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Inactivation of human cells exposed to fractionated doses of low energy protons: relationship between cell sensitivity and recovery efficiency.

Within the framework of radiation biophysics research in the hadrontherapy field, split-dose studies have been performed on four human cell lines with different radiation sensitivity (SCC25, HF19, H184B5 F5-1 M10, and SQ20B). Low energy protons of about 8 and 20 keV/micron LET and gamma-rays were used to study the relationship between the recovery ratio and the radiation quality. Each cell line was irradiated with two dose values corresponding to survival levels of about 5% and 1%. The same total dose was also delivered in two equal fractions separated by 1.5, 3, and 4.5 hours. A higher maximum recovery ratio was observed for radiosensitive cell lines as compared to radioresistant cells. The recovery potential after split doses was small for slow protons, compared to low-LET radiation. These data show that radiosensitivity may not be related to a deficient recovery, and suggest a possible involvement of inducible repair mechanisms.