Monte Carlo Simulation Applied to Uncertainties in Iodine-123 Assay And
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Monte Carlo simulation applied to uncertainties in iodine-123 assay and thyroid uptake measurement By Matthew Talboys Cardiff University, School of Engineering PhD Thesis 2016 DECLARATION This work has not been submitted in substance for any other degree or award at this or any other university or place of learning, nor is being submitted concurrently in candidature for any degree or other award. Signed ………………………………………… (candidate) Date ………………………… STATEMENT 1 This thesis is being submitted in partial fulfilment of the requirements for the degree of PhD Signed ………………………………………… (candidate) Date ………………………… STATEMENT 2 This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references. The views expressed are my own. Signed ………………………………………… (candidate) Date ………………………… STATEMENT 3 I hereby give consent for my thesis, if accepted, to be available online in the University‟s Open Access repository and for inter-library loan, and for the title and summary to be made available to outside organisations. Signed ………………………………………… (candidate) Date ………………………… Page | ii SUMMARY The Monte Carlo method can be used to simulate deterministic processes such as the interaction of nuclear particles with matter, using a probabilistic approach. For radiation transport, the method is realised using Monte Carlo radiation transport codes. Performing simulations often has a distinct advantage over experimental studies, especially when physical measurements would be impracticable or even impossible to undertake. The application of Monte Carlo methods to nuclear medicine problems can therefore be seen as advantageous. 123I is a commonly utilised radionuclide for nuclear medicine diagnostic imaging and non-imaging investigations. The radioactive decay scheme of 123I presents particular challenges in terms of accurately determining the activity of an 123I based radiopharmaceutical prior to administration. The aims of this project were to use Monte Carlo modelling techniques to investigate the uncertainties associated with 123I assay and thyroid uptake assessment using this radionuclide. A model of the UK secondary standard calibrator was created using MCNP5 code and validated against the physical calibrator. A sensitivity curve for the instrument was created through simulation and the effect of measuring 123I using a vial and a syringe investigated. Up to 21% variation was seen between the vial and the syringe geometries studied. A thyroid uptake counter model was produced to determine uncertainties in thyroid uptake assessment for a number of different parameters. Variations in collimator to Page | iii neck distance, horizontal displacement of the detector and increasing depth of the thyroid gland in tissue were shown to affect the accuracy of uptake measurement. A depth-based correction for the thyroid gland was derived from the differential counts in the x-ray and gamma peaks of 123I. Such an approach could be utilised in clinical practice to correct for the depth of the thyroid gland in the neck. Page | iv ACKNOWLEDGEMENTS I am extremely grateful to Professor Wil Evans and Professor Peter Wells for firstly affording me the opportunity to undertake these studies and for their invaluable guidance and support throughout. I would like to thank other colleagues in the Medical Physics Department of the University Hospital of Wales for their assistance at numerous instances during my studies. I would also like to acknowledge the emotional and practical support afforded me by my family members and friends. Page | v CONTENTS Declaration ................................................................................................................ ii Summary .................................................................................................................. iii Acknowledgements .................................................................................................. v Contents ................................................................................................................... vi List of abbreviations ............................................................................................... xi CHAPTER 1: Introduction ........................................................................................ 1 1.1 Nuclear medicine .......................................................................................... 1 1.2 Applications of radioiodine in nuclear medicine............................................. 5 1.3 Introduction to Monte Carlo simulation .......................................................... 7 1.4 Current Monte Carlo research in medical physics ......................................... 8 1.5 Outline of research objectives ....................................................................... 9 CHAPTER 2: Simulation of Radiation Transport using Monte Carlo Techniques ................................................................................................................................. 11 2.1 Introduction ................................................................................................. 11 2.2 Salient features of Monte Carlo radiation transport codes .......................... 12 2.2.1 Models for the physical processes ................................................................. 12 2.2.2 Random number generator ............................................................................ 17 2.2.3 Sampling routines .......................................................................................... 18 2.2.4 Tallies ............................................................................................................ 18 2.2.5 Variance reduction techniques ....................................................................... 19 2.3 Monte Carlo codes ...................................................................................... 20 2.3.1 MCNP5 .......................................................................................................... 20 2.3.2 EGS ............................................................................................................... 21 2.3.3 Geant4 ........................................................................................................... 22 2.3.4 Monte Carlo code for subsequent investigations ............................................ 23 CHAPTER 3: Assay of Radionuclides .................................................................. 24 3.1 Introduction ................................................................................................. 24 3.2 Legal requirements for accurate assay of radionuclides in the UK.............. 25 3.3 Assay methodology ..................................................................................... 27 3.4 123I production ............................................................................................. 33 3.5 123I decay scheme ....................................................................................... 35 Page | vi 3.6 Uncertainties associated with 123I assay ..................................................... 37 3.7 Aims of Monte Carlo radionuclide calibrator simulations ............................. 41 Chapter 4: Monte Carlo Simulation of a Fidelis Radionuclide Calibrator .......... 42 4.1 Introduction ................................................................................................. 42 4.2 Fidelis secondary standard calibrator performance specifications .............. 43 4.3 Reproducibility of the Fidelis ....................................................................... 45 4.4 Accuracy of the Fidelis calibration factors ................................................... 46 4.5 Activity linearity of the Fidelis ...................................................................... 47 4.6 Geometric structure of the Fidelis ............................................................... 49 4.7 Material composition of the Fidelis .............................................................. 51 4.8 Monte Carlo Fidelis model .......................................................................... 53 4.8.1 Cell card ........................................................................................................ 53 4.8.2 Surface card .................................................................................................. 54 4.8.3 Mode card ...................................................................................................... 56 4.8.4 Material card .................................................................................................. 57 4.8.5 Importance card ............................................................................................. 58 4.8.6 Source definition card .................................................................................... 59 4.8.7 Tally card ....................................................................................................... 60 4.8.8 Particle and print card .................................................................................... 62 4.8.9 Geometry visualisation ................................................................................... 63 4.9 Validation of the Monte Carlo model ........................................................... 67 4.9.1 Monte Carlo Fidelis sensitivity curve .............................................................. 68 4.9.2 Discussion of Monte Carlo Sensitivity