Response of Cancer Stem Cells and Human Skin Fibroblasts to Picosecond-Scale Electron Irradiation at 10 10 to 10 11 Gy/s.

Purpose: This study aimed to demonstrate for the first time the possibility of irradiating biological cells with gray (Gy)-scale doses delivered over single bursts of picosecond-scale electron beams, resulting in unprecedented dose rates of 10 ¹⁰ to 10 ¹¹ Gy/s.
Methods and Materials: Cancer stem cells and human skin fibroblasts were irradiated with MeV-scale electron beams from a laser-driven source. Doses up to 3 Gy per pulse with a high spatial uniformity (coefficient of variance, 3%-6%) and within a timescale range of 10 to 20 picoseconds were delivered. Doses were characterized during irradiation and were found to be in agreement with Monte Carlo simulations. Cell survival and DNA double-strand break repair dynamics were studied for both cell lines using clonogenic assay and 53BP1 foci formation. The results were compared with reference x-rays at a dose rate of 0.49 Gy/min.
Results: Results from clonogenic assays of both cell lines up to 3 Gy were well fitted by a linear quadratic model with α = (0.68 ± 0.08) Gy ^-1 and β = (0.01 ± 0.01) Gy ^-2 for human skin fibroblasts and α = (0.51 ± 0.14) Gy ^-1 and β = (0.01 ± 0.01) Gy ^-2 for cancer stem cells. Compared with irradiation at 0.49 Gy/min, our experimental results indicate no statistically significant difference in cell survival rate for doses up to 3 Gy despite a significant increase in the α parameter, which may reflect more complex damage. Foci measurements showed no significant difference between irradiation at 10 ¹¹ Gy/s and at 0.49 Gy/min.
Conclusions: This study demonstrates the possibility of performing radiobiological studies with picosecond-scale laser-generated electron beams at ultrahigh dose rates of 10 ¹⁰ to10 ¹¹ Gy/s. Preliminary results indicate, within statistical uncertainties, a significant increase of the α parameter, a possible indication of more complex damage induced by a higher density of ionizing tracks.
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