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

Abstract

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 ¹²C and ²⁰Ne ions.

Materials and methods: Chinese hamster ovary (CHO-K1) cells were exposed to high LET ¹²C (33.2 MeV, 20.3 MeV, 9.1 MeV at cell entrance) and ²⁰Ne 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/μm for ¹²C and for 1020–1600 keV/μm for ²⁰Ne 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/μm 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.

Keywords:

• Relative biological effectiveness (RBE)
• linear energy transfer (LET)
• Bragg peak
• heavy ions
• CHO-K1 cells