Two-phase carcinogenesis in hamster buccal pouch.

A two-phase mechanism of chemical carcinogenesis in hamster buccal pouch has been demonstrated. Painting of buccal pouches three times weekly with a 0.1 percent solution of 7,12-dimethylbenz(a)anthracene (DMBA) in mineral oil for 10 weeks resulted in no visible tumors nor any microscopic evidence of dysplasia after the animals had been maintained for a 20 week period (Group 1 animals). Painting with 0.1 percent DMBA for 10 weeks, no treatment for a following 6 week period, and painting with 0.5 percent DMBA for a subsequent 4 weeks resulted in epidermoid carcinomas (Group 2 animals). Painting with 0.5 percent DMBA for 4 weeks in animals without prior treatment resulted in no tumors (Group 3 animals). It is postulated that the early treatment in Group 2 may serve for initiation and the later treatment for promotion.