Tumor persistence and regression in skin carcinogenesis. An experimental study.

A lifetime study in Swiss mice showed that a single 300microgram application of 7, 12-dimethylbenzanthracene (DMBA) once, or 300 or 30microgram DMBA once followed by 3.2 microgram phorbolester for 15 weeks induced a high number of skin tumors many of which regressed spontaneously. The regression occurred mostly early in the experiment, the number of appearing and regressing tumors following a cynical pattern. The incidence of regressions was only to a limited extent associated with tumor size, tumor duration and total number of tumors. Repeated DMBA treatment, 5 microgram twice a week induced the same types of tumors in larger numbers: with a smaller incidence of regressions and only if they were transient in nature, i.e., lasting less than 3 weeks. These occurred in animals which showed a large number of persisting tumors simultaneously, but which rarely displayed multiple regressions. The results indicate the occurrence of multiple steps of neoplastic transformation from hyperplastic lesions, benign regressing tumors, presistent tumors and frankly malignant ones, the incidence as well as biological behavior depending upon inducing treatment.