The dorsal skin of 261 male albino rats was irradiated with low voltage X-ray exposures of 1200 or 2000 R in a uniform pattern and in sieve patterns with pore diameters of 0.5 or 1.6 mm. The tumor yield was markedly reduced by the sieve pattern exposure particularly with the 0.5-mm pore size, but there was no associated sieve effect on the occurrence of hair follicle damage. When the relatively small radiation exposure of 670 R was administered to the skin between the sieve holes, the sieve effect was largely abolished. The results of this and previous studies suggest that the sieve effect occurs with low LET radiation and may represent a delay in the time of tumor occurrence.

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