Dosimetric characterization of an ultra-high dose rate beam for FLASH radiotherapy through alanine EPR dosimetry

Experimental evidence is growing, supporting the evidence of a considerable normal tissue sparing effect when treatments are delivered with dose rates much larger with respect to the conventional ones [1]. In particular, an increasing of the therapeutic window has been demonstrated for dose rates over 50 Gy/s, over a large variety of in-vivo experiments [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 [3]. Ultra-high dose rate (UHDR) beams for FLASH radiotherapy present significant dosimetric challenges [4]. 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. Solid state detectors, such as diamond or Silicon Carbide (SiC) detectors, have been also recently investigated, as a valuable alternative for real-time measurements. In this work we analysed the response of alanine pellets to UHDR electron beams. Irradiations of alanine pellets with electron beams at 7 and 9 MeV, accelerated by a SIT-Sordina ElectronFlash Linac, at conventional and UHDR regimes have been carried out. Average dose rates up several hundreds of Gy/s were used for the experimental campaign, with instantaneous dose rate even more two orders of magnitudes larger. Indeed, pulse structure of the used accelerator is characterized by a pulse duration between 1-4 us and a frequency up to hundreds of Hz. The analysis of the depth dose profile performed by stacking alanine pellets along the electron beam direction allowed to evaluate whether a dependence on the dose rate is present for these UHDR beams. The experimental results were aided by computational analyses. The results will be presented and discussed in details.