Factors Influencing the Biological Effects of FLASH Irradiation

Among the methods for increasing the specificity of tumor radiotherapy, FLASH radiotherapy (FLASH-RT) stands out, having recently entered clinical trials. A distinctive feature of this treatment method is the delivery of a therapeutic dose in a fraction of a second with a typical mean dose rate greater than 40 Gy/s. In addition to improved patient comfort and a shorter hospital stay, this therapy potentially carries a lower risk of radiation-related side effects due to reduced damage to normal tissues. Numerous preclinical and in vivo laboratory trials of FLASH-RT have demonstrated that, in addition to reducing the severity of radiation-related complications, FLASH radiotherapy has antitumor efficacy similar to conventional radiotherapy. Partly reduced radiotoxicity after such a dose rate delivery obtained, in a broader radiobiological sense, an eponymous term FLASH effect. Although the first clinical trials aimed to evaluate the safety and efficiency of FLASH-RT against bone metastases (FAST-01/02), melanoma skin metastases (IMPulse, Flash-Skin I), Squamous Cell Carcinoma, or Basal Cell Carcinoma (LANCE) have already started or even finished and showed promising results (FAST-01), the radiobiological basis of the FLASH effect is far from a complete explanation. The fundamental factors explaining the nature of the FLASH effect are mainly considered to be the following: (1) changes in the balance of water radiolysis products and a decrease in the generation of stable reactive oxygen species (ROS), (2) differential oxygen depletion, depending on the initial oxygen concentration in tissues, and (3) physiological and metabolic, gene expression and probably epigenetic shifts in response to irradiation in normal and tumor cells. The main purpose of this review is the systematization of the radiobiological manifestations of the FLASH effect together with a consideration of the elementary processes laying in the basis of the FLASH effect in order to actualize rationale and future application developments of FLASH-RT.