Effect of dose rate on antitumor activity in hypoxic cells by using flattening filter free beams

Background: Recently, the Fla ening Filter Free (FFF) beams, which allow a dose rate increase of up to four mes compared to the normal dose rate, have been incorporated into radia on therapy machines. The aim of this study is to evaluate an tumor ac vity in hypoxic cells irradiated with different dose rates using FFF beams and to iden fy the casual mechanism for cellular damage during irradia on. Materials and Methods: EMT6 cells were treated with 95% N2 and 5% CO2 to maintain a hypoxic condi on. Three dose rates, namely, 6.27 Gy/min, 12.00 Gy/ min, and 18.82 Gy/min, were used to deliver the prescribed dose of 2 to 4 Gy using the TrueBeam linear accelerator. The number of colonies was counted to evaluate the cell surviving frac on. To inves gate the mode of ac on, addi onal experiments to detect reac ve oxygen species (ROS) by aminophenyl fluorescein (APF) assay, and DNA double-strand breaks (DSBs) by γH2AX assay were performed. Results: Irradia on of hypoxic cells using FFF beams increases an tumor ac vity as a func on of dose rate. The fluorescence of the APF assay was significantly increased when high dose rates were used. In addi on, results from our γH2AX assays show that the number of DNA DSBs increased as a func on of dose rate, in hypoxic cells. Conclusion: We demonstrate that there is a significant dose rate-dependent difference in an tumor ac vity in hypoxic cells, when FFF