Relative biological effectiveness of fast neutrons, observed in the skin reaction of mice, after single or fractionated irradiation

This experiment was carried out to estimate the relative biological effectiveness (RBE) of x rays and fast neutrons in reactions of irradiated skin. Experimental animals used were C/sub 3 H/ He mice, aged 8 weeks. Mean skin reaction scores for early reaction were calculated from the scores obtained from the 8th to 30th day after irradiation, and those for late reaction were from the 2nd to 10th month. The mean energy of fast neutrons was 2 MeV, and x rays was 200 kV with HVL of 1.12 mmCu. The time course of early skin reaction following fast neutrons was essentially the same as that of x rays. However, the durations of late skin reactions produced by fast neutrons were slightly longer than those produced by x rays. The RBE of early skin reactions was 1.9, 2.4, and 2.4 for the mean skin reaction scores of 1.0, 1.5, and 2.0, respectively. The RBE for late skin reactions was higher than that of early reactions and the values were 3.7, 3.8, and 4.0 for the mean skin reaction scores of 0.5, 1.0, and 1.5, respectively. By split dose experiments, D/sub 2/-D/sub 1/ values, which described the recovery of the skin injuries aftermore » irradiation, were estimated in both of early and late skin reactions. For early skin reactions, D/sub 2/--D/sub 1/ values were 500 to 600 rads at 24 hr intervals and 990 rads at 48 hr intervals for xrays, and 75 rads at 24 hrs. and 125 rads at 48 hrs for fast neutrons. In late skin reactions, the D/sub 2/-D/sub 1/ values at 24 hrs were 600 rads for x rays and 100 rads for fast neutrons. Although the recovery rate of irradiated cells was less in the case of fast neutrons than x rays, the extent of late skin reaction for the neutron cases decreased when the interval between split dose was extended up to 5 days. Fractionation schemes in the fast neutron therapy should be carefully planned to minimize late skin damage after irradiation. (auth)« less