The Woodruff School

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 to MV energy photons (<8 MeV), 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 frequency as well as continue to expand the energy range of radiation that can be used in treatments.

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 to continue studying the underlying biological mechanisms and to gain experience in anticipation of clinical FLASH radiotherapy machines.

This work focuses on the design, optimization, and experimental verification of the collimator head, resulting in a novel multilayered shielding solution, as well as collimator dimensions optimized through a flexible hybridized Genetic Algorithm (GA) and Nelder Mead Simplex Search (NMSS) Algorithm. Shielding performance is assessed using the Monte-Carlo code, FLUKA, analytic methods from National Council on Radiation Protection and Measurement Report 151, and experimental measurements. New analytic methods are developed to account for the effect of polyethylene, and the novel multilayered shielding method is developed in this work. A sensitivity analysis for the GA and NMSS Algorithm will be conducted as to the weighting of the objective function, which considers dose rate, dose profile, surface dose on the phantom, the bremsstrahlung converter temperature, dose distribution penumbra, and shielding performance, as well as other internal optimization parameters. The design and optimization of the FLASH-EXACT system will allow for experiments with clinically relevant photon beam energies and dose distributions to study the FLASH effect.

SUBJECT:	Ph.D. Proposal Presentation

BY:	Andrew Rosenstrom

TIME:	Friday, March 18, 2022, 11:00 a.m.

PLACE:	MRDC Building, 2405

TITLE:	Shielding Design And Optimization Of Novel Mv Photon Preclinical FLASH Radiotherapy System

COMMITTEE:	Dr. Shaheen Dewji, Chair (NRE) Dr. Anna Erickson (NRE) Dr. Nolan Hertel (NRE) Dr. Chris Wang (NRE) Dr. Sayed Rokni (Outside GT)

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 to MV energy photons (<8 MeV), 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 frequency as well as continue to expand the energy range of radiation that can be used in treatments. 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 to continue studying the underlying biological mechanisms and to gain experience in anticipation of clinical FLASH radiotherapy machines. This work focuses on the design, optimization, and experimental verification of the collimator head, resulting in a novel multilayered shielding solution, as well as collimator dimensions optimized through a flexible hybridized Genetic Algorithm (GA) and Nelder Mead Simplex Search (NMSS) Algorithm. Shielding performance is assessed using the Monte-Carlo code, FLUKA, analytic methods from National Council on Radiation Protection and Measurement Report 151, and experimental measurements. New analytic methods are developed to account for the effect of polyethylene, and the novel multilayered shielding method is developed in this work. A sensitivity analysis for the GA and NMSS Algorithm will be conducted as to the weighting of the objective function, which considers dose rate, dose profile, surface dose on the phantom, the bremsstrahlung converter temperature, dose distribution penumbra, and shielding performance, as well as other internal optimization parameters. The design and optimization of the FLASH-EXACT system will allow for experiments with clinically relevant photon beam energies and dose distributions to study the FLASH effect.