Radiation Protection for FLASH Proton Therapy

Abstract

The use of ultra-high dose rates of radiotherapy has been found to decrease toxicity in animal models when compared to conventional dose rate beams. Termed the FLASH effect, studies have revealed it is present for normal tissues in a range of animals and organs, and a range of radiation types, including protons. It has also been demonstrated that ultra-high dose rates do not adversely affect tumour growth delay. These results have generated great interest in translation to humans from the radiotherapy community, increased research, and instigated the use of ultra-high dose rate beams thought to be capable of the FLASH effect. However, the use of such high-dose rate beams may result in existing shielding and safety procedures not achieving their radiation protection objectives. A shielding assessment for FLASH proton beams was carried out using Monte-Carlo calculations with MCNPX, by scaling a standard dose rate survey and by conducting a FLASH survey around the proton research facility at our centre. It found that at the maximum available beam dose rate, additional access restrictions were needed, especially for prolonged test beams. This assessment served as a pilot study, and a procedure for conducting FLASH shielding assessments is proposed. Accident scenarios were also simulated and measurements were made of dose rate, which allowed recommendations on safety to be made, including a limitation of beam-on time to 25 seconds. Activation was induced in water samples for a range of energies and dose rates, to establish the likely activation of aqueous biological samples, and to assess a calculation methodology. It found that at 5 Gy exposure, samples may be able to be handled soon after FLASH exposure, but for higher doses, a delay in handling may be required. A commonly used calculation methodology predicted activation accurately. A questionnaire regarding the future use of FLASH radiotherapy and expected radiation protection implications was created, and respondents were invited from around the world. A total of 52 respondents completed the survey. It found that protons, linacs, and very high energy electrons are thought to be the most likely to deliver FLASH, that a range of centres could deliver it and that it is likely to be in use within a decade. It also found that support regarding radiation protection for FLASH radiotherapy would be welcomed and provided recommendations of the most useful methods of support. Finally, the methodologies and research techniques used in this project are appraised, future work in radiation protection for FLASH proton therapy is discussed, and an innovation is proposed to support the radiotherapy community to introduce FLASH radiotherapy safely.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Taylor (Supervisor) & Mike Merchant (Supervisor)