Abstract
Ultra-high dose rate radiotherapy defined as FLASH radiotherapy is a potential technology to improve local tumor therapeutic gain ratio. It relies on linear accelerator capable of delivering large doses in a single microsecond pulse (>40 Gy/sec). This therapy would lead to sparing of normal tissue which has been termed the FLASH effect. As significant reduction of radiation-induced toxicity, a greater dose of FLASH radiotherapy could be administered in tumor region. Some evidences prove the relation between FLASH effect and oxygen. Yet, the underlying physicochemical and biological mechanism remain to be fully demonstrated. The current hypotheses that may explain the normal and tumor tissue different response were We summarized and the future direction of study and clinic implementation was proposed.