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Data collected from approximately 1800 male and 1800 female Sprague-Dawley (SD) rats used in 104-week carcinogenicity studies were archived in a historical control database at Labcorp Early Development, Inc, and the neoplastic microscopic observation data from these rats were retrospectively evaluated. Historical control data can provide useful information on the range and incidence of spontaneously occurring background neoplasms in the species and strain of the test animal used in different types of toxicity studies, including studies of differing lengths, delivery of test article, and test animal. Some of the most common malignant findings noted included fibrosarcoma of skin/subcutis and thyroid C-cell carcinoma in males (2.1% each) while mammary gland carcinoma and pituitary carcinoma (25% and 2.6%) were most common in females. Pituitary adenoma of pars distalis was found to be the most prevalent benign neoplasm in both males and females (56.4% and 77.1%). Fibroadenoma of mammary gland (35.6%) and thyroid C-cell adenoma (8.5%) were the second and third most common benign tumors in female SD rats. In males, the thyroid C-cell adenoma (10.9%) and benign pheochromocytoma (8.9%) were the second and third most common tumors. [ABSTRACT FROM AUTHOR]