Answer: Oncocytic renal neoplasm

Histology: The needle biopsy consists of nests and occasional tubules of cells set within a fibromyxoid background. Cytologically, the cells have very bland uniformly round nuclei without prominent nucleoli. The cytoplasm is abundant eosinophilic having the appearance of oncocytes.

Discussion: The histology in this case is typical of a renal oncocytoma. One of the characteristic architectural patterns of renal oncocytoma is nests or tubules set in a fibromyxoid background as seen in the current case. The key nuclear finding to establish the diagnosis of oncocytoma and rule out a renal cell carcinoma with abundant eosinophilic cytoplasm is that the nuclei in oncocytomas are perfectly round. The nuclei in oncocytomas may show very prominent nuclei. The only deviation from round nuclei seen in oncocytomas is that oncocytomas may contain clusters of cells with pleomorphic nuclei where the nuclei have degenerative atypia consisting of smudgy hyperchromatic chromatin yet lacking the cytologic features of truly malignant nuclei. Although the histological findings on this needle biopsy are classic for oncocytoma, one should not in my opinion definitively establish a diagnosis of oncoytoma on a needle biopsy. We have seen numerous cases and cases have been reported in literature where there are classic features of oncocytoma in some areas of the lesion yet others show chromophobe renal cell carcinoma or papillary renal cell carcinoma. Although the likelihood is that the current lesion is a pure oncocytoma given the findings on needle biopsy, one cannot be entirely certain given the limited sampling. We diagnose these cases as “renal oncocytic epithelial neoplasm” with a comment stating that this lesion histologically has the appearance of oncocytoma yet rarely one can have renal cell carcinomas with oncocytic areas closely resembling oncocytoma. Clinical management in these cases often depends on the radiological findings, co-morbidity, and findings in the other kidney. In a young individual with a solitary renal mass with the needle biopsy findings as seen in the current case, if the lesion is amenable to a wedge resection or partial nephrectomy, often that is what will be performed. In an older individual with either poor renal function or co-morbidity who is a poor operative risk these lesions may either be left alone if the imaging features are typical of an oncocytoma or they may be ablated in vivo using various techniques such as hyperthermia.