Biological effectiveness of (12)C and (20)Ne ions with very high LET.

PURPOSE To determine the relationship between the relative biological effectiveness (RBE) for cell inactivation and linear energy transfer (LET) in the Bragg peak region of (12)C and (20)Ne ions. MATERIALS AND METHODS Chinese hamster ovary (CHO-K1) cells were exposed to high LET (12)C (33.2 MeV, 20.3 MeV, 9.1 MeV at cell entrance) and (20)Ne ions (56.2 MeV, 34.7 MeV, 15 MeV at cell entrance) and to low LET x-rays. Technical details of the irradiation facility are presented which is based on the Monte Carlo simulation of the lateral spread of heavy ions as a result of the multiscattering small-angle process in physical conditions of the experimental set-up. RESULTS RBE has been measured for LET values close to the Bragg peak maximum, i.e., 440-830 keV/microm for (12)C and for 1020-1600 keV/microm for (20)Ne ions. RBE values at several levels of survival were estimated and were found to decrease with increasing LET. The inactivation cross sections were calculated from the final slope of dose-response curves and were found to increase with increasing LET. CONCLUSIONS The RBE decreases with increasing LET in the range between 440 and 1600 keV/microm for the two types of radiations forming a single line when plotted together, pointing towards LET as the single determinant of RBE. The inactivation cross section describing the killing efficiency of a single particle at the end of particle range comes close to the size of the cell nucleus.