Characterization of a low-energy cyclotron-based proton beam for preclinical radiobiological studies

Proton therapy is considered an attractive alternative to conventional radiotherapy in oncology, as its dose-depth curve favors tumor control while minimizing the risk of radiation-induced side effects in healthy tissue. Preclinical investigation into proton Relative Biological Effectiveness (RBE) is imperative to improve the current clinical RBE standard and subsequently optimize therapeutic efficacy. A low-energy cyclotron-based proton irradiation set-up for in vitro research was optimized at ICNAS-University of Coimbra. The system dosimetry was assessed using a calibration curve from a standard radiotherapy linear accelerator (LINAC) applied to proton-irradiated Gafchromic™ EBT4 films, while recording integrated beam charge in real time. Pulsed dose rate measurements were performed by determination of target exposure time. As proof of concept, glioblastoma cell lines (U373 and U87) were subjected to proton irradiation for quantification of cell survival and DNA damage. Homogeneous dose profiles were achieved on a 21 mm-diameter circular area at the target region for an incident proton energy of 14 MeV. A linear relation was found between proton dose at the target and integrated beam charge for pulsed dose rates from 10.8 to 16.2 Gy/s and a proton flux of ~ 10 7 protons/(s ∙ cm 2 ). Proton irradiation of U373 cells yielded effects on cell survival comparable to kilovoltage X-ray exposure. U87 cells exhibited unrepaired DNA damage following proton exposure. A cyclotron-based pulsed proton beam was successfully optimized for in vitro radiobiological research, as evidenced by the first irradiation studies for evaluation of cell survival and DNA damage in glioblastoma cellular models.