Mouse skin was irradiated with either cyclotron-accelerated helium ions or 230 kV X-rays. Single-dose exposures to helium ions were performed in the plateau region and in the Bragg peak region of ionization after modification of the peak region by insertion of a moving copper ridge filter into the beam path ("spread-out" Bragg peak). The degree of recovery occurring between equal fractions of helium ions (using the spread-out Bragg peak) given with an interval of 24 hr was compared to recovery after X-irradiation also with a 24-hr interval. Single doses of helium ions in the plateau portion of ionization gave skin responses not significantly different from X-ray responses, as determined by comparing average skin responses over the period of 8-30 days postirradiation. Helium ion irradiations in the spread-out Bragg peak region of ionization yielded a relative biological effectiveness of about 1.3 as compared to that yielded by X-irradiation. Recovery after helium ion irradiation in the Bragg peak region was not statistically different from X-ray recovery values, which were about 500 rads. Evaluation of late skin reactions (95-105 days postirradiation) indicated that the late RBE of spread-out Bragg peak helium ions was 1-1.2 and the RBE of plateau helium ions was 1.0. Comparison of split doses of spread-out Bragg peak helium ions and X-rays showed that the skin reaction dose responses were not significantly different at 95-105 days postirradiation, when RBE for single doses was allowed for.

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