Effects of heavy ions on rabbit tissues: alopecia.

During extended investigations of the radiation responses of optic and proximate tissues of New Zealand white rabbits (Oryctolagus cuniculus), the skin surrounding the eyes was exposed to the Bragg plateau regions of 530 MeV/amu Ar ions and 365 MeV/amu Ne ions, and also to 60Co gamma rays. The linear energy transfers (LET infinity s) for the radiations were 90 +/- 5, 35 +/- 3, and 0.3 keV/micron, respectively. Alopecia was measured after irradiation with subjective indices of baldness (0-5) that were calibrated against hair loss in other rabbits irradiated with Ne and Ar ions at the central region of the ear. The patterns of post-irradiation alopecia, which conformed to the model proposed by Casarett (1963) for the long-term expression of radiation damage in proliferating animal tissues, were modified by radiation quality in the following ways: (1) For acute (early) losses, the magnitude of the recovery phase decreased as the LET infinity of the incident radiation increased; (2) In the intermediate or plateau phase of radiation response, the values of relative biological effectiveness (r.b.e) approached those reported for proliferating cells in culture, but differences were found that may be related to the changes in the magnitude of the recovery phase; (3) For a given level of intermediate damage, the onset of the late degenerative phase of radiation response, which was expressed years after irradiation, occurred earlier the higher the LET infinity of the incident radiation. Taken overall, the most important finding of these experiments was that the r.b.e.s for alopecia varied with post-irradiation time.