grains moulded on hot plate ................................. 92 Figure 4.22: PMMA N1(composite)sample..... .................................................................................. 93 Figure 4.23 :Material free PMMA sample.............................................................................................93 Figure 4.24 : PMMA N8 (composite) sample ...................................................................................... 93 Figure 4.25 :PMMA Al2O3 (composite) sample .................................................................................. 94 Figure 4.26 : PMMA BeO (composite) sample......................................................................................94 Figure 4.27 : The final plastic sample of N1125 µm with 40 to 60 ratio with PMMA (wt/wt),collected from melt mixing machine......................................................................................................................95 Figure 4.28 :The final plastic sample of N1125 µm with 40 to 60 ratio with PMMA (wt/wt),as plastic sheet (2 mm, obtained after hot press of sample...................................................................................95 Figure 4.29 SEM pictures of the final PMMA N1 (grounded,125 μm),40/60 (wt/wt) sample ............ 96 Figure 4.30: Ratio of normalised curves of optical transparencies of vial with oil, irradiated with .. 97 400 G y/day dose of 9 MeV electrons, with respect to un irradiated. .............................................. 97 Figure 4.31 Ratio of normalised curves of optical transparencies of material free PMMA ............... 97 (2 mm) cut out irradiated with different 9 MeV electrons with respect to un irradiated..................... 97 Figure 4.32: Ratio of optical transparencies of un irradiated N1raw to ............................................. 99 N1(125 μm) grounded .......................................................................................................................... 99 Figure 4.33: Ratio of optical transparencies of un irradiated raw N8 to N8 .................................... 100 Grounded (63 µm) .............................................................................................................................. 100 Figure 4.34: Optical transparency curves (normalised) of N8 and N1 (raw, unexposed),samples .. 101 in plastic vial immersed in clear oil .................................................................................................. 101 Figure 4.35: Ratio of the relative optical transparencies of un irradiated, N1(raw .......................... 101 to grounded N1, to N8 (raw to grounded N8 ) ................................................................................. 101 Figure 4.36: Ratio of normalised curves of optical transparencies Al2O3 in vial with oil, ............... 103 irradiated with 2 Gy dose of 6 MV photons, with respect to un irradiated sample ........................... 103 Figure 4.37: Ratio of normalised curves of optical transparencies BeO in vial with ........................ 104 oil, irradiated with 2 Gy dose of 6 MVphotons, with respect to un irradiated sample ...................... 104 Figure 4.38: Ratio of normalised curves of optical transparencies Potash alum in .......................... 104 vial with oil, irradiated with 2 Gy dose of 6 MV photons, with respect to un .................................. 104 irradiated sample ............................................................................................................................... 104 Figure 4.39 : Ratio of normalised curves of optical transparencies of ammonium ........................... 105 Chloride in vial with oil irradiated with 2 Gy dose of X6, with respect to ....................................... 105 un irradiated sample ......................................................................................................................... 105 Figure 4.40: Ratio of normalised curves of optical transparencies Bamboo salt in .......................... 106 vial with oil, irradiated with 2 Gy dose of 6 MV photons, with respect to un ................................... 106 irradiated sample .............................................................................................................................. 106 Figure 4.41: Ratio of normalised curves of optical transparencies N1 salt ....................................... 107 in vial with oil ,irradiated with 2 Gy dose of 6 MV photons, with respect ........................................ 107 to un irradiated sample ...................................................................................................................... 107 Figure 4.42: Ratio of normalised curves of optical transparencies N2 salt in................................... 108 vial with oil, irradiated with 2 Gy dose of 6 MV photons, with respect to un .................................. 108 irradiated sample ............................................................................................................................... 108 Figure 4.43: Ratio of normalised curves of optical transparencies N3 salt in vial with .................... 108 oil, irradiated with 2 Gy dose of 6 MV photons, with respect to un irradiated sample ..................... 108 Figure 4.44: Ratio of normalised curves of optical transparencies N4 salt in vial with oil, .............. 109 irradiated with 2 Gy dose of 6 MV photons, with respect to un irradiated sample ........................... 109 Figure 4.45: Ratio of normalised curves of optical transparencies N5 salt in vial with .................... 109 oil, irradiated with 2 Gy dose of 6 MV photons, with respect to un irradiated sample ..................... 109
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Figure 4.46: Ratio of normalised curves of optical transparencies N6 salt in vial with oil, .............. 110 irradiated with 2 Gy dose of 6 MV photons, with respect to un irradiated sample ........................... 110 Figure 4.47: Ratio of normalised curves of optical transparencies N7 salt in vial with oil, .............. 111 irradiated with 2 Gy dose of 6 MV photons, with respect to un irradiated sample ........................... 111 Figure 4.48 : Ratio of normalised curves of optical transparencies N8 salt in vial with oil, ............. 111 irradiated with 2 Gy dose of 6 MV photons, with respect to un irradiated sample ........................... 111 Figure 4.49: Ratio of optical transparencies of plastic cut outs of PMMA (0.5mm thick)................ 112 embedding Al2O3 grains irradiated to 2 Gy of 6 MV photons to its un irradiated state .................. 112 Figure 4.50: Ratio of optical transparencies of plastic cut outs of PMMA (0.5mm thick)................ 113 embedding BeO grains irradiated to 2 Gy of 6 MV photons to its un irradiated state .................... 113 Figure 4.51: Ratio of optical transparencies of plastic cut outs of PMMA (0.5mm thick)................ 114 embedding N1 grains irradiated to 2 Gy of 6 MV photons to its un irradiated state ....................... 114 Figure 4.52 :Ratio of optical transparencies of plastic cut outs of PMMA (2 mm) .......................... 114 embedding N8 grains irradiated to 5 Gy of 6 MV photons to its un irradiated state ........................ 114 Figure 4.53 :Ratio of normalised optical transparencies curves of Al2O3 (in vial with oil), ........... 116 irradiated with 2 Gy dose of 6 MV photons with respect to un irradiated sample ............................ 116 Figure 4.54: Ratio of normalised optical transparencies curves of Al2O3 (in vial with .................... 116 oil), irradiated with 2 Gy dose of 6 MV photons (post irradiation of 2 k Gy of E9 ), ...................... 116 with respect to un irradiated sample .................................................................................................. 116 Figure 4.55 : Ratio of normalised optical transparency curves of Al2O3 (in vial with oil), irradiated with 2 Gy of 6 MV photons (post irradiation of 2 k Gy of 9 MeV 2 Gy dose of 6 MV photons unexposed to electrons ....................................................................................................................... 117 Figure 4.56 : Ratio of normalised curves of optical transparencies of BeO (in vial with oil), ......... 118 irradiated with 2 Gy dose of 6 MV photons, with respect to un irradiated sample ............................ 118 Figure 4.57: Ratio of normalised optical transparencies curves of BeO (in vial.............................. 118 oil), irradiated with 2 Gy dose of 6 MV photons post irradiation of 2 k Gy of 9 MeV ...................... 118 electrons, with respect to un irradiated sample ................................................................................. 118 Figure 4.58: Ratio of normalised optical transparencies curves of BeO (in vial with oil), .............. 119 irradiated with 2 Gy dose of 6 MV photons post irradiation of 2 k Gy of 9 MeV electrons ............. 119 with respect to 2 Gy dose of 6 MV photons unexposed to electrons ................................................. 119 Figure 4.59 :Ratio of normalised optical transparencies curves of ammonium chloride ................. 119 ( Noshadar) in(vial with oil), irradiated with 5 Gy of 6 MV photons, to unexposed sample ............. 119 Figure 4.60: Ratio of normalised optical transparencies curves of Bamboo salt (vial with .............. 120 oil), irradiated with 2 Gy of 6 MV photons, with respect to unexposed sample ............................... 120 Figure 4.61: Ratio of optical transparencies of potash alum, 2 Gy of X6 exposed to post .............. 121 bleaching of 80 Gy of E9 ................................................................................................................... 121 Figure 4.62: Ratio of optical transparencies of potash alum 2 Gy of X6 exposed to post ................ 122 bleaching of 200 Gy of E9 ............................................................................................................... 122 Figure 4.63: Ratio of normalised optical transparencies curves of N8 (vial with oil, average of ..... 123 three samples),irradiated with 2 Gy of 6 MV photons, post irradiation of first E9 treatment of ..... 123 80 Gy of 9 MeV electrons and optically bleached, with respect to unexposed sample ................... 123 Figure 4.64: Ratio of normalised curves of N1( vial in oil ) optical transparencies, post 2 Gy dose of 6 MV photons and unexposed sample ............................................................................................ 124 Figure 4.65: Ratio of normalised curves of N1( vial in oil )optical transparencies, after 2 Gy dose 124 Figure 4.66 : Ratio of normalised curves of N1( vial in oil ) photons, post optical bleaching of .... 125 1600 Gy of 9 MeV electrons with respect to 2 Gy of 6 MV photons delivered to fresh sample ...... 125 Figure 4.67:Average of the Ratio of optical transparencies of N1( vial in oil ) after 2 Gy of X6 .... 125 photons (post 200 Gy E9 treatment )with respect to 2 Gy of X6 un irradiated sample ................... 125 Figure 4.68: Average of the Ratio of optical transparencies of N1( vial in oil ) after 2 Gy ............. 126 of 6 MV photons ( post 200 Gy of E9 treatment) respect to 2 G yX6 to fresh sample ...................... 126 Figure 4.69: Average of the Ratio of optical transparencies of N1(oil in vial ) after 2 Gy ............. 126 of 6 MV photons ( post 200 Gy of E9 treatment) respect to 2 Gy of X6 to fresh sample ................ 126 Figure 4.70: Average of the Ratio of optical transparencies of N1oil in vial after 2 Gy ................. 127 6 MV photons post 100 Gy of E9 treatment with respect to 2 Gy of X6 to fresh ............................. 127 sample ................................................................................................................................................ 127 Figure 4.71: Ratio of optical transparencies post 5 GyX6 to un exposed sample ............................ 127
xii
Figure 4.72: Ratio of optical transparencies of 5 Gy X6 ( post bleaching of 400 Gy of E9) to 5 Gy 6 MV photons exposed, fresh sample..........................................................................................128 Figure 4.73:Ratio of optical transparencies of samples ( post treatment of 800 Gy of E9 ) to ......... 128 fresh samples, for 5 Gy dose of 6 MV photons .................................................................................. 128 Figure 4.74: Ratio of optical transparencies of, 5 Gy X6 ( post bleaching cumulative 1600 Gy ..... 129 E9) to 5 Gy 6 MV photons exposed, fresh sample ........................................................................... 129 Figure 4.75: Ratio of optical transparencies of, samples ( post treatment with 2000 Gy of ............ 129 E9 to fresh samples, for 5 Gy dose of 6 MV photons. ..................................................................... 129 Figure 4.76:Ratio of optical transparencies of, samples ( post treatment with 2400 Gy of .............. 130 E9) to fresh samples, for 5 Gy dose of X6 photons. ....................................................................... 130 Figure 4.77: Ratio of optical transparencies of samples ( post treatment with 2800 Gy .................. 130 of E9) to fresh samples for 5 Gy dose of X6 photons .................................................................... 130 Figure 4.78: Optical transparency of N1 (final plastic sample 2 mm exposed with 0.5 Gy of X6 post treatment with E9 single dose of 800 Gy ......................................................................................... 132 Figure 4.79: Optical transparency of N1 final plastic sample 2 mm exposed with 1 Gy X6 post .... 132 treatment with E9 single dose of 800 Gy .......................................................................................... 132 Figure 4.80: Optical transparency of N1 final plastic sample (2 mm) exposed with 1.5 Gy of X6 . 133 post treatment with E9 single dose of 800 Gy .................................................................................. 133 Figure 4.81: Comparison of N1 (2 mm) samples ratios of optical transparencies for different X6 doses (after optical bleaching from 800 Gy single dose of E9) to its freshly bleached state ..................... 133 Figure 4.82: Optical transparency of N1 (final plastic sample 2 mm) exposed with 0.5 Gy X6 post treatment with E9 single dose of 1200 Gy ........................................................................................ 134 Figure 4.83 : Optical transparency of N1 (final plastic sample(2 mm)) exposed with 1.5 Gy X6 post treatment with E9 single dose of 1200 Gy ........................................................................................ 134 Figure 4.84 : SEM images of 800 Gy (single)dose exposed N1 final) plastic sample ..................... 135 Figure 4.85: Ratio of the average optical transparencies, of N1(40/60 2 mm) for 0.5 Gy of ........... 136 6 MV photons post treatment with 800 Gy of E9, to its bleached state ............................................ 136 Figure 4.86: Ratio of the average optical transparencies, of N1 (40/60 2 mm) for 1 Gy .................. 136 of 6 MV photons post treatment with 800 Gy of E9, to its bleached state .................................... 136 Figure 4.87: Ratio of the average optical transparencies, of N1(40/60 2 mm) for 1.5 Gy of ........... 137 X6 photons post treatment with 800 Gyof E9, to its bleached state .............................................. 137 Figure 4.88: Ratio of the average optical transparencies, of N1(40/60 2 mm) for 2 Gy of ........... 137 6 MV photons post treatment with 800 Gy of E9, to its bleached state ......................................... 137 Figure 4.89: Ratio of the average optical transparencies, of N1(40/60 2 mm) for 3 Gy of .............. 138 X6 post treatment with 800 Gy of E9, to its bleached state .......................................................... 138 Figure 4.90: Ratio of the average optical transparencies, of N1(40/60 2 mm) for 5 Gy ................. 138 X6 photons post treatment with 800 Gy of E9, to its bleached state ............................................ 138 Figure 4.91: Ratio of the average optical transparencies, of N1(40/60 2 mm) for 7.5 Gy of ........... 139 X6 photons post treatment with 800 Gy of E9, to its bleached state ............................................... 139 Figure 4.92: Ratio of the average optical transparencies of N1(40/60 2 mm) for10 Gy of .............. 139 6 MV photons post treatment with 800 Gy of E9, to its bleached state ........................................... 139 Figure 4.93: Ratio of the average optical transparencies of N1(40/60 2 mm) for 15 Gy ................ 140 of X6 photons post treatment with 800 Gy of E9, to its bleached state .......................................... 140 Figure 4.94: Ratio of the average optical transparencies, of N1(40/60 2 mm) for 20 Gy ............... 140 of 6 MV photons post treatment with 800 Gy of E9, to its bleached state. ..................................... 140 Figure 4.95: Ratio of optical transparencies of 0.5 Gy X6 (post bleaching of fractioned 800 Gy E9) to 0.5 Gy X6 given to unexposed sample .......................................................................................... 141 Figure 4.96: Ratio of optical transparencies of 1 Gy X6 (post bleaching of fractioned 800 Gy E9) to 1 Gy X6 given to unexposed sample ................................................................................................. 142 Figure 4.97: Ratio of optical transparencies of N1 plastic sample for 2 Gy of 6 MV photons (post 142 bleaching from fractionated dose of 800 Gy of 9 MeV electron) to the same dosed fresh sample .... 142 Figure 4.98: Ratio of optical transparencies of N1 plastic sample for 5 Gy of 6 MV photons (post 143 bleaching from fractionated dose of 800 Gy of E9) to the same dosed fresh sample ........................ 143 Figure 4.99: Ratio of optical transparencies of N1 plastic sample for 7.5 Gy of 6 MV photons (post bleaching from fractionated dose of 800 Gy of E9) to the same dosed fresh sample ....................... 143 Figure 4.100 :Ratio of optical transparencies of N1 plastic sample for 15 Gy of 6 MV photons (post . bleaching from fractionated dose of 800 Gy of E9) to the same dosed fresh sample ...................... 144
xiii
Figure 4.101: Ratio of optical transparencies of N1 plastic sample for 20 Gy of 6 MV photons (post bleaching from fractionated dose of 800 Gy of E9) to the same dosed fresh sample ....................... 144 Figure 4.102: Optical transparency of fresh N1 (final sample) for 0.5 Gy of X6 with respect to their un irradiated fresh sample transparency ........................................................................................... 145 Figure 4.103 : Optical transparency of fresh N1 (final sample) for 1 Gy of X6 with respect to their un irradiated fresh sample transparency ........................................................................................... 145 Figure 4.104 : Optical transparency of fresh N1 (final sample) for 2 Gy of X6 with respect to their un irradiated fresh sample transparency ........................................................................................... 146 Figure 4.105 : Optical transparency of fresh N1 (final sample) for 3 Gy of X6 with respect to .. 146 their un irradiated fresh sample transparency ................................................................................... 146 Figure 4.106 : Optical transparency of fresh N1 (final sample) for 5 Gy of X6 with respect to their un irradiated fresh sample transparency .......................................................................................... 147 Figure 4.107 : Optical transparency of fresh N1 (final sample) for 7.5 Gy of X6 with respect to their un irradiated fresh sample transparency ................................................................................... 147 Figure 4.108 : Optical transparency of fresh N1 (final sample) for 10 Gy of X6 with respect to .. 148 their un irradiated fresh sample transparency ................................................................................... 148 Figure 4.109 : Optical transparency of fresh N1 (final sample) for 15 Gy of X6 with respect to their un irradiated fresh sample transparency ........................................................................................... 148 Figure 4.110 : Optical transparency of fresh N1 (final sample) for 20 Gy of X6 with respect to .. 149 their un irradiated fresh sample transparency ................................................................................... 149 Figure 4.111: Normalized optical transparency curve of fresh N1 plastic sample (2 mm) post irradiation of 20 Gy of X6 .............................................................................................................. 150 Figure 4.112 : Normalized optical transparency curve of fresh N1 plastic sample (2 mm) post irradiation of 15 Gy of X6 ............................................................................................................... 150 Figure 4.113 : Normalized optical transparency curve of fresh N1 plastic sample (2 mm) post irradiation of 10 Gy of X6 ....................................................................................................... 151 Figure 4.114 : Normalized optical transparency curve of fresh N1 plastic sample (2 mm) post irradiation of 7.5 Gy of X6 ..................................................................................................... 151 Figure 4.115 : Normalized optical transparency curve of fresh N1 plastic sample (2 mm) post irradiation of 5 Gy of X6 ......................................................................................................... 151 Figure 4.116 : Normalized optical transparency curve of fresh N1 plastic sample (2 mm) post ....... 152 irradiation of 3 Gy of X6 ................................................................................................................ 152 Figure 4.117 : Dose versus response curve of fresh N1 plastic samples ......................................... 153
xiv
xv
List of tables
Tables Page no
3.1 Summarized dosimetric experiments .....................................................................69 - 74
xvi
List of abbreviations BANG BIS Acrylamide Nitrogen Gelatin BeO Beryllium Oxide CT Computed Technology CW-OSL Continuous waves optically stimulated dosimetery CCD Charge coupled device EBT External Beam Therapy E9 9 MeV Electrons HDR High Dose Rate HVLP High volume low pressure IMRT Intensity modulated Radiation Therapy LM-OSL Linearly modulated optically stimulated luminescence LVLP Low volume low pressure MOSFET Metal Oxide semi-conductor Field Effect Transistor MRI Magnetic Resonance imaging MV Million volt OCT Optical CT scan OD Optical Density OSL Optically stimulated luminescence PC Poly carbonates PE Poly ethylene PMMA Poly methyl meta acrylate PMT Photo multiplier tube PVA Poly vinyl alcohol RIA Radiation induced attenuation RTC Relative transparency curve SRS Stereo radio surgery TL Thermoluminescence TLD Thermo Luminescence dosimeter TPS Treatment planning system UV Ultra violet XO Xylenol Orange X6 6 MV photons
xvii
FABRIKASI DOSIMETER GUNASEMULA SINAR-X
BERASASKAN PLASTIK UNTUK RADIOTERAPI
ABSTRAK
Tujuan kajian ini adalah untuk menghasilkan dosimeter komposit plastik untuk
mengukur dos sinaran foton sinar-X yang digunakan dalam radioterapi. Kriteria
untuk dosimeter yang dicadangkan adalah kos yang rendah, penggunaan yang
mudah, bahan bukan toksik, dan boleh diguna semula. Sebagai dosimeter, ia harus
mempunyai sensitiviti dan kebolehpercayaan yang cukup dalam lingkungan ukuran
dos kepentingan dalam radioterapi.
Dalam membuat dosimeter berasaskan plastik, zarah bahan aktif yang sensitif kepada
sinaran tertanam secara seragam, di dalam posisi yang tetap, dalam plastik lut sinar.
Perubahan dalam spektrum transmisi optik bagi bahan telah digunakan sebagai
kaedah bacaan, menggunakan Shimadzu UV 1800 spektrofotometer. Perubahan nilai
ini dikalibrasi untuk mendapatkan nilai dos serapan. Pelunturan optik sampel yang
telah menerima dedahan sinaran dilakukan dengan menggunakan dua buah mentol
biasa berkuasa 100 watt dengan dedahan selama 18 jam.
Antara tiga belas bahan yang telah dipilih untuk kajian kesesuaian mereka sebagai
bahan aktif terhadap sinaran, natrium klorida tulen gred penyelidikan didapati
merupakan bahan yang paling sesuai. Bahan-bahan lain yang turut dikaji ialah
berilium oksida, tujuh jenis garam yang boleh dimakan dari di pasaran tempatan,
aluminium oksida tulen, kalium alum (tawas), ammonium klorida (Noshadar) dan
garam buluh. PMMA (polymethyl metaacrylate) telah dipilih sebagai bahan hos.
Sampel plastik yang baik telah dapat dihasilkan menggunakan Haake poly drive
pencampur dalaman dan mesin tekan panas, berbentuk lembaran plastik setebal
xviii
2 mm, dengan zarah natrium klorida (125 μm) tertanam secara seragam dalam nisbah
berat 40/60 natrium klorida kepada PMMA. Kesemua penilaian bahan aktif telah
dijalankan dengan sampel diletakkan di dalam botol plastik lutsinar yang
mengandungi minyak yang lut sinar. Botol plastik ini didapati mempunyai ciri-ciri
o p t i k y a n g s a n g a t d e k a t s a m p e l P M M A t a n p a b a h a n a k t i f .
Pemerhartian mendapati bahawa rawatan sampel menggunakan dos elektron 9 MeV,
sehingga 2800 Gy, tidak mengubah sensitiviti sampel terhadap sinaran. Sampel
plastik akhir yang dinyatakan di atas, mengandungi natrium klorida yang dilabelkan
sebagai N1, telah menunjukkan tindak balas kepada foton 6 MV, dengan tindak balas
dos didapati linear dalam julat 3 Gy hingga 10 Gy. Sensitiviti kelihatan menurun
antara 10 Gy hingga15 Gy, selepas itu sensitiviti kelihatan kembali untuk dos
terakhir yang dinilai, iaitu 20 Gy. Penurunan sensitiviti pada 15 Gy berkemungkinan
besar berpunca daripada pengurangan elektron terperangkap, sepatutnya
menghasilkan penyerapan optik dalam julat 395 kepada 550nm, disebabkan
penukaran-ke atas pendarfluor hasil daripada penyerapan semasa ukuran, dalam julat
570-625 nm. Nilai Zeff bagi dosimeter berasaskan plastik PMMA yang mengandungi
40% N1 mengikut berat telah dikira dan dipastikan sebagai 10.51 amu. Sampel N1
yang mendapat dos sinaran 5 Gy 6 MV foton telah menunjukkan nilai kepudaran
16.92% untuk satu minggu dan 48.06% untuk satu bulan. Kebolehulangan dos
dianggarkan daripada tiga ujian menggunakan tiga sampel yang berasingan dengan
nilai dos serapan 5 Gy 6 MV foton. Dos sinaran telah diberikan selang 4 hari selepas
pelunturan optik selama 18 jam. Kawasan lembah pada spektra transmisi optik yang
sepadan dengan dos serapan didapati mempunyai nilai purata 96.4 unit-arbitrari
dengan sisihan piawai 4.85%. Kawasan lembah di dalam lekuk transmisi tersebut
terletak dalam julat 460 nm hingga 472nm. Dosimeter berasaskan plastik yang
xix
terhasil didapati mantap, murah, mudah untuk digunakan dan boleh diguna semula.
Ia didapati sesuai untuk mengukur dos X-ray foton, dalam julat respons dos linear,
dari 3 Gy hingga 10 Gy, dan untuk julat dos 15 Gy dan 20 Gy.
xx
FABRICATION OF A REUSEABLE PLASTIC BASED X-RAY
DOSIMETER FOR RADIOTHERAPY
ABSTRACT
The aim of this study was to fabricate a plastic composite dosimeter for the
measurement of radiation doses of X -ray photons used in radiotherapy. The criteria
for the proposed dosimeter were low cost, ease of use, non toxic, and reusable.
As a dosimeter, it should have acceptable sensitivity and reliability in the dose
measurements range of interest in radiotherapy.
In making plastics based dosimeters, radiation sensitive material grains were
uniformly embedded in transparent plastic host where they are kept spatially
fixed. Changes in optical transmission spectra of the materials was used as readout
method, using Shimadzu UV 1800 spectrophotometer. These changes were than
calibrated to get the absorbed doses. Optical bleaching of the irradiated samples were
performed with two 100 watts ordinary bulbs for 18 hours. Among the thirteen
materials that were selected for the study of their suitability as the embedding
material, research grade pure sodium chloride was found to be the most suitable
material. Other materials studied were beryllium oxide, seven different types of
edible salts available in local markets, un doped aluminium oxide, potash alum
(tawas), ammonium chloride (Noshadar) and bamboo salt. PMMA (polymethyl
metaacrylate) was chosen as the host material. The final plastic sample was produced
using Haake poly drive internal mixer and hot press machine, as 2 mm thick plastic
sheet, with uniformly embedded grounded (125µm) pure sodium chloride grains in
40/60 sodium chloride to PMMA wt/wt ratio. All the evaluations of embedding
materials were carried out with their samples placed inside a transparent plastic vial
xxi
containing optically clear oil. This plastic vial was found to have optical properties
very close to the final, material free, PMMA sample. It was observed that treatment
of samples using 9 MeV electron doses of up to 2800 Gy does not changed their
sensitivity. The aforementioned final plastic sample, containing sodium chloride
labelled as N1, have shown response to 6 MV photons with a linear dose response
from 3 Gy to10 Gy. The sensitivity appeared to be decreased between 10 Gy to15
Gy, after which appeared to recover for the last observed dose of 20 Gy. The
decrease in 15 Gy response is postulated to be due to the depletion of the trapped
electrons of interest, supposed to produce optical absorption at 395 to 550nm, due to
fluorescence up-conversion resulting from 570 to 625 nm absorption during
measurements. The Zeff of the PMMA plastic based dosimeter containing 40% N1
by weight was calculated to be 10.51 a.m.u. The N1 samples irradiated with 5 Gy
6MV photons has shown fading of 16.92% and 48.06% for one week and one month
respectively. The dose reproducibility was estimated with the average response of
three fresh samples to 5 Gy 6 MV photons. The doses were delivered for alternate 4
days post optical bleaching of 18 hours. The area of dip in the percentage
transmission curve corresponding the absorbed dose was found to have an
average value of 96.4 arbitrary area units with standard deviation of 4.85%. The
peak values of the dips were found to be between 460 nm to 472 nm. The fabricated
plastic based dosimeter was found to be robust, cheap, easy to use and reusable and
suitable for measuring doses of X-ray photons, in the dose ranges of linear responses,
3 G y to10 G y and fo r obse rved h igher doses , 15 G y and 20 G y.
1
CHAPTER 1
INTRODUCTION
1.1 Background A variety of dosimeters are in use for different purposes these days. They vary in
their sensitivities for different radiations, range of measurable doses, physical form
and suitability for a particular usage. Although achievements have been made in
developing materials that are very sensitive and can work at very high doses but
they fall in the advanced materials category and have very costly.
Some introduced dosimeters have high dose threshold and they are difficult to
develop due to, their toxicity (plastic dosimeters), their sensitivity to impurities and
atmospheric conditions (Fricke chemical dosimeters and Fricke gel dosimeter,
polymer gel dosimeter).
To use chemical and gel dosimeters the introduction of the container not only
made its routine usage difficult but required corrections in dose estimation
made its usage difficult . Further the extraction of dose information in most of the
dosimeters required diagnostic machines such as MRI, CT scan and Ultra sound
which may be difficult due to availability of these machines at for immediate
dosimetric use. Regarding reusability only ionization chambers, OSLD's and TLD's
are reusable.
2
1.2 Statement of the problem
Although the radiation dosimeters used these days have covered specific dosimetric
requirements of a particular usage, none of them fit in the criteria of a tissue
equivalent, reusable, easy to use, low cost and reliable dosimeter for routine
dosimetry of therapeutic ranges of doses.
For example Ionization chambers with all of their merits for dosimetry, have
drawbacks in terms of size, requirements of correction factors and stable
electrical. On the other hand semiconductor dosimeters such as diodes, and
MOSFETS, are non tissue equivalent and their performances are, affected by
temperature, dose rates and cumulative doses.
Among the tissue equivalent dosimeters, the diamond detectors are affected by dose
rates and the cumulative doses, Silver halide and radio chromic films on the other
hand are sensitive to dose rates and require energy corrections. Furthermore silver
halide films showed non linear behaviour that needs to be corrected.
Tissue equivalent synthetically prepared dosimeters such as, LiF:Mg,Ti,
LiF:Mg,Cu,P, Li 2 B 4 O 7 :Mn although are good for the therapeutic range of
dosimetry, but are expensive and require dedicated readout systems for their dose
data acquisitions. Other tissue equivalent dosimeters, such as chemical and plastic
dosimeters have shown high dose thresholds. The gel dosimeters, although
provides good results in the therapeutic range of doses, suffered with the corrections
required for its containers.
Recently introduced, tissue equivalent plastic dosimeter, PRASAGE (Zeff varies from
6.4 to 16.3 depending on the compositions), has established as a good dosimeter
qualifying for dosimetry of therapeutic range of doses.
3
The methods of producing all these tissue equivalent dosimetrs are not simple and in
most of the cases, handling of toxic materials and steps of preparation has potential
hazards for health. The simple and non toxic preparation and the reusability of a
dosimeter results in its low cost.Except ionization chambers, doped materials, diodes
and MOSFETS, all the tissue equivalent materials are one time useable. Furthermore
their readout methods require either expensive dedicated equipments or
sophisticated diagnostic machines, which cannot be used on routinely in most
medical centres.
Hence, fabrication of a tissue equivalent radiation dosimeter, which could be used
on routine basis in radiation therapy centres with acceptable accuracy and reliability
in its dose measurements of therapeutic ranges, which is reusable, non toxic, using
easily available low cost ingredients , easy fabrication method and have ease in use,
is still a challenge for the researchers.
1.3 Purpose of the study It was planned to fabricate a tissue equivalent, nontoxic, easy to develop, low cost
and robust dosimeter, which should be capable of handling therapeutic dose ranges
with acceptable accuracy and confidence of reproducibility in its routine
use. The purpose of the study is to develop a tissue equivalent robust plastic
composite dosimeter suitable for validating dose ranges used in radiotherapy.
Its non-toxicity, ease in its, preparation, handling, simple readout method and its
reusability was of prime concern.Search for the most suitable low cost materials and
simple methods for the preparation and dose data acquisitions were also considered.
4
1.4 Objectives of the study
The aim of the study is to fabricate a reusable tissue equivalent plastic composite
dosimeter with acceptable accuracies, that could serve the routine dosimetric
requirements of a radiation therapy centre. The simple preparation technique easy
read out methods were of prime concern. The cost of the dosimeter was also
considered. The sample was supposed to be of PMMA, uniformly embed sieved
grains of the selected radiation sensitive material. To achieve this goal, smaller goals
for this work have been identified.
First, this work seeked to identify the most suitable material to be used as the
ingredient in the plastic composite mix. This could be achieved through independent
evaluations of the respective materials for their responses to radiation in term of
changes in their optical properties.
Secondly, this work seeked to determine whether the sensitivity of any of these
materials can be improved by synthetic introduction of crystal defects. This can be
done by using high dose of particulate radiation such as neutron or electron.
Thirdly, methods of sample preparations were to be evaluated. The aim of this
evaluation is to determine the most suitable technique that can produce plastic
composite samples having favourable properties such as uniform physical
dimensions and uniform distribution of the embedding materials. Once the sample
preparation technique has been identified, the fourth minor objective is to actually
fabricate the samples for the rest of evaluation needed in this work.
Finally, these samples would be evaluated for their radiation dosimetric properties
using X-ray photon beam from a linear accelerator. They would be evaluated mainly
for radiation sensitivity, repeatability, fading, and dose linearity. Further dosimetric
studies, such as energy and dose rate dependency, would be studied if time allowed.
5
1.5 Scope of the study
The selected radiation sensitive material grains should be embedded uniformly in the
body of the plastic as the optical response depends on it. Non-uniformities in either
the number of particles per unit mass of the plastic composite or the thickness of the
sample may cause error in observations.
Since the selected materials are supposedly light sensitive, any exposure to light even
for a small duration, from the time of irradiation until readouts may result in loss of
response signal. Performing the whole procedure in light tight condition helps in
getting the true dose versus response relationship.
The UV Spectrophotometer, which uses intensity based CCD detectors, is a less
sensitive instrument compared to fluorescence spectrophotometer, which uses PMT's
as detectors. Hence the radiation sensitivity of the final plastic composite dosimeter
may appear to be less with UV spectrophotometer than with fluorescence
spectrophotometer.
6
CHAPTER 2
LITERATURE REVIEW
2.1 Radiation dose and dosimetry Ionizing radiations interacts with matter in different ways. The possibility of a
mode of interaction depends upon the type and energy of radiation and the physical
properties of the interacting matter. Ionizing radiations may lose its energy in
matter either by ionizing, exciting or by making the atoms or molecules
vibrate. Deposition of radiation energy within the interacting material is termed as
dose, whose unit is Gray (Gy), defined as joule of deposited energy per unit
mass(kg) (Khan, 2010).
Dosimetry is the measurement or estimation of dose, ideally at a defined point
within the interacting material, but it could also be performed for a plane of 2D
or for a volume of 3D. The properties of ionizing radiations, their interacting
mechanisms, and the relationship between doses versus different material's
responses, has been successfully exploited in health services, different industries,
scientific research and technologies. The use of ionizing radiation in plastic
industry, sterilizing of medical stuff and food items, quality assurance of different
goods such as metal sheets, environmental monitoring, archaeological and
geological dating, forensic applications, development of new materials for space
crafts and in medical sciences are examples of its usage.
7
2.2 Medical dosimetry
The dose measurements in medical sciences was rapidly advancing and becoming
ever complexing. The needs of radiation dosimetry for, personal monitoring or
radiation protection, diagnostic radiology or for therapeutic radiology were different
leading to different methods, and related sophistications. Although the basic
concern of medical dosimetry is to estimate the biological damages resulting from
ionizing radiation exposures, the radiation therapy differs in that, the exposures were
pre planned.
The goal of radiation therapy is to deliver uniform and homogeneous prescribed
dose to treatment volume but at the same time, save the adjacent healthy tissues
from radiation exposure or keep the doses to un-avoidable healthy tissues, well
below their radiation tolerance (Khan, 2010).
2.3 Importance of dosimetry for radiation therapy
The challenges in radiation dosimetry is due to the variety in cancer tumours, types,
its anatomical site, its shape, and adjacent healthy tissues. The diagnostic images
used for marking treatment volumes may not be used for a particular radiation
treatment plan as the geometries of organs may change during the planned posture.
The cure or control of cancerous tumour, depends upon the coverage of the
treatment volume in the prescribed dose. Hence radiation treatments were needed
to be planed and must be validated prior to the actual treatments to optimize the
goals of radiation therapy.
Ideally, the 3D dose distribution of a treatment plan, within a tissue equivalent
phantom, similar to patients treated anatomy, is needed to be validated. New types
8
of radiation therapy machines and techniques generally made the dose delivery faster
and its distribution over a treatment volume more precise, however due to the
sophistications involved, it is increasingly desirable that the actual dose delivered to
the patient be checked possibly for each treatment plan.
In more complicated treatment techniques such as stereotactic radio surgery,
conformal radiotherapy, Intensity modulated radiation therapy (IMRT),
Brachy therapy, in which are situations may arise where junctions between two fields
or regions of steep dose gradient exists, or the un-avoidable inclusion of some vital
organ or healthy tissues in the prescribed radiation fields. In such conditions 3D
dosimetry will be very useful, with spatially detailed point to point accurate dose
measurements, for the quality assurance of a treatment plan and hence to get the
ultimate benefits of the radiation therapy.
2.3.1 Treatment planning, treatment planning systems and dosimetry The treatment planning systems (TPS) used the dose distribution data in water
phantom. These doses were delivered with standard protocols and field sizes, used
in therapeutics. The TPS then interpolate or extrapolate the dose values for
different depths to calculate three dimensional doses distributions for a particular
treatment plan.
The beam data of every machine for a given field size is generally different,
due to different scatter factors, may be due to different machine design and
variations during production. Since TPS used interpolated or extrapolated
values of parameters, it might show different dose distributions, from the actual
treatment machines even for standard field sizes.
9
Serious dosimetric errors such as under dosing, over dosing, missing the treatment
volume or inclusion of healthy tissues or vital organs, which were considered in the
treatment plan but may wrongly appeared in TPS. Such errors especially may
occurred at the junction of different density tissues or at the field junctions where
dose gradient appeared. Such errors ultimately cause unsuccessful treatment.
2.3.2 Requirement of validating treatment plan The treatment plans are made with the patients radiographic images X-rays, CT scan
and MRI. The requirement for validating a treatment plan is also to match the
patient's planed and the actual treatment setup. This matching of the patient's
planned treatment positioning with the actual treatment set-up is very important
as the treatment volumes are related to the surface marking of the beam entrance
points. This set up matching has to be done with a tissue equivalent phantoms
similar to the patient's treatment anatomy, whose 3D dose measurement will then
validate a treatment plan. Further, in the cases where the treatment volume or
the surface anatomy changed post irradiations, any modification in the plan needed
to be verified the same way. Due to current treatment complexity raising the risk for
target miss or unnecessary irradiation of healthy tissues, pre-treatment dosimetric
verification of each treatment plan is highly recommended.
Ideally, validations of the treatment plans are performed on real treatment machines
with real treatment parameters using tissue equivalent phantoms that faithfully
represents the anatomy of the patients treatment volume .The 3D dose distribution in
these phantoms should accurately indicates the actual dose distribution.
10
2.4 Over view of the dosimeters Generally, dosimetry is performed using radiation sensitive volumes, whose
responses should be dose dependent and measurable using suitable read out system.
The response of any new dosimeter should be able to be confirmed and calibrated
with a standard dosimetry system, i.e. using ionization chambers and electrometers.
The performance of a dosimetry system is the net performances of its two
components, that is the radiation sensitive material and its response read out system.
2.4.1 Ionization chambers Among all types of dosimeters, ionization chambers are accepted to standard
dosimeters in clinical applications. Generally, ionization chambers have basic
construction of a cylindrical shell of air equivalent material that contain a suitable
gas of known volume and density. There is a fine electrode at the centre of this
cylindrical construction, maintained at high voltage, normally around
300-400 volts (Khan, 2010).
The thickness of the solid shell was made such that all the charges produced by the
ionization due to photons within a range of specified energies, reached the
electrodes and an electronic equilibrium establishes between the outgoing and
incoming electrons in measuring volume. The charge collected was proportional
to absorbed dose.Using relation between average energy required for producing
an ion pair in air as 33.85 eV / ion pair (Khan, 2010), amount of charge
collected can be converted to absorbed dose.
The doses of different energy photons could by measured by using a build up cap
whose thickness corresponds with the range of electronic equilibrium to the
11
measuring volume of the chamber. Ionization chambers with build-up caps were
near to tissue equivalent and considered as standard point dose detectors .For dose
measurement temperature pressure and humidity corrections are required.
2.4.2 Film dosimetry Radiographic film was originally used for diagnostic radiography, later it found its
uses in personal monitoring. Now the use of radiographic films were also common
for portal imaging in therapeutic radiology.
2.4.2.1. Silver halide Radiographic films Radiographic films consists of a transparent thin base cellulose acetate or
polyester resin sandwiched between emulsion layers put together using thin layers
of adhesive. The emulsion is a mixture of gelatine and photosensitive grains of
silver halide micro crystals. The finished surfaces of film was protected with
layers known as super coating. The silver iodo bromide crystals embedded in
layers are sensitive to light and ionizing radiations. Unfortunately, the sensitivity
to ionizing radiations is generally much more lower to that of light.
The crystal defects enable silver ions Ag+ which moves freely with in the crystal.
Upon radiation exposure, bromide ion of the crystal absorbs light and gives off
electron, and left in gelatine as neutral bromide atom.
The electrons are then captured by the wandering silver ions converting them to
silver atoms. Depending on the amount of radiation received ,the silver atoms form
clumps together, forming latent image, i.e. a map of the radiation intensity.
Upon irradiation the exposed crystals were reduced to grains of metallic silver.
During the fixation of film processing, the unexposed grains are washed away,
12
leaving behind a clear film. The silver grain within the film appeared black hence,
areas with higher radiation exposures have higher accumulation of silver grains,
leading to appearance.
Densitometers with suitable calibration are used to estimate the doses of the
different areas of exposed film, using the varying optical transparencies in a unit
known as optical density (Akselrod et al.) (Thomas III et al., 1984).
Although the radiographic film's response to radiation is nonlinear and depend on
radiation energy and processing parameters, their ease of use, inexpensiveness, high
spatial resolution and long period record keeping capability, keeps them in use for
general dsoimetric field checks of megavoltage therapy machines, both for photons
and electrons (Khan, 2010). With suitable choice of filters, radiographic films served
as personal monitoring device.
2.4.2.2. Radiochromic films
Radio chromic films are commonly used in therapeutic dosimetry. These films
are transparent and almost tissue equivalent. They contain grain less dye, which
upon irradiation gets polymerized and turn to blue colour. The change in optical
transparency was then determined with densitometer with proper calibration, the
results can be used to estimate absorbed dose. These films are self-developing,
thus they do not require chemical processing.
The response is dose rate independent. In standard to ambient conditions ,i.e. except
excessive humidity, they are comparatively less energy dependent than silver halide
films. Although radio chromic films are generally less sensitive than silver halide
films, however they provide higher resolution dose measurement, attributed due
to grain less feature of the dye, therefore they are found to be valuable in validating
13
multi field radiotherapy plan with high dose gradient regions. Gafchromic HS (high
sensitivity), EBT (External beam therapy), films were found to be useful in the
dose ranges 0.1 Gy to 8 Gy and 0.5 Gy to 40 Gy and ranges respectively
(Jordan, 2006). These and many other radio chromic films with little differences
in, sensitivity, energy dependency, linearity and read out methods were
successfully used in therapeutic radiology.
2.4.3. Luminescence dosimetry
Luminescence is the process of emission of visible and near visible light from
certain materials, upon stimulation . A more detailed classification of luminescence
according to the with emission delay includes fluorescence and phosphorescence
with emission times ~ 10 - 8 s and >> 10-8 s respectively. It was found t hat some
solids, after irradiating with ionizing radiations upon heating or illuminating with
lights of suitable frequencies, emits light. In most cases, the light intensity was found
to be proportional to the absorbed doses.
This property of stimulated luminescence was exploited for dosimetry and named
according to the used mode of stimulation, as thermally simulated and optically
stimulated, luminescence dosimetry. The stimulated luminance used for dosimetry
was explained with the band theory of solids (Podgorsak, 2003).
2.4.3.1. Thermoluminescence dosimetry (TLD) In solids insulators, defects in the crystal lattice modifies the forbidden energy
bands between valence and conduction band by creating discontinuous energy levels
within the forbidden band, and create cites (L,T) where a hole or electron could be
tapped. Figure 2.1 gives a simplified model of these defects in term of the energy
14
band. These defects may be intrinsic, extrinsic or result of some ionizing
radiation exposures, causing displacement of negative ion and hence generating a
vacancy for an electron. When such an insulator is irradiated with ionizing
radiations, the generated electrons and holes from the valence band may trapped
in these sites.
Figure 2.1. Model for thermo luminescence
If the natural de-trapping is negligible and the energy depth 'E' of the electron
trapping site below the conduction band is large, the life time of the trapped electrons
would be significantly large, establishing a meta stable state. When the irradiated
material is heated with constant rate of elevating temperature trapped electrons get
energized after receiving their energy from lattice vibrations leading to electron
ejection. The radioactive recombination of these liberated electrons with the
trapped holes, known as luminescence centres, gives out luminescence that
corresponds with some temperature is known as glow peaks. These glow peaks
could give information about the traps and the structural defects of the material
(Mandavia, 2011).
After heating the TLD to a specific high temperature, all the traps are emptied.
The TLD was then termed as zeroed as it was prepared for its re usage .
Thermo luminescence was originally used for measuring nuclear radiations by
15
Daniel in early 1950's. Successful use of TLD for archaeological dating was done in
1960's by Aitken and Mejdahl and for geological dating in 1980's by Wintle
and Huntley, ( Botter-Jensin, 1997).
TLD's having favourable properties such as high sensitivity, reusability, reliability
and ease of use are now established as a popular tool for dosimetry. Nowadays
commercially TLD'S are available that varies in the physical forms(rods, tablets ,
ribbons, small size grain, powder), tissue equivalency, sensitivity, operational
conditions making them suitable for a wide range of dosimetric application . In
medical dosimetry, near tissue equivalent TLD's such as LiF: Mg, Ti, LiF:
Mg, Cu, P, Li2B4O7: Mn and non tissue equivalents such as CaSO4: Dy,
Al2O3: C, CaF2:Mn , as most sensitive dosimeters, are in common use
(Mandavia, 2011).
2.4.3.2. Optically stimulated dosimeters
The phenomenon of optically stimulated luminescence is the same as thermally
stimulated luminescence except that the mode of de-excitation which is optical
rather than thermal. This technique was originally used for dating by Huntley
in 1985, and further studied by, Hutt in 1988 , Aitken and Smith in 1988,
Spooner and Questiaux in 1990, Poolton and Bailif in 1989, Bùtter-Jensen in 1991.
Bùtter-Jensen and Duller in 1992, established it as a tool for dating, and
Godfrey-Smith and Haskell in 1993, Haskell in 1993, Bailiff in 1995, Bøtter- Jensen
in 1995 used for dosimetry for Hiroshima and Chernobyl nuclear events.
An insulator with defects in its lattice, either by some impurity or dislocation of
an ion, naturally present or synthetically produced, when irradiated with
ionizing radiation, generates electrons by leaving behind holes in the valence band.
16
These electrons and holes may get trapped at the cites of defects, below the
conduction band and above the valence band respectively. Upon Illumination with
suitable frequency of light, luminescence may be observed. The emitted light may be
longer or of shorter wave length than stimulation wave length. The emitting light is
a characteristic of the material but the intensity was found to be proportional to the
absorbed dose. Different modes of optical excitations used were namely,
Continuous wave optically stimulated dosimetry (CW-OSL), Linear modulated
(LM-OSL) and Pulse OSL (P-OSL).
2.4.3.2.a. CW- OSL
In this mode of stimulation, samples were stimulated with constant intensity light
source which may be either laser or a broad band source. The luminescence was
measured simultaneously with the stimulating illumination. The luminescence
wavelength are distinguished from excitation wave lengths, using mono chromoters.
The decay curve of luminescence may or may not be exponential, revealing
different de trapping modes of trapped electrons and information about the
lattice defects. The total luminescence i.e. the total area under luminescence
curve, was proportional to the absorbed dose (McKeever, 2001).
2.4.3.2.b. LM-OSL
In this technique, instead of constant intensity of excitation wave lengths,
continuously increased intensity was used. The continuous increase in the intensity
of light, emptied trapped electrons, at their different intensity values and hence can
easily distinguished. This intensity with verses emptied electrons gives information
about the depth of the traps. The luminescence peaks at different points in time
17
of a linearly increased intensity ramp, gives information about the contribution
of different electron traps of different photo ionization cross section ,to the total
luminescence (McKeever, 2001).
2.4.3.2.c. Pulsed OSL In Pulsed OSL, the excitation wavelengths are used to illuminate the samples for
such short intervals of time that suits the life time of the luminescence
(Bøtter-Jensen, 1997).
Since luminescence does not depleted much in a short pulse, multiple observations
could be made for single dose (McKeever, 2001). Further, since measurement
was done after illumination, the stimulation and emitted wavelengths are easily
distinguishable without any aid of filters, which simplifies the method and
increased the measuring efficiency of the luminescence.
2.4.3.3. OSD VERSUS TLD The general features of the two modes of luminescence dosimeters i.e, TLD and
OSLD can be summarized as follows :-
1. Thermo luminescence dosimetry, as a single time dose measuring technique, with
comparatively complex method and thermal quenching, is a less sensitive
technique than non-destructive optically stimulated luminescence dosimetry, which
offers multiple observations for single dose .
2. Thermoluminescence output is a whole sample's response, where as in OSL, with
thin laser beams as optical exciting tool, even a grain could be selected from the
whole sample to get an optical response. This ease of site selection from a sample
for dose estimation could be exploited in re assessment of absorbed or in dose
mapping .
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3. It was observed that TL observations could be taken after taking OSL
observations from the same sample and that in many cases separate samples with
same dose gave same response, either use TL or OSL. This suggests that the
electron traps for TL and OSL may not be the same.
2.4.4. Semi conductor detectors
Semi-conductors have offered a good choice for many dosmetric applications.
Commonly used semi-conductor radiation detectors includes, PN diodes, MOSFET
(Metal-Oxide Semiconductor field effect Transistor), and Diamond detector
(Rosenfeld, 2011).
2.4.4.a. Diodes
Semiconductor diodes has been used as radiation detectors since last 30 years.
Due to their merits, of being very small sized and hence offering high resolution,
and ease of use for real time dosimetry, they are popular dosimeters for quality
assurance in radiation therapy (Podgorsak, 2003).
2.4.4.b. PN diodes
Silicon diodes are commercially available as n-type and p-type. They are fabricated
by counter doping the surfaces to produce opposite type material. The pn junction in
semi-conductor diodes, provides an in build, very high electric field (103 V/m)
across the junction.
When diode was exposed to ionizing radiations, electrons and holes pairs were
generated in the diode, which then diffused through the pn junction and collected by
an electrometer. This induced current through the diode was known as radiation
19
current and was proportional to dose rate. The total current was proportional to
absorbed dose rate in silicon. The silicon diode and MOSFET have near tissue
equivalency in MV energy range as silicon to water mass stopping collision power
ratio is almost independent of energy in MV range for photons and electrons
conventionally used in radiotherapy. There small sensitive volume and low
ionization energy compare to that of air made them 18000 times more sensitive
than same volume ionization chamber (Rosenfeld, 2011).The Diodes are commonly
used in vivo dosimetry as relative dosimeter, for example in bladder and rectum
dose measurements, entrance and exit dose. It is used with beam entrance dose
measurements routine checks of treatment parameters and setup errors such as
correct technique (SSD OR SAD), use of correct radiation and applicator, dose
rate change are able to be performed (Yorke et al., 2005).
As a relative dosimeter, they have proven to be suitable for dose measurements in
phantoms , especially for small field, high dose gradient regions as in radio surgery,
for depth dose measurements and for beam profiles of photons and electrons, in
commercially available 2D and 3D systems serves as quality assurance tools for
IMRT treatment plans. In spite of its merits, commercially available diodes could
not be used for surface dosimetry, because of its packing which behaves as build up
region. Diodes responses are sensitive to dose rate, temperature, accumulated doses
and the material used. Diodes suffers radiation degradation over time, the degree of
which depend on type of radiation and dose. Hence the use of diodes as
radiation detectors needs calibration against many factors. The response of
diodes were found directional dependent and also affected by spectral
composition of beam (Rosenfeld, 2011).
20
2.4.4.c. MOSFET
MOSFET Metal-Oxide Semiconductor Field Effect Transistor, due to its very small
size 1μm, and sensitivity to full range of photons and electrons, has been
successfully used for measuring relative doses in brachy therapy, radio surgery,
surface dose measurements diagnostic radiology.
These short life detectors have the same drawbacks as the diodes detectors have
Their responses were dependent on temperature, dose rate, direction of detection,
radiation quality, absorbed doses, and bias voltage during irradiation
(Podgorsak, 2003).
2.4.4.d. Diamond detector
Diamonds, either natural or artificial, when exposed to ionizing radiations,
shows change in its resistance. The detector with a suitable biased voltage
100 V and charge integrating amplifier, when exposed to ionizing radiations
gives signal proportional to the charge induced electron hole pair, which in turn
proportional to the absorbed dose.
Diamonds are low Z material and near to tissue equivalent therefore almost no
energy correction is required. These detectors, have small effective volumes order
of few mm3,very small directional dependency, negligible ; dose rate and
temperature 0.1 % per oC dependence and high radiation damage resistance. Prior
irradiation of the detectors were required for stable responses for each usage,
(Podgorsak, 2003).These detectors are specially suitable for small field high dose
gradient regions such as in radio surgery (SRS), routine treatment set up
checks, and relative dosimetry, (Rosenfeld, 2011).
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2.4.5. Chemical dosimetry
Chemical dosimeters are the oldest of all kinds of dosimeters. Originally
the chemical dosimeters were in liquid form but now later it was modified as a
gel dosimeters
2.4.5.a. Frick dosimeter
Fricke & Morse 1927 developed a ferrous sulphate solution as radiation
dosimeter with the irreversible oxidation of ferrous ion to ferric ion by ionizing
radiations (Nemtanu et al., 2008). The absorbed doses were measured by the Fe+3 ion
concentration which changes the absorption of UV radiation of wave lengths
224 nm and 303 nm with spectrophotometer. The absorbance of light at those
particular wavelengths and is directly proportional to the absorbed dose. It was used
for the determination of the doses between 40 Gy to 400 Gy (Soares et al., 1987).
The dosimeter was dose rate radiation quality ( photon and electron) independent
and relatively temperature 0.12% per 0C. It was also found to be very precise
0.15%. However it is easily affected by contaminations and does not show very
good stability over the periods of few years.
Being a contamination sensitive solution dosimeter it is difficult to use it on a
routine bases. Furthermore the Fricke dosimeter could not be used for
measurement requiring high spatial resolution. Since it is an irreversible chemical
process Fricke dosimeter could not be recycled. Other chemical dosimeters
includes ferrous–cupric dosimeter measurable range of doses 5×102 to 104 Gy,
with accuracy of ±02% and found to be LET dependent, Ceric sulphate
dosimeter dose range 102 to 2×105 with accuracy ±02% and the ethanol – chloro
benzene dosimeter dose range 4×102 Gy to 5 ×102 6 × 104 Gy, accuracy ±03%.
22
2.4.6. Gel dosimetry Gel dosimetry is relatively a new technique. Many different kinds of Gels are used
for the purpose . Most of them are modified formulas of the few basic gels originally
introduced. They offer 3D dosimetry with good accuracy.
2.4.6.a. Fricke gel dosimetry
Fricke chemical dosimeters Ferrous sulphate solution could be considered as the
first dosimeter with potential to be extended as 3D dosimeters .Gore in1984, have
showed that Fricke dosimeters could be probed with MRI to have 3D dose
distribution , (Baldock et al., 2010).
Gels such as gelatine, Agarose were used with infused ferrous sulphate ions to have
some stable 3D dose distribution, the concentration of radiation induced Fe+3 ions,
which is proportional to the absorbed doses, changes the colour of gel. The Fe+3
concentration remained fix in gelatine matrix for some time and could be probed
with suitable technique such as MRI, OCT, Ultra sound. (Baldock et al., 2010)
Fricke gels are water equivalent, hence considered as tissue equivalent, for a large
range of therapeutic radiation (Schreiner, 2004a). It is easy to prepare, inexpensive,
non-toxic since it is prepared in liquid form the dosimeter can have complex
phantom shapes and shows very good reproducibility.
However the Fricke gels have high threshold such as 50 Gy to 75 Gy making
it not suitable for low or intermediate radiation doses applications as encountered
in hospitals. Furthermore, the ferrous ions tends to diffuse in the gel after irradiation,
losing the spatial distribution information within a few hours. Attempts to solve the
problem of ferrous ion diffusion were made through the use of different gels such
as ge la t ine , agarose , sephadex and po lyvinyl a lcohol PVA.
23
Chelating agent agents such as Xylenol Orange XO were tried, which made a small
improvement in the stability ( Baldock el at 2010) but reduces the sensitivity of the
dosimeter (Schreiner, 2004b).When optical CT (OCT) was used noticed for that
readout the dose sensitivity depends on the wave lengths chosen. Optical
CT for dose readout of Fricke-Agarose –Xylenol Orange gel uses a fast
CCD camera to acquire the image. (Luciani et al., 2006).
The results of OCT readout were found to have good linear relationship in the
dose range up to 10 Gy and with spatial resolution approximately 0.5 mm
(Viti et al., 2006). It was observed that the use of MRI for dose data acquisition
requires relatively longer time than OCT readout , hence the optical technique
was better in limiting the effect of diffusion the 3D gel dosimeter.
2.4.6.b. Polymer gel dosimetry
Historically, the polymer gel dosimetry was successfully introduced by Hoecker and
Watkins in 1958. Boni in 1961, used poly acrylamide as a gamma dosimeter.
(Baldock et al., 2010).
The polymer gel dosimetry was based on radiation induced polymerization in
solutions of monomers and polymers (McJury et al., 2000). The polymerization was
proportional to the absorbed dose. The extent of polymerization thus changes the
density of the set materials. This change in density is proportional to the absorbed
dose. The change in density detectable upon viewing with X-ray CT scanners.
The colour or opacity of irradiated gels also changes and hence could be detected
and measured with optical devices such as optical CT OCT. Additionally MRI
was also used as a readout method. Maryanski in 1996 introduced polymer gel
dosimetry which was based on the polymerization of acrylamide AAm and
24
Bis monomers infused in an aqueous agarose matrix.
He gave it the name BANANA, (Baldock et al., 2010). The dosimeter was found to
be a tissue equivalent and its performance evaluation a 3D dosimeter was done
using stereotactic radio surgery and HDR brachy therapy plans. MRI scans were
used for 3D dose data acquisition and was found in good agreement with the
planned doses (Maryanski et al., 1996). With small change in the formulation of
BANANA, Maryanski in 1994 introduced BANG. This name later changes to PAG
to distinguish it with from the in house manufactured gels, (Baldock et al., 2010).
BangTm3 and PAG gels were investigated for use in 3D dosimetry with XCT
(Oldham et al., 2001) OCT and MRI (Audet et al., 2002), and was found good
for the purpose. Although the polymer gels were tissue equivalent and do not have
the problem of ion diffusion as in Fricke gels, it is very sensitive to atmospheric
oxygen which inhibited the polymerization process. For this reason the polymer
gels has to be prepared in oxygen free environment. This is found to be an in-
convenient procedure for daily use.
Fong in 2001, introduced a normoxic gel ‘MAGIC’ in which atmospheric oxygen
was bounded in metallo organic compound matrix without the gel for its
preparation (Baldock et al., 2010). Thus gel does not require for an Oxygen free
environment.
Different gels with little changes in the formulation were investigated with different
techniques. Mather in 2002 make use of Ultrasound for acquiring 3D dose
distribution data, Rintoulet in 2003 showed how to use Raman spectroscopic
technique to probe depth dose distribution in electron beams for PEG
(Baldock et al., 2010). Bheag gel (Rabaeh et al., 2008) uses MRI to evaluate its
dosimetric performance, Anthromorphic Barex (Duthoy et al., 2004) was
