On the capabilities of conventional x-ray tubes to deliver ultra-high (FLASH) dose rates

Abstract

Purpose: By means of Monte Carlo (MC) simulations and indirect measurements, we have evaluated the maximum dose rates achievable with conventional x-ray tubes and related them to FLASH therapy dose rates of >40 Gy/s. Methods: Monte Carlo models of two 160 kV x-ray tubes, the 3-kW MXR-160/22 and the 6-kW MXR-165, were built in the EGSnrc/BEAMnrc code. The dose rate in a water phantom placed against the x-ray tube surface, located at 3.7 and 3.5 cm from the focal spot for the MXR-160/22 and MXR-165 x-ray tube, respectively, was calculated with DOSXYZnrc. Dose delivered with the 120-kV beam in a plastic water phantom for the MXR-160/22 was measured and calculated. Gafchromic EBT3 films were placed at 15 and 18 mm depths in the plastic water phantom that was irradiated with a low tube current of 0.2 mA for 30 s. Results: The maximum 160-kV phantom surface dose rate was determined to be FLASH capable, calculated as (114.3 ± 0.6) Gy/s and (160.0 ± 0.8) Gy/s for the MXR-160/22 and MXR-165 x-ray tubes, respectively. The dose rate in a 1-cm diameter region was found to be (110.6 ± 2.8) Gy/s and (151.9 ± 2.6) Gy/s and remained FLASH capable to depths of 1.4 and 2.0 mm for the MXR-160/22 and MXR-165 x-ray tube, respectively. The 120-kV dose profiles measured with EBT3 films agreed with MC simulations to within 3.6% for regions outside of heel effect and at both measurement depths; this presented a good validation data set for the simulations of phantom surface dose rate using the 160-kV beam. Conclusions: We have indirectly determined that, with a careful experimental design, conventional x-ray tubes can be made suitable for use in FLASH radiotherapy and dosimetry experiments.