Ultra-high dose rate (FLASH) carbon ion irradiation: dosimetry and first cell experiments.

Abstract Purpose To establish a beam monitoring and dosimetry system to enable the FLASH dose-rate carbon ion irradiation and investigate, at different oxygen concentrations, the in vitro biological response in comparison to conventional dose-rate. Methods and Materials CHO-K1 cell response to irradiation at different dose-rates and at different levels of oxygenation was studied using clonogenic assay. The XXX† Heidelberg Ion-Beam Therapy Center (HIT) synchrotron, after technical improvements, was adjusted to extract ≥5 × 108 12C ions within approximately 150 ms. The beam monitors were filled with helium. Results The FLASH irradiation with beam scanning yields a dose of 7.5 Gy (homogeneity of ±5%) for a 280 MeV/u beam in a volume of at least 8 mm in diameter and a corresponding dose rate of 70 Gy s−1 (± 20%). The dose repetition accuracy is better than 2%, the systematic uncertainty is better than 2%. Clonogenic assay demonstrates a significant FLASH sparing effect which is strongly oxygenation-dependent and mostly pronounced at 0.5% O2 but absent at 0% and 21% O2. Conclusion The FLASH dose-rates >40 Gy/s were achieved with carbon beams. Cell survival analysis revealed FLASH dose rate sparing in hypoxia (0.5% - 4% O2).