SUMMARY

FLASH radiotherapy is an emerging modality that takes advantage of a biological mechanism that occurs at dose rates > 35 Gy/s. While the so-called FLASH Effect has been shown to occur using electrons, protons, and low-energy photons (< 600 keV), the underlying biological mechanism is still disputed. In order to obtain greater clarity regarding the biological mechanism, preclinical, experimental systems must be created with irradiation parameters that span a wide range of achievable dose rates and pulse frequencies. The FLASH-Experimental X-ray small Animal Conformal Therapy (FLASH-EXACT) system, as a part of the Pluridirectional High-energy Agile Scanning Electronic Radiotherapy (PHASER) project, is a preclinical external beam radiotherapy device that uses high-energy bremsstrahlung (> 10 MeV) produced by a novel compact linear accelerating structure built at SLAC National Accelerator Laboratory. Due to the high-energy bremsstrahlung radiation and high workload, shielding and beam-shaping solutions are needed to create a safe and well-characterized experimental apparatus in anticipation of clinical FLASH radiotherapy machines. This work focuses on the design and metaheuristic optimization of the treatment head as well as the experimental verification of methods used in the design of the treatment head. The product is the novel application of multilayered shielding to produce efficient shielding as well as collimator dimensions optimized through a flexible hybridized Genetic and Nelder-Mead Simplex Search Algorithm. Shielding performance is assessed using the Monte-Carlo code FLUKA, empirical methods from the National Council of Radiation Protection and Measurement (NCRP) Report 151, and experimental measurements. Assessments and sensitivities of the vacuum window and bremsstrahlung converter to the highly transient thermal and structural loading are assessed using two finite element analysis codes to ensure there is a reasonable safety margin for the operation of the FLASH-EXACT system.