Doctor of Philosophy (PhD)


Physics and Astronomy

Document Type



The growing population of cancer survivors at risk of radiation induced side-effects is a public health concern. These side-effects include serious conditions such as second cancers, the majority of which occur outside of the primary treatment volume. Radiotherapy treatment planning systems systematically underestimate the dose to tissues out-of-field. Attempts to predict and reduce the risks of radiogenic side effects require accurate and personalized knowledge of the out-of-field radiation dose to patients. The long-term goal of this research is to provide clinical and research tools necessary to reduce the risk of radiotherapy side effects and improve the health outcomes of radiotherapy patients. The goal of this dissertation was to characterize the stray radiation from external beam radiation therapy, including megavoltage x-ray therapy and proton therapy. Chapter 1 gives a brief primer on radiation therapy and a summary of the state of knowledge regarding stray radiation exposures. In Chapter 2, we developed an analytical model of leakage neutron exposures from passively scattered proton therapy based on Monte Carlo simulations and measurements from two proton therapy facilities. Predicted neutron equivalent doses agreed with simulations and measurement to within 15%. In Chapter 3, we developed a broadly applicable model of stray photon radiation from conventional x-ray therapy. Predicted doses agreed with corresponding measurements to within 10% for two treatment machines and five photon beam energies. In Chapter 4, we report measurements, simulations, and a physics-based analytical model of stray photon radiation that realistically models transmission through complex collimator shapes. A gamma index analysis comparing predicted and measured doses found an 89.3% passing rate for criteria of 3-mm distance-to- agreement, 3% dose difference in-field, and 3-mGy/Gy dose difference out-of-field. Chapter 5 presents a model of photoneutron exposures from x-ray radiotherapy. Predicted absorbed doses agreed with simulations within 10%. In Chapter 6, we describe the simulation of external and internal neutron radiation from a compact proton therapy facility. External neutrons were the greatest source of dose out-of-field, but internal neutron dose was greater than external neutron dose near the field-edge. Chapter 7 summarizes the results presented in this dissertation. Our major finding is that fast, accurate analytical models of stray radiation dose are feasible.



Committee Chair

Newhauser, Wayne

Available for download on Saturday, June 04, 2022