Correlation between gross observations of tumors and neoplasms diagnosed microscopically in carcinogenesis bioassays in rats.

Microscopic diagnoses of spontaneous and induced neoplasms in F344 rats in two carcinogenesis bioassays were correlated with the gross observations made at the time of necropsy. The results for a negative and a positive compound with reference to carcinogenicity, indicate that of the gross descriptions indicative of tumors, 37 percent and 24 percent respectively, were not diagnosed as neoplasms microscopically. However, some of the gross lesions were diagnosed histologically as nonneoplastic lesions. Of the neoplasms diagnosed microscopically only 70 percent and 76 percent, respectively, were observed grossly. Endocrine tissues, liver and lung had the lowest correlation rates. The implications of these findings are discussed.