My thesis work concerns the study of FLASH radiotherapy. Recent in vivo results have shown prominent tissue sparing effect of radiotherapy with ultra-high dose rates (FLASH) compared to conventional dose rates (CONV). To nowadays several hypotheses are being investigated to correctly explain the FLASH effect. It has been suggested, among the most relevant theories, that this apparently beneficial effect is due to differences in oxygenation in tumor and healthy tissues. The differential response between FLASH-RT and CONV-RT may be due to radiochemical oxygen depletion at very high doses and the resulting radioresistance conferred on the irradiated tissue. It is largely accepted that hypoxic tissues are more radioresistant than well-oxygenated tissues.