FLASHradiotherapy using high-energy X-rays: Critical dosimetry design and characteristic of FLASHradiation platform based on compact LINAC

Compared with conventional dose-rate radiotherapy (CONV-RT), ultrahigh dose-rate radiotherapy (FLASH RT) can reduce damage to normal tissues while effectively killing cancer cells. To meet clinical requirements and hospital installation requirements, developing a room-temperature compact FLASH linear accelerator is crucial. The FLASH-RT platform, built by the Institute of Applied Electronics, China Academy of Engineering Physics, and Zhongjiu Flash Medical Technology Co., Ltd., can produce ultrahigh dose rate X-rays. In this study, we measured the dose in an experimental platform and verified our design. EBT4 films were mounted in solid water to measure both the percentage depth dose (PDD) curves and dose distribution profiles for various radiation field sizes. Based on the experimental platform and initial beam parameters, a Monte Carlo model was established using the Monte Carlo N-particle transport code MCNP6 (Version 6) to optimize the conversion target thickness and measure the PDDs and beam profiles. To meet the requirements of users for different dose rates and perform the same machine experiment, the possibility of several dose rate adjustment methods was analyzed using the Monte Carlo model. The results showed that the maximum mean dose rate was over 330 Gy/s and the in-pulse dose rate was approximately 7500 Gy/s at a surface-source distance (SSD) of 50 cm on the X-ray FLASH-RT experimental platform, which was much higher than the conditions for triggering the FLASH effect (≥40 Gy/s). The discrepancy between the Monte Carlo simulation results and the experimental data was less than 3% for all PDD curves and profiles. Furthermore, gamma analysis (2 mm/2% criteria) demonstrated pass rates exceeding 95%, confirming that the results were well within the clinically acceptable tolerances. To optimize the thickness of the conversion target material, the average dose obtained by combining the principle of maximum conversion efficiency was increased by approximately 12.7% compared to the Continuous Slowing Down Approximation (CSDA) range. The method of adjusting only the average dose rate is recommended to change the pulse frequency, and the method of changing the average and instantaneous dose rates simultaneously involves adding the corresponding thickness of the shielding material before the secondary collimator. This dose rate range meets the requirements for conducting CONV-RT and FLASH-RT on the same machine, minimizing experimental biological variability.