RBE-LET relationship for the survival of V79 cells irradiated with low energy protons

The survival of V79 Chinese hamster cells irradiated with proton beams with energies of 0.73, 0.84, 1.16, 1.70 and 3.36 MeV, corresponding to LET values, evaluated at the cell midplane, of 34.5, 30.4, 23.9, 17.8 and 10.6 keV/micron respectively, have been studied in the dose range 0.5-6.0 Gy. As a reference, the survival curve obtained with 200 kV X-rays was used. The initial shoulder, typical of survival curves obtained with sparsely ionizing radiation, decreases as the LET increases and completely disappears at 23.9 keV/micron. This value corresponds to the maximum of the RBE, expressed as the initial slope ratio. In the energy range we have considered, the RBEs for protons are higher than those reported for other ions of comparable LET and the RBE-LET relationship results shifted to lower LET values. Our data seem to indicate that the RBE-LET curve depends on the type of radiation and this could imply that LET is not a good reference for the dose-effectiveness relationship.