Relationship of lung adenoma prevalence and growth rate to acute urethan dose and target cell number

Outbred Swiss Cox mice of both sexes were given single ip injections of 0.5--2.0 mg urethan/g body weight. Lung adenomas began to grow about 7 days after urethan administration. The relationship of plateau tumor level to dose was slightly concave upward. However, the relationship of average tumor number per 10(6) surviving target cells or alveolar type II cells to dose was markedly concave upward. Thus the definition of the urethan dose-lung adenoma prevalence curve should take into account the urethan cytotoxicity. Tumor diameters followed a log normal distribution pattern. The rate of change of geometric mean tumor diameter was independent of sex and urethan dose. In another experiment Swiss Cox mice received ip injections of 1.5 mg urethan/g body weight, and 4 weeks later chronic exposure to urethan in the drinking water was begun. The growth rate of adenomas induced by the single urethan injection was unaffected by subsequent chronic-urethan administration. The results support the common assumption that tumor growth rate is independent of the rate of chronic administration of a carcinogen.