Not All Tumors Are Alike: Varying Efficacy of FLASH Across Tumor Types and Oxygenation Status in Spheroid Models

Objectives Ultra-high dose rate irradiation (UHDR) has been shown to spare normal tissue in various model systems. This study evaluates its potential to sterilize cancer cells using spheroid tumor models. Methods Spheroids from glioblastoma (U87), hypopharyngeal squamous cell carcinoma (two sizes, FaDusmall and FaDularge) and breast adenocarcinoma (T47D) cells were irradiated with electron beams using UHDR (>200Gy/s) or conventional dose rate (CONV,∼0.1 Gy/s) exposures under ambient or reduced oxygen (1%) conditions. U87 and FaDusmall were also irradiated with protons. Spheroids were monitored using imaging for up to 100 days to determine the dose required to cure 50% of spheroids (SCD50). These data were used to calculate dose-modifying factor estimates for UHDR at the 50% survival level (DMFSCD50). Results A total of 3,230 spheroids were analyzed. Under ambient oxygen tension, UHDR and CONV showed no significant differences in U87 (DMFSCD50=0.98, p = 0.47), FaDusmall (DMFSCD50=1.01, p = 0.75), and T47D (DMFSCD50=1.04, p = 0.25), regardless of electron or proton irradiation. Under reduced oxygen levels, significantly higher UHDR doses were required to sterilize the spheroids, with DMFSCD50 1.14 (U87, p < 0.01), 1.07 (FaDusmall, p = 0.02) and 1.13 (T47D, p < 0.01) . FaDularge-spheroids irradiated under ambient oxygen showed a DMFSCD50 of 1.66 (p < 0.001). Conclusion Using spheroid tumor models with long follow-up, we demonstrate that efficacy of UHDR varies across cancer types and conditions. Whereas small spheroids exhibit iso-efficacy, both reduced oxygen tension and increased spheroid size lead to higher DMF. Advances in knowledge This preclinical study suggests that tumor iso-efficacy with UHDR may not hold true for all cancer types and is associated with oxygen level.