Radiobiological Effectiveness of Charged Particle Therapeutic Beams: Experimental Derivation,Application for Treatment Optimization and Radiation Protection of Astronauts

This thesis explores various applications of silicon-on-insulator (SOI) microdosimeters, specifically for space and aviation application, neutron monitoring in mixed radiation fields, as well as relative biological effectiveness (RBE) derivation for treatment planning systems (TPS) verification in proton and heavy ion therapy (HIT). Microdosimetry investigates the stochastic deposition of energy in a micron sized site in order to determine the biological effects that would be experienced by human cells in various radiation fields. SOI microdosimeters were conceived and developed by the Centre for Medical and Radiation Physics (CMRP) at the University of Wollongong (UOW), in Australia in order to supplement the current gold standard in microdosimetric measurements - the tissue equivalent proportional counter (TEPC) for radiation protection purposes in space, and address shortcomings of the TEPC for routine clinical applications in particle therapy.

With an increasing push towards space exploration, it is vital to fully understand the risks associated with space travel not only to the astronauts, but also to the electronics on board the spacecraft. The presence of mixed radiation fields in space, makes it difficult to accurately distinguish between components, as heavy ions, while being less abundant, possess high linear energy transfer (LET) which can cause greater biological damage to cells. While it is possible to measure the macro doses associated with space travel, determining the biological effects on cells is difficult on a micron scale. The ability of an SOI based microdosimetry system to be utilized in a low dose rate, mixed radiation fields is investigated in the first part of this thesis. By utilizing several heavy ion radiation fields, it was possible to replicate the radiation field encountered in space and investigate the quality of the radiation and assess the potential biological impact on human cells. The response of the SOI microdosimeter in a mixed radiation field containing neutrons is also investigated. Neutron are difficult to detect in mixed radiation field environments. With the aid of a ¹⁰B₄C thin film converter, it was found to be possible to separate the epithermal, thermal and fast neutron component from other radiation components. Additionally, a new 2 μm thick active layer SOI microdosimeter is extensively characterized and applied for thermal neutron detection in a heavy ion beam with successful detection of the thermal neutron component.

The next part of this thesis, explores the radiation hardness and charge collection properties of a new layout design SOI microdosimeters to determine the limits of their exposure to ionizing radiation and whether the device could withstand typical doses in space and QA measurements. An ion beam induced current (IBIC) technique is utilized using a microbeam probe at the Centre for Accelerator Science (CAS) at the Australian Nuclear Science and Technology Organization (ANSTO) with low energy carbon ions possessing high LET which is raster scanned over the microdosimeters. By mapping the position of the ion beam with time, it is possible to generate charge collection efficiency (CCE) maps and better understand the charge collection properties of the devices. It was found that the microdosimeters with this new design were fabricated with different specifications, i.e. on a support wafer if higher resistivity, leading to unwanted charge collection from the substrate of the device. To alter the intrinsic properties of the substrate, the microdosimeters were irradiated with high doses of gamma radiation from a Co-60 source. Following this irradiation, the microdosimeters were then rescanned by the IBIC beam, and new CCE maps were generated to further investigate the unwanted charge collection. The findings of this part of the thesis showed important recommendations for future designs of SOI microdosimeters, as well as showing their ability to tolerate typical doses encountered in space and particle therapy, furthering the case for their applicability for space radiation monitoring and QA measurements in particle therapy.

The third part of this work, explores the ability of the SOI microdosimeters to predict the RBE of various heavy ion beams at various clinical, and experimental facilities, as well as demonstrating the first ever experimental verification of LET_d optimized plans for proton therapy. This final section of the thesis demonstrates the ability to accurately predict RBE values for heavy ion therapy (HIT) in real time using the modified microdosimetric kinetic model (mMKM) and an improved biological weighting function (IBWF) known as the V79-RBE10 BWF, while also accurately measuring LET_d in proton therapy - something that has never been shown before.

Overall, this thesis successfully shows and justifies the use of SOI microdosimeters for monitoring of radiation effects for astronauts during space missions, as well as its viability in a new domain of QA for particle therapy.