Radiation treatments for cancer promise a targeted approach that treats tumors while avoiding irradiation of healthy tissue. Such treatment requires a careful calibration of the radiation dose, with excellent spatial resolution, using a measurement device (dosimeter) that ideally mimics human tissue (i.e., water) and does not affect the path of the radiation. By exploiting Hydrated electron dosimetry we propose to create a highly sensitive, and fast dosimeter with water as the sensing medium. Hydrated electron dosimetry consist of measuring absorbed radiation dose to water by monitoring concentration of hydrated electrons, which are short-lived radicals produced by the radiolysis of water – using absorption spectrophotometry. In this phase of the study we will asses the suitability of the technique in the very low dose-per-pulse regime for conventional radiotherapy as well as for the FLASH radiotherapy. Radiation hardness of all the involved material will be investigated in detail. In addition, software to calculate the radiation chemical yield for hydrated electrons, which is the number of chemical entities produced by ionizing radiation, as well as software for automatic acquisition, analysis and reporting of the absorbed dose will be developed. Future work will focus on miniaturizing this technology into a fiber-coupled micro-cavity for the purpose of an in vivo radiotherapy dosimeter – evaluation of the dose delivered to patient during course of treatment.