Further evidence that renal oncocytoma has malignant potential.

We report a case of bilateral renal oncocytoma. The right tumor infiltrated the renal capsule focally, and the left tumor penetrated through the capsule into the surrounding perinephric fat and extended into the left renal vein. Karyotyping of the left oncocytoma showed loss of chromosomes 1 and Y (-1,-Y). These karyotypic anomalies, along with the capsular, perinephric fat and renal vein involvement, indicate that renal oncocytomas, despite similar gross and microscopic pathological characteristics, are a heterogeneous group of tumors, some of which have malignant (local invasiveness) potential.