Persistence and growth of rat liver neoplastic nodules following cessation of carcinogen exposure.

Altered (hyperplastic) foci and neoplastic (hyperplastic) nodules identified by their resistance to iron accumulation were induced in the livers of F344 rats by limited feeding of N-2-fluorenylacetamide for three, four, or five cycles of 4-week intervals separated by 1 week of a basal diet. Foci were present by the end of three feeding cycles and increased in number with further carcinogen exposure. No nodules were present at the end of three or four cycles, but they appeared at later intervals after removal of the carcinogen. Nodules were present at the end of five cycles of feeding and increased in number later. Thus nodules were found to be persistent and to have the progressive growth ability in situ that is characteristic of neoplasms.