Determination of RBE of 450 MeV/nucleon carbon ions using the micronucleus test and survival of mice after irradiation in different regions of the Bragg curve

Abstract

Purpose Determination of the value of relative biological effectiveness (RBE) of heavy charged ions in vivo is an important task for their optimal use in particle radiotherapy. The aim of this study was to determine the RBE value of a beam of carbon ions with an energy of 450 MeV/nucleon in different regions of the Bragg curve in irradiation of mice at low, medium, and high doses in comparison with X-ray radiation. Materials and methods SHK mice (n = 330) were irradiated in three regions of the Bragg curve in the dose range of 0–1.5 Gy for cytogenetic damage detection and at a dose of 6.5 Gy for determination of 30-day survival. For irradiation of mice in the Bragg peak, two widths of a spread-out Bragg peak (SOBP) were used: 10 mm (LET ∼100 keV/µm) and 30 mm (LET ∼39 keV/µm). Results The RBE value was 0.8–0.9 before the Bragg peak (LET ∼15 keV/µm) and 0.8 after the peak (LET ∼5 keV/µm), and did not depend on the determination method, despite the differences in LET values. The RBE value determined by the micronucleus test was 1.1–1.7 for the 10-mm-wide SOBP and 1.0–1.3 for the 30-mm-wide SOBP, with the highest RBE value obtained in the low-dose region upon irradiation of mice in the 10-mm-wide Bragg peak. The RBE values in the high-dose region determined by the 30-day survival test lay in the range from 1.4 to 2.6 depending on the width of the Bragg peak and the chosen criterion for calculating the value. The RBE values in the 10-mm-wide Bragg peak (LET ∼100 keV/µm) were higher than those in the 30-mm-wide Bragg peak (LET ∼39 keV/µm) at all used criteria. Conclusions The present findings suggest that there is the complex relationship between LET and organism response to accelerated charged particle radiation, and the contribution of specific factors and mechanisms must be further considered.