Effect of dose rate on the induction of experimental lung cancer in hamsters by alpha radiation

The effect of dose rate on the induction of lung cancer in Syrian hamsters by 5.3 MeV ..cap alpha.. particles was examined by varying the number of weekly intratracheal instillations of carrier-free /sup 210/Po. By this technique, most of the radiation dose to the lungs was delivered over intervals ranging from 10 to 120 days. Protraction of exposure over 120 days was slightly more carcinogenic at lower total lung doses (24 rad), but slightly less carcinogenic at higher doses (240 rad), than exposure limited to a 10-day interval. No synergism was observed between very low radiation exposures (2.4 rad) and simultaneously administered benzo(a)pyrene. The carcinogenic effect of a single intratracheal instillation of /sup 210/Po in isotonic saline was markedly enhanced by subsequent weekly instillations of 0.2 ml of saline alone, emphasizing the importance of noncarcinogenic secondary factors in the expression of radiation-induced lung cancer.