[Renal capsuloma: description of a case with predominantly muscular differentiation].

The Authors report a case of Renal Capsuloma with a histologic biphasic pattern constituted of epithelioid and spindle cells. The lesion is observed in a nephrectomy specimen as incidental finding concurrent with a Papillary Transitional Cell Carcinoma of calices and renal pelvis. The lesion presents a distinct muscular immunophenotype (actin+, desmin+), with coexpression of the melanogenesis marker (HMB45+) in a lower cellular population. This combined immunophenotype is a characteristic feature of the Perivascular Epithelioid Cell's (PEC) lesions. Actually Renal Capsuloma is a PEC's lesion (Pecoma) closely related to Angiomyolipoma (AML). Particularly Capsuloma is thought as a variant of AML predominantly composed by muscular spindle cells (monotypic spindle cell-leiomyoma like variant of AML). In the presented case the spindle cell component is associated with an epithelioid cellular population which also shows a distinct muscular differentiation. The Authors have not found in the literature the association between Renal Capsuloma and Papillary Transitional Cell Carcinoma of the renal pelvis.