Radiation protection considerations for ultra-high dose rate proton beam FLASH research: consensus from multi-institutional experience.

The use of ultra-high dose rate (UHDR) radiation for radiotherapy could potentially improve outcomes for cancer patients by increasing healthy tissue sparing, known as the FLASH effect. As UHDR use expands in FLASH research and clinical translation, reassessing existing radiation safety programs is essential to determine whether UHDR beams introduce any new radiation safety requirements. This manuscript outlines the lessons learnt through investigations performed by members of the Varian FLASH forward consortium radiation protection working group. The information contained within is the consensus of the working group, as agreed, through discussions and reports during the monthly meetings. The paper brings together multi-institutional radiation protection experience from five Varian ProBeam type facilities engaged in preclinical and clinical proton FLASH research. Three main aspects of radiation protection are considered: shielding, activation, and accident scenarios. Lessons learned from the collective experience gained are presented to benefit those embarking on new proton FLASH research programs.