Metanephric adenoma-like tumors of the kidney - Report of 3 malignancies with emphasis on discriminating features

Background.-Metanephric adenoma is a very rare benign renal tumor; only 80 well-documented cases have been reported to date. We have seen several rena l tumors that were originally incorrectly diagnosed as metanephric adenoma. Design.-We present 3 unusual renal tumors (2 primary and 1 metastatic), eac h of which illustrates important pathologic features useful in discriminati ng metanephric adenoma from malignant mimics. Results.-Case 1 involved a 46-year-old man with multiple small, cortical, s olid, papillary (chromophil) renal cell carcinomas in his right kidney; the patient developed multiple, histologically identical, solid, papillary (ch romophil) carcinomas in the opposite kidney 17 months later. Case 2 involve d a 32-year-old woman with a 14-cm right renal tumor who developed soft tis sue and bone metastases over a 17-year period. Case 3 involved a 52-year-ol d woman who presented with a 1.8-cm corticomedullary renal nodule, which ev entually proved to represent a metastasis from a poorly differentiated (ins ular) carcinoma of the thyroid. All 3 tumors superficially resembled metane phric adenoma and consisted of primitive, dark-staining cells arranged in t ubules or sheets. Each tumor, however, also had features inconsistent with the diagnosis of metanephric adenoma, including multifocal lesions with a v ariable nuclear-cytoplasmic ratio and diffuse cytokeratin 7 and epithelial membrane antigen immunopositivity in case 1, a 14-cm-diameter tumor with oc casional mitoses in case 2, and a distinct fibrous capsule with capsular an d vascular invasion in case 3. In addition, all 3 tumors lacked the cytolog ic features of bland overlapping nuclei with imperceptible cytoplasm consis tently seen in metanephric adenoma. Conclusion.-Adherence to strict histopathologic criteria will discourage mi sdiagnosis of a malignant or potentially malignant renal neoplasm as the ra re and always benign metanephric adenoma.