An Unusual-Appearing Metanephric Adenoma: Features Distinguishing It From Other Small Blue Cell Tumors in the Adult Kidney

Abstract

This article reports a “small blue cell tumor” with an unusual microscopic appearance in an asymptomatic 63-year-old man. Examination of the right partial nephrectomy specimen showed a 5 × 3 × 3-cm mass with fleshy cut surface and focal necrosis. The mass was confined in the kidney with no margin involvement. Microscopically, the peripheral part of the tumor showed bland-appearing small blue cells forming tightly packed small acini, a feature typical of metanephric adenoma. The central portion had larger blue cells arranged as crowded long tubular or somewhat papillary structures. Mitoses and small necrotic foci also were seen occasionally in the central portion. No lymphovascular invasion or capsular involvement was identified. All the tumor cells were positive for keratin markers (AE1/AE3 and CAM5.2). All the peripheral cells and 20% of the central cells were reactive with vimentin. Cells at both the peripheral and central portions stained negatively for neuroendocrine markers (synaptophysin and chromogranin). Differential diagnosis includes several small blue cell tumors that can be found in the adult kidney. The pertinent literature is reviewed.