Metanephric adenoma of the kidney. A clinicopathological, immunohistochemical, flow cytometric, cytogenetic, and electron microscopic study of seven cases.

We report seven examples of a distinctive adenomatous tubular cortical neoplasm of the adult kidney. The average age of the patients (six women and one man) was 48.6 years (range, 38-64 years). In six patients the tumor was discovered during investigation of unrelated conditions, and all were treated with total nephrectomy. One tumor was found at autopsy. The tumors were well-circumscribed, nodular, tan-pink masses localized to the kidney, ranging in size from 0.6 to 8 cm. Histological examination demonstrated orderly, closely packed, small round tubules lined by bland, darkly staining oval cells with little cytoplasm, merging with rounded nests of similar cells. Occasional branching, elongated tubules, and papillary infoldings of glomeruloid-like bodies were present but blastema was absent. The tumor cells were immunoreactive for Leu 7 (three of five cases) and vimentin (four of six cases), and a few tumors were immunoreactive for cytokeratin (two of six cases), epithelial membrane antigen (one of six cases), and muscle-specific antigen (one of six cases). Ultrastructural examination of two tumors revealed tubular and solid nests of epithelial cells surrounded by basal lamina, with prominent cell junctions, microvilli, and apical secretory granules. DNA content analysis by flow cytometry yielded diploid histograms (four of four cases). Cytogenetic analysis of one case revealed a normal male karyotype. Clinical follow-up, available for six patients, revealed no evidence of recurrence (mean follow-up, 60.8 months). We believe this is a benign tumor, best classified as a metanephric adenoma because of its embryonic architectural and cytological appearance, that can be recognized by its very characteristic pathological features.