Papillary and metanephric adenomas of the kidney.

Papillary renal adenomas are the commonest neoplasms of the renal tubular epithelium, occurring in up to 40% of adults. If these criteria are met, the diagnosis of adenoma can be made with confidence: (1) papillary, tubular, or tubulopapillary architecture; (2) diameter less than or equal to 5 mm; and (3) does not histologically resemble clear cell, chromophobe, or collecting duct renal cell carcinomas. They are composed of epithelial cells with variable amounts of cytoplasm with variable tinctorial qualities including clear. Metanephric adenoma and metanephric adenofibroma are closely related neoplasms that appear to be benign. Although they have a morphological resemblance to Wilms' tumor, there is some genetic evidence relating them to papillary adenoma and papillary renal cell carcinoma. Metanephric adenoma and metanephric adenofibroma occur at all ages, have a 2:1 predominance of female patients, and are associated with polycythemia. No instance of malignant behavior has been reported. The adenomatous component is composed of uniform small cells arranged in tubular or papillary architectural patterns. The fibromatous component is moderately cellular and composed of bland spindle cells.