Dose Rate Control of FLASH Proton Beam for Animal Testing: Monte Carlo Simulation

Abstract

<h3>Purpose/Objective(s)</h3> Compared to conventional proton therapy (PT), ultra-high dose rate (FLASH) PT can reduce the toxicity to normal tissues while maintaining tumor control. The biological effect of FLASH beam will depend on its dose rate. In this study, we proposed a method that can freely change the dose rate of FLASH proton beam and deliver its uniform dose to mice for animal testing. <h3>Materials/Methods</h3> A FLASH proton beam nozzle was built in our proton therapy system for animal experiments. Ultra-high dose rate higher than 200 Gy/s can be obtained at the plateau region in the pristine Bragg peak of 230 MeV, 300 nA proton beam. The nozzle consisted of two main parts: a graphite scatterer that provides a uniform dose distribution from a Gaussian proton beam with ultra-high dose rate and a lead scatterer that can control the dose rate of the FLASH proton beam. A variable collimator system and an animal irradiation stage were added for the study of FLASH effects. The graphite scatterer was designed to obtain a uniform dose distribution with a size of 2 cm without reducing the dose rate significantly. It had a Gaussian shape and was installed downstream of a window extracting proton beam into the air. The lead scatterer was designed to control the dose rate of proton beam by changing its thickness and position. Its position was adjustable from 10 to 50 cm from the isocenter. It had a flat plate shape and was installed at the end of the proton beam nozzle. The graphite scatterer is slidable and the lead scatterer is detachable not to interfere with the existing nozzles. The dose rates and lateral dose profiles were calculated using a simulation toolkit. <h3>Results</h3> A FLASH proton beam nozzle was designed to control the dose rate under the ultra-high dose rate condition and to partially irradiate a mouse with uniform dose. Dose rates exceeding 40 Gy/s for 230 MeV proton beam were obtained at the plateau region in the pristine Bragg peak. The dose rate could be changed from 100 to 44 Gy/s, and the field size with ±10% dose uniformity was about 2 cm. <h3>Conclusion</h3> Our FLASH proton beam nozzle can conduct animal testing on the flash effect without interfering with existing proton beam nozzle. In addition, various studies of the FLASH effect depending on dose rate became possible. This approach designing a FLASH proton beam nozzle may be applicable to other proton therapy facilities. In the near future, animal experiments to demonstrate the FLASH effect will follow in our institute.