Oncocytic papillary renal cell carcinoma: A clinicopathological and genetic analysis and indolent clinical course in 14 cases.

A sort of PRCC with distinct eosinophilic cytoplasm named Oncocytic Papillary Renal Cell Carcinoma (OPRCC) has been increasingly attracting the attention of researchers recently. However, owing to the rarity of OPRCC, the clinicopathological and genetic features of the tumor have still not been well elucidated and whether it should be regarded as an independent subtype of PRCC remains controversial. Herein, a cohort of 14 OPRCCs was studied with the aim of revealing the distinct clinicopathological features, facilitating the classification and correct diagnosis of OPRCC. Men and women each accounted for a half of the cohort with the median age of 64 years old. The majority of patients (9/14) were identified by medical examination and the remaining presented with macroscopic haematuria or lumbar pain. Grossly, tumors were well demarcated and varied from 1.5 to 9cm in diameter. Microscopically, typical OPRCC possessed fine papillary structures with delicate fibrovascular cores, lined with a single layer cell with large, deeply eosinophilic granular cytoplasm and round or polygonal-shaped nucleus exhibiting low nuclear grade in 10 cases (WHO/ISUP grade I-II). Most cases (12/14) possessed hemosiderin-laden and foam-like cells. Focal necrosis presented in 5 cases. Furthermore, solid oncocytoma-like areas appeared in 5 cases and focal sarcomatoid differentiation was identified in 1 case. Immunohistochemically, the majority of tumors presented high expression rates of alpha-methylacylCoA racemase (AMACR), CD10 and vimentin, which were similar to type 2 PRCC. The immune markers including cytokeratin-7 (CK7), KSP-cadherin and EMA exhibited variable positive immunostaining. Genetically, FISH analysis demonstrated trisomy of chromosome 7 in 7 OPRCCs and trisomy of chromosome 17 in 6 OPRCCs. Among 7 male cases, loss of chromosome Y was revealed in 2 cases. Follow-up data was available in 13 patients and only 1 patient died of bone metastasis of the tumor. The other 12 patients were all alive uneventfully at a mean follow-up time of 37 months, indicating that OPRCC is a unique subtype of PRCC with indolent clinical behavior. In conclusion, OPRCCs show a single layer tumor cell of low nuclear grade similar to type 1 PRCC, and abundant eosinophilic cytoplasm resembling type 2 PRCC. Furthermore, the tumor presents the same immunophynotype as type 2 PRCC but the same genetic features and prognosis as type 1 PRCC. OPRCC should be classified as an independent subtype of PRCC with different features from both type 1 and type 2 PRCC.