Development and characterization of phantoms to investigate the Flash effect with Drosophila melanogaster at an ultra-high dose rate radiotherapy linac.

Background and purpose: Ultra-high dose rate (UHDR) radiotherapy may widen the therapeutic window thanks to the Flash effect. Experimental linear accelerators have been converted to UHDR to collect pre-clinical evidence. Increasing the accessibility, throughput and investigating additional biological endpoints is key for deciphering the mechanism of the Flash effect. The aim of this study was to develop and characterise an experimental platform for UHDR experiments with Drosophila melanogaster, i.e. the fruit fly. Materials and methods: A clinical linear accelerator was modified to deliver 16 MeV electron beams in UHDR and conventional (CONV) mode. Two phantoms were developed to irradiate Drosophila melanogaster. The characterization was based both on active (ultra-thin ion chamber prototype, scintillator) and passive detectors (radiochromic films, OSLD). Moreover, the UHDR capabilities for megavoltage photon were investigated with an additional dedicated phantom. Results: The electron UHDR irradiations provided average dose rates in the range of 200---7500 Gy/s. The beam spatial uniformity within a single vial was better than ± 5 %. The dose delivered to Drosophila melanogaster in different configurations and beam modalities was confirmed to the ± 5 % level. The average dose rate achieved with photon megavoltage UHDR radiation reached beyond 40 Gy/s. Conclusions: This high-throughput experimental platform on a converted clinical linear accelerator could be used to compare CONV to UHDR for up to 500 animals per week for biological endpoints at up to 1000 Gy. The production of photon megavoltage UHDR radiation was also demonstrated for the first time at a converted clinical linac.