Myxoma of the renal capsule.

Myxomas are uncommon soft-tissue neoplasms, which are extremely rare in the kidney, with only five cases documented in the intraparenchymal location. However, renal capsular myxoma has not yet been reported. We describe a unique case of a clinically detected renal myxoma arising in the capsule. A 37-year-old man receiving treatment for epididymitis sought medical assistance for infertility. A radiological examination incidentally discovered a right renal tumor. The mass intruded into the perirenal tissue and measured 6 cm in major diameter. The resected kidney contained a well-circumscribed gelatinous capsular tumor. It was composed of sparse, bland, slender, spindle-shaped cells scattered in large amounts of basophilic interstitial mucoid material. The tumor cells showed diffuse immunoreactivity for vimentin. Occasional cells stained for alpha-smooth muscle actin and calponin. Reactivity was negative for S100 protein, epithelial membrane antigen, pancytokeratin, neurofilament protein, and h-caldesmon antibodies. Ultrastructural examination revealed fibroblast-like cells with long thin cytoplasmic processes, prominent rough endoplasmic reticulum, a well-developed Golgi complex, and secretory vesicles. No basal lamina was identified around the tumor cells. The differential diagnosis includes many other benign and malignant soft-tissue lesions exhibiting prominent secondary myxoid features. It is important to consider a renal capsular myxoma when examining lesions at this anatomic site to avoid misdiagnoses and to ensure that the patient receives appropriate treatment and prognostic information.