Spontaneous neoplasms in control Wistar rats: a comparison of reviews

Spontaneous neoplasms in 930 control Wistar rats from five carcinogenicity bioassays conducted between 1990 and 1995 were reviewed and compared with review findings in studies between 1980 and 1990. Mean survival at 104 weeks was 55% for males and 60% for females, similar to that of the previous review. A total of 1599 neoplasms was diagnosed in 361 (78%) male and 415 (89%) female rats; 1293 (81%) of these were benign and 306 (19%) were malignant (11% with metastases). Sixty-eight percent of all neoplasms were in endocrine and integumentary systems, similar to 74% seen in the previous review. Most common neoplasms (affecting > 7% of either sex) were pituitary adenoma (34% of males, 50% of females), benign adrenal pheochromocytoma (10% of males, 1% of females), thyroid C cell adenoma (6% of males, 8% of females), mammary fibroadenoma (3% of males, 36% of females), keratoacanthoma (11% of males, 0.6% of females), testicular interstitial cell tumor (11% of males), uterine stromal polyp (16% of females), pancreatic acinar cell adenoma (13% of males, 0.6% of females), and benign thymoma (3% of males, 8% of females). Seventeen neoplasms affecting 2 to 6.9% of either sex included adrenal cortical adenoma, thyroid follicular adenoma, pancreatic islet cell adenoma, pituitary carcinoma, mammary adenoma, mammary adenocarcinoma, fibroma, fibrosarcoma, dermal papilloma, uterine schwannoma, uterine granular cell tumor, pancreatic acinar cell carcinoma, hepatocellular adenoma, lymphoma, granular cell meningioma, renal mesenchymal tumor, and hemangiosarcoma. Remaining neoplasms occurred in fewer than 2% of animals. Mean tumor incidence did not differ significantly between our two reviews. Ratios of benign to malignant neoplasms were similar in both reviews and percentages of survival at 104 weeks were similar. Between the two reviews, greater than threefold increase in frequency of some neoplasms occurred only in males and included keratoacanthomas, pancreatic acinar cell adenomas/carcinomas, and astrocytomas. Frequencies of remaining neoplasms were within twofold or within 10% of previous frequencies. Some neoplasms diagnosed in this review but not in the previous review included cardiac schwannoma, pilomatrixoma, parathyroid adenoma, and prostatic adenoma but incidence was approximately 1% for any one tumor. Based on these reviews, Wistar rats appear to have a predilection to pituitary neoplasms and mammary fibroadenomas (females).