Doctor of Philosophy
School of Physics
Hadron therapy, referring to treating cancer with protons and heavier ions, provides many advantages over conventional X-ray radiotherapy, including better dose conformity and dose sparing to healthy tissue. One open problem associated with hadron therapy is that the radiobiological effectiveness (RBE), which is an important input parameter in the clinical treatment planning, changes significantly along the Bragg peak/spread out Bragg peak. It is paramount to be able to estimate the RBE to improve the treatment in terms of clinical outcome. The solution proposed by the Centre For Medical Radiation Physics (CMRP), University of Wollongong, is given by silicon-based microdosimetry technology, which offers a powerful solution to estimate the RBE with sub-mm spatial resolution. Such high spatial resolution is particularly important at the distal edge of the Bragg peak/spread out Bragg peak where organs at risk may be positioned.
Microdosimetry is conventionally performed using tissue equivalent proportional counters which feature complex and bulky operation and do not achieve a sub-mm spatial resolution. Silicon microdosimetry offers a more simple compact design, which is more well suited to the sharp dose gradients of hadron therapy beams and for routine quality assurance measurements. However, silicon microdosimetry is not without its difficulties, namely the measurement is not tissue equivalent and the design is not angularly independent.
This thesis describes the in-silico characterisation and design optimisation of novel silicon microdosimeters developed at the CMRP. The study has been performed by means of the Geant4 Monte Carlo Toolkit, which has been validated against experimental measurements in this project to quantify its accuracy for hadron therapy and for microdosimetric studies.
The tissue equivalence and angular dependence have been investigated. A method to convert the response of the detector from silicon to tissue was developed, which is now routinely used at the CMRP to convert experimental microdosimetric measurements to tissue, in proton and carbon ion therapy.
Due to the strong directionality of hadron therapy beams and the angular dependence of the silicon microdosimeter designs, it was found that the traditional method of converting the energy deposition to lineal energy using the mean chord length of the silicon sensitive volumes (SVs) of the device was inappropriate. Instead, the mean path length was found to be more appropriate to generate the lineal energy deposition. Based on the results of this project, the SV design was optimised to reduce the variance of the path length to reduce the angular dependence.
Bolst, David, Silicon microdosimetry in hadron therapy using Geant4, Doctor of Philosophy thesis, School of Physics, University of Wollongong, 2019. https://ro.uow.edu.au/theses1/619
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.