Tumor induction after local irradiation of mice legs with single doses of carbon ions: an Interim report

(INTRODUCTION) We previously reported tumor induction in mice legs after fractionated irradiation with carbon ions (J.Radiation Res., 46, 185-190, 2005), reporting that the RBE value of carbon ions was 2.9. What puzzles us is that dose responses of carbon ions as well as reference gamma rays are linear without decrease at large doses, which is contrary to other reports using whole body irradiation. (PURRPOSE) We here studied and reported tumor induction in the same radiation conditions/endpoints after single doses. (MATERIALS and METHODS) Right legs of female C3H mice were irradiated with either Cs-137 gamma rays or 290 MeV/u carbon ions of Spread-Out-Bragg peak (SOBP). Dose averaged LET of carbon ions used was 15 (entrance plateau), 45 (proximal SOBP) or 75 (distal SOBP) keV/microm. Number of mice used was 50 for each dose, and a total of 1650 mice were irradiated. Irradiated legs were once a week observed and palpated to detect tumor induction up to 800 days. Incidence of tumor induction was analyzed against doe and time after irradiation. (RESULTTS and DISCUSSION) Tumor induction frequency for any radiation qualities increased with an increase of dose up to 40 Gy. The frequency saturated to increase at higher doses, and rather decreased at 80 Gy of gamma rays and at 60 Gy of 75 keV/microm carbon ions. These results indicate that the irradiate site of leg is the primary target of tumor induction, and not its margin which receives lower doses than prescribed. RBE values of carbon ions were calculated by using a dose to induce 20 % tumor, and were 1.0, 1.2 and 1.9 for 15, 45 and 75 keV/microm, respectively. Maximum frequency of tumor induction by 15 keV/microm carbon ions was lower than that by gamma rays. Time between irradiation and tumor appearing depended on dose except 45 keV/microm carbon ions, and was longer for low dose groups than high dose groups. (Conclusion) Carcinogenic effects of carbon ions are quantitatively and qualitatively different from gamma rays. Low LET carbon ions could be less dangerous than gamma rays.COSPAR Colloquium : Mutagenic consequences of the space environmen