In this paper we show our preliminary study results in which it was planned to investigate the response of selected cell lines (MCF-7 and MCF-12A) at the protein level, to doses of ionizing radiation typical for conventional and set towards ultra-high radiotherapy in normoxic and hypoxic conditions. The irradiations were performed with a dedicated linear accelerator for intraoperative radiotherapy (IORT). The cells were irradiated with 0,0833 Gy/s and 10 Gy/s dose rates. Western blot analysis was performed. We also investigated the proteins involved in the protection of cells against oxidative stress and the regulation of reactive oxygen species (ROS) – superoxide dismutase 1 (SOD 1), catalase and thioredoxin (Trx). In general, no significant differences were observed between conventional and ultra-high dose rate types of radiotherapy, but comparing the results of cells cultured in normoxia with those cultured in hypoxia, we noted a difference in the level of thioredoxin expression. We will definitely continue our research with a beam with a dose rate of at least 40 Gy/s considered to be required for FLASH radiotherapy but, by this study, we demonstrate that FLASH-related effects could be observed at a lower dose rate.