High dose proton FLASH irradiation under hypoxic conditions results in reduced DNA damage in normal pancreatic cells

Ultra-high dose rate (UHDR) irradiation spares normal tissues while achieving similar tumour control compared to conventional dose rate (CONV) irradiation in a preclinical setting. However, the underlying biological mechanisms remain unclear. In this study, we examined the relationship between DNA damage and UHDR proton irradiation in both normal and cancerous pancreatic cells.

Normal human pancreatic cells (H6c7) and human pancreatic adenocarcinoma cells (PANC-1) were exposed to 3 and 15 Gy of 4 MeV protons (LET = 10 keV/µm) at 0.025 Gy/s (CONV) or 375 Gy/s (UHDR) under normoxic (21% O2) or hypoxic (1% O2) conditions. DNA damage was assessed by yH2AX immunofluorescence and by the alkaline comet assay.

Following 3 Gy irradiation, no significant differences in DNA damage were found between UHDR- and CONV-irradiated cells. In contrast, 15 Gy of UHDR irradiation significantly reduced DNA damage in H6c7 cells compared to CONV irradiation under hypoxia but not normoxia. No statistically significant FLASH sparing was found in PANC-1 cells irradiated with 15 Gy under either normoxic or hypoxic conditions.

UHDR proton irradiation shows potential in reducing radiation-induced DNA damage in normal but not cancer cells compared to CONV proton irradiation at relatively high doses and low oxygen levels.

This is the first study that compared the responses of normal and cancerous cells-from the same background-to UHDR and CONV proton irradiation. In addition, our data confirm the importance of dose and oxygen concentration for observing the FLASH effect in normal cells.