A new calibration method of an array of plastic scintillating fibers for dosimetry in electron FLASH Radiotherapy

The challenge of saturation at the high dose rate employed in FLASH radiotherapy and the lack of realtime 2D and 3D dosimeters create an opportunity for the use of plastic scintillators. This study presents the development of an online dosimetric system designed for electron FLASH radiotherapy applications: an array of dosimeters, made by plastic scintillating fibers, each one coupled to an optical fiber, was evaluated as a proof- of-concept using a LINAC providing 9 MeV electrons, at the Centro Pisano Flash Radiotherapy. Signal linearity was established up to 10 Gy/pulse, with a pulse duration of 4 mu s. We also measured the signal variation across the beam profile using different applicators (30 mm, 50 mm and 100 mm in diameters) and we developed a geometrical model that accounts for the different amount of dose absorbed by the plastic scintillating fibers and the optical fibers. By fitting this model to the data, we estimated both the inter-calibration factors of the dosimeters, as well as the intrinsic ratio (i.e. for equal irradiated volumes) of spurious light in the optical fiber respect to the scintillation, which is equal to (4.7 +/- 0 . 1. +/- 1 . 0 . ) %.