Historical data are presented for neoplasms and related proliferative lesions from 1,170 Sprague-Dawley rats that served as controls in 9 carcinogenicity (2 year) studies conducted in the Safety Evaluation Facility of Ciba-Geigy Corporation, Summit, New Jersey. The most common neoplasm was pituitary adenoma, which occurred in 62.2% of the male and 84.7% of the female rats. Incidences of other neoplasms that occurred in more than 6.0% of the rats were, for males, benign pheochromocytoma (19.0%), cutaneous keratoacanthoma (7.9%), pancreatic islet cell adenoma (7.5%), benign testicular interstitial cell tumor (6.5%), and thyroid C-cell adenoma (6.5%). For females these incidences were mammary fibroadenoma (31.3%), mammary adenocarcinoma (16.8%), and mammary adenoma (6.5%). Focal cortical hypertrophy/cystic degeneration of the adrenal, a focal nonneoplastic lesion of zona fasiculata cells that often degenerate into large cysts, was present in 23.4% of all male and 82.7% of all female rats. Criteria for the differential diagnoses of selected neoplasms and related lesions are presented.
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