Incidental detection of oncocytosis in a renal biopsy specimen for idiopathic nephrotic syndrome.

Oncocytosis (oncocytomatosis) is a rare condition characterized by the presence of innumerable oncocytic nodules in one or both kidneys, usually associated with the presence of a dominant nodule. An incidental detection of a renal tumor in a renal biopsy performed for the diagnosis of a medical disease of the kidney is exceptional. We report herein a case of a 58-year-old woman presenting with idiopathic nephrotic syndrome. Histological examination of the percutaneous renal core biopsy revealed a minimal-change glomerular lesion and oncocytosis. A contrast-enhanced abdominal CT scan showed two solid round masses, one in each kidney, measuring 1.5 and 2 cm. The nephrotic syndrome was responsive first to prednisone and finally to cyclosporine. Pathological study of both renal dominant masses confirmed that they were oncocytomas. As far as we are aware, renal oncocytosis incidentally discovered in a biopsy performed for the diagnosis of a medical disease of the kidney has not been reported.