Proliferative lesions in the male reproductive system of F344 rats and B6C3F1 mice: incidence and classification.

The incidences of all primary neoplasms in the testes and accessory reproductive organs of 51,230 male Fischer 344 (F344) rats and 46,752 male B6C3F1 mice were obtained from the pathology data base of more than 300 long-term toxicity/carcinogenicity studies performed for the National Toxicology Program. The overall incidence of reproductive system neoplasms in male F344 rats was 81.5%. The most common neoplasms were interstitial cell adenoma of the testis (76.6%), adenoma/carcinoma of the preputial glands (2.9%), mesothelioma of the tunica vaginalis (1.5%), and adenoma of the prostate gland (0.3%). The combined incidence (0.1%) of 59 other rare neoplasms consisted of 18 different types that occurred with a frequency of 4 or less. In contrast to the rats, male B6C3F1 mice had a low overall incidence (0.6%) of neoplasms in reproductive organs. The most common neoplasm was the interstitial cell adenoma (0.4%). Thirty additional types of neoplasms were identified. Each of these uncommon neoplasms occurred at a frequency of 12 or less with an overall combined incidence of 0.2%. Morphological features are described for neoplasms in the rat and mouse, and criteria are presented for differentiation of testicular interstitial cell adenoma, prostatic adenoma, and mesothelioma from hyperplastic changes.