title: Bioassay of 1,1,2-trichloroethane for possible carcinogenicity.
abstract: A bioassay of technical-grade 1,1,2-trichloroethane for possible carcinogenicity was conducted using Osborne-Mendel rats and B6C3F1 mice. 1,1,2-Trichloroethane in corn oil was administered by gavage, at either of two dosages, to groups of 50 male and 50 female animals of each species, 5 days a week for a period of 78 weeks, followed by an observation period of up to 35 weeks for rats and up to 13 weeks for mice. The high and low time-weighted average dosages of 1,1,2-trichloroethane were, respectively, 92 and 46 mg/kg/day for male and female rats, and 390 and 195 mg/kg/day for the male and female mice. For each species, 20 animals of each sex were placed on test as vehicle controls. These animals were gavaged with corn oil at the same rate as the high dose group of the same sex. Twenty animals of each sex were placed on test as untreated controls for each species. These animals were not intubated. No neoplasms were observed at statistically significant incidences in male or female rats. In both male and female mice, administration of 1,1,2-trichloroethane was associated with a significantly increased incidence of hepatocellular carcinomas. Hepatocellular carcinomas were observed in 2/17 (12 percent) untreated control males, 2/20 (10 percent) vehicle control males, 18/49 (37 percent) low dose males, and 37/49 (76 percent)high dose males. Hepatocellular carcinomas were also observed in 2/20 (10 percent) untreated control females, 0/20 vehicle control females, 16/48 (33 percent) low dose females, and 40/45 (89 percent) high dose females. Both the Fisher exact test comparing tumor incidences of dosed to control groups and the Cochran-Armitage test for positive dose-related trend indicated a highly significant (P<0.001) association between hepatocellular carcinomas in all mouse groups and the administration of 1,1,2-trichloroethane. A positive dose-related association between administration of 1,1,2-trichloroethane and the incidence of pheochromocytoma of the adrenal gland was indicated by the Cochran-Armitage test for mice of both sexes. Fisher exact tests confirmed these results for high dose female mice but not for other mouse groups. There were no other neoplasms for which statistical tests indicated a positive association between dosage and tumor incidence in mice. The results of this study do not provide convincing evidence for the carcinogenicity of 1,1,2-trichloroethane in Osborne-Mendel rats. Under the conditions of this bioassay 1,1,2-trichloroethane is carcinogenic in B6C3F1 mice, causing hepatocellular carcinomas and adrenal pheochromocytomas.