The clinicopathological, ultrastructural, genetic features and diagnosis of small cell variant renal oncocytoma

The small cell variant renal oncocytoma is until now a rarely described and easily misdiagnosed subtype of renal oncocytoma. The tissue morphology, immunohistochemical profile, ultrastructural and molecular characteristics of four cases of small cell variant renal oncocytoma were analyzed and the literature was reviewed. The patients were three women and one man with ages ranging from 51 to 76 years. The size of the tumors ranged from 3 to 8.5 cm in diameter. The follow-up duration ranged from 6 to 58 months. All patients lived uneventfully without tumor recurrence or metastasis. The tumors were grayish yellow to brown, well demarcated, with a central scar or cystic change. Microscopically, the tumor was arranged in lobular structure containing dense small acini or tubular structures, with small cells featuring weak eosinophilic and scant cytoplasm, small round nuclei and inconspicuous nucleoli. No mitotic figures or necrosis were discerned. The immunohistochemical profile of small cell variant of renal oncocytoma is partially consistent with classic oncocytoma, expressing EMA, CK18, CD117 and E-cad. However, MITO and S-100A1 were intensively expressed in classic RO, but neither of them was expressed in small cell variant RO. Ultrastructurally, a small number of organelles was revealed in the tumor cell, including a few mitochondria, lysosomes and microvilli, less than those in the classic oncocytoma. No genetic aberrations were found in all cases regardless of clinicopathological characteristics and tissue types. Small cell variant renal oncocytoma of the kidney is frequently difficult to be differentiated from other benign and malignant small cell tumors with eosinophilic cytoplasm. The immunohistochemical profile, ultrastructural and genetic features of the tumor are integrally presented here for the first time. Acquaintance with the special characteristics of the tumor could facilitate the correct diagnosis of the tumor.