Small-cell variant of renal oncocytoma with dominating solid growth pattern: a potential diagnostic pitfall

Abstract

Dear Editor, Renal oncocytoma is a distinct benign renal tumour accounting approximately for 4% of all renal tumours. This neoplasm is easily recognizable in its classic form— polygonal cells with abundant granular eosinophilic cytoplasm, arranged in tubuloalveolar cystic pattern— whereas diagnostic difficulties may arise when it exhibits small cells with scant cytoplasm and hyperchromatic nuclei (so-called “oncoblasts”) [2]. Although these small cells are rarely detectable in an otherwise classic renal oncocytoma [1, 3], there is the possibility that they may represent the dominating cell component [2]. One of us (MM), by reviewing a large series of renal oncocytomas, found only 3 out of 134 tumours with a prominent (80% of the entire tumours in two cases and 50% in the other case) small-cell component, and the term “small-cell variant of renal oncocytoma” was proposed for these cases [2]. We report a rare case of renal oncocytoma with a predominant small-cell component, accounting for about 80% of the entire tumour. The unusual extensive solid growth pattern of the tumour may represent a potential diagnostic pitfall with primary or metastatic malignant small cell tumours. A 58-year-old woman presented with a vague right-flank pain. Ultrasonography and computerized tomography examination revealed a renal mass, measuring 3.5 cm in greatest dimension and located in the upper pole. A radical nephrectomy with abdominal lymph node dissection was performed. Grossly, a well-circumscribed, nonencapsulated tumour mass, tan in colour, was observed. No central scar or haemorraghic or necrotic areas were seen. Histologically, the tumour was mainly (about 80% of the entire tumour) composed of small cells with scant pale to slightly eosinophilic cytoplasm and small hyperchromatic round-shaped nuclei, arranged in a solid growth pattern (Fig. 1a,b). Only focally, the tumour cells exhibited a tubular and alveolar growth pattern. After an extensive search, in some tumour areas, we observed that neoplastic small cells gradually merged with larger cells with abundant granular eosinophilic cytoplasm similar to oncocytes as seen in classic oncocytoma (Fig. 1c). In other areas, the transition between the small cells and the classic oncocytes was abrupt (Fig. 1d). Only the small foci of tumour were composed exclusively of typical oncocytes (Fig. 1e). Nuclear pleomorphism was focally observed in classical oncocytes, as well as binucleation. Mitoses, necrosis, and nuclear pleomorphism were absent. Perirenal adipose tissue invasion and lymph node metastasis were not seen. The patient is well, without evidence of tumour after a 2-year follow-up. Immunohistochemical analyses revealed a faint immunoreactivity to antimitochondrial antibody MITO-113 (Biogenex) in the small-cell component (Fig. 1f), whereas a stronger staining was observed in typical oncocytes. All neoplastic cells were positive to epithelial membrane antigen and pancytokeratins. No reactivity was obtained with chromogranin A, synaptophysin, CD56, neuron-specific enolase, leucocyte common antigen, CD34 and CD99. Differential diagnosis mainly revolved around malignant small-cell tumours. Among these, metastatic or primary neuroendocrine carcinomas (small cell G. Magro (*) Dipartimento G.F. Ingrassia, Anatomia Patologica, Universita di Catania, Via S. Sofia 87, 95123 Catania, Italy e-mail: g.magro@unict.it Tel.: +39-95-3782024 Fax: +39-95-3782023