RBE for carbon track-segment irradiation in cell lines of differing repaircapacity

Purpose: The LET position of the RBE maximum and its dependence on the cell ular repair capacity was determined for carbon ions. Hamster cell lines of differing repair capacity were irradiated with monoenergetic carbon ions. R BE values for cell inactivation at different survival levels were determine d and the differences in the RBE-LET patterns were compared with the indivi dual sensitivity to photon irradiation of the different cell lines. Material and methods: Three hamster cell lines, the wild-type cell lines V7 9 and CHO-K1 and the radiosensitive CHO mutant xrs5, were irradiated with c arbon ions of different energies (2.4-266.4 MeV/u) and LET values (13.7-482 .7 keV/mu m) and inactivation data were measured in comparison to 250 kV x- rays. Results: For the repair-proficient cell lines a RBE maximum was found at LE T values between 150 and 200 keV/mu m. For the repair-deficient cell line t he RBE failed to show a maximum and decreased continuously for LET values a bove 100 keV/mu m. Conclusions: The carbon RBE-LET relationship for inactivation is shifted to higher LET values compared with protons and alpha-particles. RBE correlate d with the repair capacity of the cells.