Tumor incidence in rat skin after proton irradiation in a sieve pattern

The dorsal skin of 183 male, albino rats was irradiated with 8.0 MeV protons at various doses in a uniform pattern and at two doses in sieve patterns with pore diameters of 5.0, 2.0, and 0.5 mm. The depth-dose curve of the proton beam was modified so that it was approximately linear with a maximum penetration of about 0.8 mm. Skin lesions were noted every 4 weeks for 80 weeks and at the end of the experiment were examined histologically. The results indicated that the sieve pattern did not significantly affect the tumor incidence per unit irradiated area or the incidence of damaged hair follicles at any pore size. This contrasts to the results of earlier experiments with electrons when a sieve pattern produced a significant protective effect at a pore diameter of 2.5 mm. Possible explanations for this discrepancy are discussed.