Abstract
A large renal mass was identified by ultrasound during a regular health check on an intact adult female chimpanzee (Pan troglodytes). Cytology revealed a neoplastic population of clusters of polygonal cells with distinctive pink, granular cytoplasm. The primary differentials were oncocytoma and renal chromophobe cell carcinoma. The mass was surgically resected and grossly it effaced >75% of the right kidney and had a central scar. Histology confirmed an oncocytoma based on characteristic morphology, diffuse expression of cKIT (CD117), and limited expression of CK7, both assessed via immunohistochemistry. Transmission electron microscopy identified the granular component of the cytoplasm as mitochondria. Renal oncocytoma has not been reported previously in a great ape, to our knowledge. Our case appears nearly identical to the frequently reported and well-characterized human counterpart.