Collision Tumor Composed of Renal Oncocytoma and Mucinous Tubular and Spindle Cell Carcinoma: Not a Rare Occurrence

Primary renal collision tumors have been rarely reported in the literature. Our institution reported the first case of a renal collision tumor that comprised oncocytoma and mucinous tubular and spindle cell carcinoma (MTSCC). In this study, upon reviewof this index case and a second case encountered in our practice, we performed molecular characterization of each component using next-generation sequencing, which did not detect any somatic mutations. In addition, we reviewed resection cases of oncocytomas for previously undetected MTSCC components. While no additional definitive oncocytoma-MTSCC collision tumor cases were identified, 2 cases with entrapped tubules within the central scar of the oncocytoma that possessed some features of MTSCC were encountered, raising the differential diagnosis of oncocytoma-MTSCC collision tumors versus entrapped renal tubules. We propose the following histomorphologic criteria to distinguish a true MTSCC component: (1) expansile mass lesion greater than 0.5 cm; (2) dense proliferation of tubules lined by plump cells; and (3) background of loose myxoid collagenous stroma. In contrast, entrapped tubules tend to form a smaller, stellate-shaped areawith atrophic tubuleswithin a dense collagenous stroma. Nevertheless, the occurrence of the 2 described cases provides evidence that the incidence of this collision tumor may not be extremely rare. Therefore, we recommend careful examination and generous sampling of oncocytomas, particularly from the central scarred region or in cases exhibiting atypical gross appearance. Further studies with additional cases are required to better document the relationship and molecular pathogenesis of MTSCC from the central scars of oncocytomas.