Characterization of real-time dose measurement using monitor ionization chamber in electron beam FLASH radiotherapy

FLASH radiotherapy (RT) reduces the level of irradiation damage to normal tissue by utilizing an ultra-high dose rate to deliver the dose into the tumor target area in millisecond-order time. The ultra-high dose rate (>= 40 Gy/s) renders existing online dose monitors largely ineffective or dose-rate dependent. In this study, the dose distribution based on a 9 MeV ultra-high dose rate electron beam FLASH irradiation platform was investigated using Gafchromic EBT3 film, and its flatness deviation within +/- 5 % and symmetry deviation within +/- 3 % meets the standard requirements. We also explored the saturation effect observed in ionization chambers (IC) used for online monitoring by reducing the sensitive volume of the chamber and its air pressure. Real-time dose monitoring was successfully achieved with the electron beam at a source skin distance (SSD) of 100 cm and a dose rate of 250 Gy/s, demonstrating a linearity deviation within +/- 2 %. The IC correction factor was calculated according to the IAEA TRS-398 standard. In conclusion, this study successfully established quality assurance for electron beam irradiation at ultra-high dose rates using Gafchromic EBT3 film and enabled effective online dose monitoring. This work provides a dosimetric foundation for future scientific research and clinical applications of FLASH RT.