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Spontaneous Lipomatous Tumors in the Kidney of the Crl:CD(SD)BR Rat

Primary tumors of the rat kidney are uncommon (0.05%) and are most of ten seen in older rats. Nephroblastoma is the most commonly diagnosed tumor, but benign and malignant tumors of epithelial cell and mesenchymal cell origin occur. Lipomatous tumors are occasionally seen, especially in aged rats, and are controversial lesions. They have been variously classified in the literature in lipomas, liposarcomas, and lipomatous hamartomas. In my experience, the range of diagnoses has also included post-inflammatory change and lipidosis. The lipomatous tumors are the focus of discussion for the purpose of defining criteria for the various diagnostic categories. The implications of these diagnostic categories for safety assessment are also discussed.