Categorizing Renal Oncocytic Neoplasms on Core Needle Biopsy: a Morphologic and Immunophenotypic Study of 144 Cases with Clinical Follow-Up

Summary There is limited literature on renal oncocytic neoplasms diagnosed on core biopsy. All renal oncocytic neoplasm core biopsies from 2006-2013 were retrospectively reviewed. Morphologic features and an immunohistochemical panel of CK7, c-KIT, and S100A1 were assessed. Concordance with resection diagnosis, statistical analysis including a random forest classification, and followup were recorded. The post immunohistochemical diagnoses of 144 renal oncocytic core biopsies were: favor oncocytoma (67%); favor renal cell carcinoma (RCC) (12%); cannot exclude RCC (21%). Diagnosis was revised following immunohistochemistry in 7% of cases. The most common features for oncocytoma (excluding dense granular cytoplasm) were nested architecture, edematous stroma, binucleation and tubular architecture; the most common features for favor RCC were sheet-like architecture, nuclear pleomorphism, papillary architecture, and prominent cell borders. High nuclear grade, necrosis, extensive papillary architecture , raisinoid nuclei , and frequent mitoses were not seen in oncocytomas. Comparing the pathologist and random forest classification, the overall out of bag estimate of classification error dropped from 23% to 13% when favor RCC and cannot exclude RCC were combined into one category. Resection was performed in 19% (28 cases) with a 94% concordance (100% of favor RCC biopsies and 90% of cannot exclude RCC biopsies confirmed as RCC; 83% of favor oncocytomas confirmed); ablation in 23%; surveillance in 46%. Follow-up was available in 92% (median follow-up 33 months) with no adverse outcomes. Renal oncocytic neoplasms comprise a significant subset (16%) of all core biopsies and the majority (78%) can be classified as favor oncocytoma or favor RCC.