Objective:To establish a rapid and precise dose measurement method with EBT4 film and ensure its measurement accuracy to be within the required range through strict operational procedures for the purpose of addressing the two essential issues of poor measurement accuracy and timeliness of EBT film under FLASH conditions.Methods:After storing under different humidity conditions for a certain period of time, the film was exposed to radiation for analyzing the influence of air humidity on the intrinsic performance of EBT film. By means of repeated scanning operations and the film angle rotation, the influences of repeated scanning and film placement angle were analuzed. Parabolic correction method was used to reduce the spatial position influence during the scanning process. By analying the relationship between net optical density (netOD) and absorbed dose through the comparison of three fitting method, the optimal fitting curve was selected. After irradiation of the same batch of films for 5 min and 24 h, the film doses were calibrated and then compared with ionization chamber-measured result. The rapid and precise film dosimetry method was used to measure both the percentage depth dose from X-rays at ultra-high dose rate and the dose distribution at a depth of 2 cm in water.Results:Air humidity had the greatest influence on the intrinsic performance of EBT film (approximately 20%). The average deviation of repeated scans is within 0.5%. The angle at which the film is placed significantly affected the readouts of the film with the maximum influence approximately 70%. The net optical density combined with polynomial fitting can control the fitting residuals of 1-16 Gy within 3%. The change rate of light channels at 5 min already mostly met the requirements of the rapid mode (< 0.5%). Compared with the measurement result obtained using the reference ionization chamber, the deviations of the 5 min or 24 h dose calibration curves were all within 2%.Conclusions:The EBT4 film can be employed as a precise dosimeter to quickly measure the FLASH radiation dose. Rapid and precise FLASH dose measurements can meet the stringent requirements of both preclinical and clinical FLASH research.

Objective:To investigate the radiation field distribution characteristics in a 9 MeV electron FLASH (e-FLASH) linear accelerator treatment room.Methods:The Geant4 Monte Carlo program was employed for physical simulating of the radiation field distribution inside and outside the treatment room under a single-beam delivery condition with a total dose of 50 Gy at the reference point and a dose rate of 250 Gy/s. High-sensitivity radiation detectors (AT1123) were used to validate the measurements at key points.Results:The dose rate at the reference point was approximately 9×10 11 μSv/h. Due to the scattering and shielding effects, the deviation of the attenuation rate from the inverse-square law was observed and the isodose lines exhibited spatial drift. Measured dose rates at key points showed good agreement with the simulation result, with a maximum deviation within 30%. Conclusions:The complex radiation field distribution can be effectively simulated using Geant4 in an e-FLASH treatment room. This indicated the Geant4 is not only applicable for the shielding calculations in e-FLASH radiotherapy facilities, but also for the design optimization through, reduction of trial-and-error iterations and engineering costs.

Objective:To establish a rapid and precise dose measurement method with EBT4 film and ensure its measurement accuracy to be within the required range through strict operational procedures for the purpose of addressing the two essential issues of poor measurement accuracy and timeliness of EBT film under FLASH conditions.Methods:After storing under different humidity conditions for a certain period of time, the film was exposed to radiation for analyzing the influence of air humidity on the intrinsic performance of EBT film. By means of repeated scanning operations and the film angle rotation, the influences of repeated scanning and film placement angle were analuzed. Parabolic correction method was used to reduce the spatial position influence during the scanning process. By analying the relationship between net optical density (netOD) and absorbed dose through the comparison of three fitting method, the optimal fitting curve was selected. After irradiation of the same batch of films for 5 min and 24 h, the film doses were calibrated and then compared with ionization chamber-measured result. The rapid and precise film dosimetry method was used to measure both the percentage depth dose from X-rays at ultra-high dose rate and the dose distribution at a depth of 2 cm in water.Results:Air humidity had the greatest influence on the intrinsic performance of EBT film (approximately 20%). The average deviation of repeated scans is within 0.5%. The angle at which the film is placed significantly affected the readouts of the film with the maximum influence approximately 70%. The net optical density combined with polynomial fitting can control the fitting residuals of 1-16 Gy within 3%. The change rate of light channels at 5 min already mostly met the requirements of the rapid mode (< 0.5%). Compared with the measurement result obtained using the reference ionization chamber, the deviations of the 5 min or 24 h dose calibration curves were all within 2%.Conclusions:The EBT4 film can be employed as a precise dosimeter to quickly measure the FLASH radiation dose. Rapid and precise FLASH dose measurements can meet the stringent requirements of both preclinical and clinical FLASH research.

Objective:To investigate the radiation field distribution characteristics in a 9 MeV electron FLASH (e-FLASH) linear accelerator treatment room.Methods:The Geant4 Monte Carlo program was employed for physical simulating of the radiation field distribution inside and outside the treatment room under a single-beam delivery condition with a total dose of 50 Gy at the reference point and a dose rate of 250 Gy/s. High-sensitivity radiation detectors (AT1123) were used to validate the measurements at key points.Results:The dose rate at the reference point was approximately 9×10 11 μSv/h. Due to the scattering and shielding effects, the deviation of the attenuation rate from the inverse-square law was observed and the isodose lines exhibited spatial drift. Measured dose rates at key points showed good agreement with the simulation result, with a maximum deviation within 30%. Conclusions:The complex radiation field distribution can be effectively simulated using Geant4 in an e-FLASH treatment room. This indicated the Geant4 is not only applicable for the shielding calculations in e-FLASH radiotherapy facilities, but also for the design optimization through, reduction of trial-and-error iterations and engineering costs.