RENAL ONCOCYTOMA - A CLINICOPATHOLOGICAL STUDY OF 70 CASES

We reviewed 954 primary nonurothelial epithelial renal neoplasms with primary resection at Memorial Sloan-Kettering Cancer Center between th e years 1980 and 1995 and classified 70 cases (7%) as renal oncocytoma s. The study population was composed of 39 men and 31 women, and the m ean age was 65 years (range 25 to 86 years). Fifty-six patients (80%) were asymptomatic at presentation, six (4%) had flank pain, six (4%) p resented with a mass, and two (3%) had hematuria. Sixty-one were treat ed with total or radical nephrectomy, nine with partial nephrectomy. T he right kidney was involved in 35 cases (50%), the left kidney in 32 (46%). Three cases (4%) were bilateral. Sixty-one cases (87%) were uni focal, nine (13%) multifocal. All the tumors were well circumscribed b ut unencapsulated. Forty-five (64%) were described as brown or red, wh ereas the remainder were variously described as tan to yellow. Central fibrosis or scar was described in 23 cases (33%), and gross areas of hemorrhage or cystic changes in 14 (20%). The mean size was 5.2 cm and median 5.0 cm (range 1.5 cm to 14 cm). Histologically, the tumors wer e characterized by a mixture of architectural patterns: compact cellul ar nests and acini embedded in a hyalinized, hypocellular stroma were present in 62 cases (89%), a solid nested architecture in 47 cases (67 %), and a variable tubular component in 50 cases (71%). Small papillae , pseudopapillae, and intratubular epithelial tufts were seen in 19 ca ses (27%). Cytologically, the neoplasms also showed a mixture of cell types, the most common being the classic oncocyte, which consisted of round or polygonal cells with moderate to abundant granular, eosinophi lic cytoplasm, and small round nuclei with evenly dispersed granular c hromatin. Small basophilic nucleoli were visible in many of these cell s in all cases. Thirty-one cases (44%) had a variable number of oncocy tic cells with pyknotic nuclei and 20 (30%) contained clusters of smal l cells with a high nuclear/cytoplasmic ratio and dense hyperchromatic nuclei (so-called oncoblasts). Foci of tubules with clear cells embed ded in a hyalinized stroma were present in six cases (9%). Cellular at ypia was evident in 42 cases (60%) and was marked in 21 (30%). Eleven cases (16%) exhibited mitotic activity, albeit low. No case had atypic al mitoses or necrosis. Twenty-two cases (31%) had areas of calcificat ion within the hyalinized stroma, 12 (17%) had calcospherites, and thr ee (4%) had osseous and myeloid metaplasia. Vascular invasion was pres ent in three cases (4%), and invasion of perinephric fat in 14 (20%). One patient presented with liver metastasis. Fourteen cases (20%) were pT1, 42 (60%) pT2, and 14 (20%) pT3. After a mean follow-up of 58 mon ths (range 1 to 181), 62 patients (89%) were alive with no evidence of tumor, six (9%) had died of other causes, one was alive with stable m etastatic disease in the liver 58 months after diagnosis, and one died with metastatic disease to bone and liver. We conclude that renal onc ocytomas have a varied morphologic appearance and their pathologic dia gnosis should be based on a constellation of architectural and cytolog ic features. The overwhelming majority of cases behave in a benign fas hion, although in rare instances they can metastasize. The presence of atypical morphologic features do not alter the excellent prognosis as sociated with oncocytomas and do not predict an aggressive clinical co urse.