Could alanine/EPR dosimetry be useful for ultra-high dose rate beams used for FLASH radiotherapy?

In the last years a large interest has aroused towards radiation therapy treatments with dose rates much larger with respect to the conventional ones since experiments support the evidence of a considerable normal tissue sparing effect. Indeed, in-vivo experiments showed an increasing of the therapeutic window for dose rates over 50 Gy/s [2]. If confirmed, the ‘FLASH effect’ has the potential to re-shape the future of radiation treatments, with a significant impact on many oncology patients. Significant dosimetric challenges should be dealt with for Ultra-high dose rate (UHDR) beams for FLASH radiotherapy [4]. In particular, ionization chambers are affected by ion recombination effects, although novel approaches for decreasing or correcting for this effect are being proposed [5]. Passive dosimeters, as radiochromic films and alanine [6], could be used for UHDR measurements, although dose determination is typically time consuming. Detectors for real-time measurements are under investigations such as solid state detectors (mainly diamond or Silicon Carbide (SiC) detectors). This work aims at investigating the response of alanine pellets exposed to UHDR electron beams. The electron beams used with energies of 7 and 9 MeV, accelerated by a SIT-Sordina ElectronFlash Linac, at conventional and UHDR regimes were used. High average dose rates (up to several hundreds of Gy/s) were adopted for the experimental campaign, characterized by instantaneous dose rate even more than two orders of magnitudes larger. Pulse structure of the used accelerator is characterized by a pulse duration between 1-4 μs and a frequency up to hundreds of Hz. In order to investigate a possible dependence of alanine response on the dose rate for these UHDR beams, the depth dose profile accomplished by stacking alanine pellets along the electron beam direction was analysed. Monte Carlo simulations were performed and compared with experimental results. The results will be presented and discussed in details.