Renal oncocytoma: preoperative diagnosis by aspiration biopsy.

The cytologic, histologic and ultrastructural features of an oncocytoma of the right kidney with the primary diagnosis by thin needle aspiration biopsy are presented. These uncommon tumors are generally considered to be of renal tubular origin and are thought to be benign. In the case reported below, two separate tumor masses were present within the diseased kidney; one of them extended to the perirenal adipose tissue and was accompanied by satellite tumor nodules. This presentation of oncocytoma, associated with focal cellular atypia, suggests that malignant behavior of at least some of these tumors may be anticipated.