Various in vivo experimental works carried out on different animals and organs have shown that it is possible to reduce the damage caused to healthy tissue still preserving the therapeutic efficacy on the tumor tissue, by drastically reducing the total time of dose delivery (<200 ms). This effect, called the FLASH effect, immediately attracted considerable attention within the radiotherapy community, due to the possibility of widening the therapeutic window and treating effectively tumors which appear radioresistant to conventional techniques. Despite the experimental evidence, the radiobiological mechanisms underlying the FLASH effect and the beam parameters contributing to its optimization are not yet known in details. In order to fully understand the FLASH effect, it might be worthy to investigate some alternatives which can further improve the tools adopted so far, in terms of both linac technology and dosimetric systems. This work investigates the problems and solutions concerning the realization of an electron accelerator dedicated to FLASH therapy and optimized for in vivo experiments. Moreover, the work discusses the saturation problems of the most common radiotherapy dosimeters when used in the very high dose-per-pulse FLASH conditions and provides some preliminary experimental data on their behavior.

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We present a new facility dedicated to radiobiology research, which has been implemented at the Trento Proton Therapy Centre (Italy). A dual-ring double scattering system was designed to produce irradiation fields of two sizes (i.e. 6 and 16 cm diameter) starting from a fix pencil beam at 148 MeV. The modulation in depth was obtained with a custommade range modulator, optimized to generate a 2.5 cm spread-out Bragg peak (SOBP). The resulting irradiation field was characterized in terms of lateral and depth-dose profiles. The beam characteristics and the geometry of the setup were implemented in the Geant4 Monte Carlo (MC) code. After benchmark against experimental data, the MC was used to characterize the distribution of dose-average linear energy transfer (LET) associated to the irradiation field. The results indicate that dose uniformity above 92.9% is obtained at the entrance channel as well as in the middle SOBP in the target regions for both irradiation fields. Dose rate in the range from 0.38 to 0.78 Gy/min was measured, which can be adjusted by proper selection of cyclotron output current, and eventually increased by about a factor 7. MC simulations were able to reproduce experimental data with good agreement. The characteristics of the facility are in line with the requirements of most radiobiology experiments. Importantly, the facility is also open to external users, after successful evaluation of beam proposals by the Program Advisory Committee.