Effect of oxygen and nanoparticles on human skin and colon cells exposed to synchrotron-based X-ray FLASH beams

FLASH radiotherapy involves delivering relatively high radiation doses at ultra-high dose rates (UHDRs) that are several orders of magnitude greater than those used in conventional radiotherapy (40 Gy s^-1 versus 0.5-5 Gy min^-1, respectively). Previous in vivo studies have shown that doses delivered with such UHDRs result in significant tumour killing while having less effect on normal tissues. Most of these in vivo studies were based on the use of charged particles such as electrons and protons. In this study we exposed cells [human epidermal melanocytes (HEM), MM96L melanoma cells, CCD841 colon epithelial cells and CaCo2 colorectal adenocarcinoma cells] grown in vitro to synchrotron-based X-ray beams delivered at either low dose rates or UHDRs to validate the FLASH effect. The FLASH effect that has been reported to occur under hypoxic conditions was also investigated using HEM and MM96L cells. Significant cell killing was observed at 48 h post-irradiation, when the cells were exposed to high dose (≥10 Gy) UHDRs compared with low dose rate beams in both groups of cells. MM96L melanoma cells were ∼10% less resistant to UHDR than were HEM cells. A similar result was observed in CCD841 and CaCo2 cells. When the hypoxic melanocytes were exposed to (≥10 Gy) UHDRs a minimal loss of cell viability was observed; however, when hypoxic MM96L cells were irradiated, significant cell losses were observed. These results show that a FLASH effect is evident in these skin and colon cells. Moreover, when MM96L melanoma cells were pretreated with 1 mM gold nanoparticles and exposed to 10 Gy UHDR X-rays there was a 50% dose enhancement observed where only 15% was observed at low dose rates.