Case ReviewsCollision Tumor Composed of Renal Oncocytoma and Mucinous Tubular and Spindle Cell Carcinoma: Not a Rare OccurrenceLin, Michelle S. MD∗; Jacobi, Elizabeth M. MD∗; Mullick, Seema S. MD†; Nguyen, Christopher V. MD‡; Thomas, Jessica S. MD, PhD, MPH∗; Olsen, Randall J. MD, PhD∗; Shen, Steven S. MD, PhD∗; Ayala, Alberto G. MD∗; Ro, Jae Y. MD, PhD∗Author Information From the ∗Department of Pathology and Genomic Medicine, Houston Methodist Hospital, Houston †Houston Methodist Sugar Land Hospital, Sugar Land ‡Trinity Pathology Associates, Tyler, TX. Reprints: Jae Y. Ro, MD, PhD, Department of Pathology and Genomic Medicine, Houston Methodist Hospital, Weill Medical College of Cornell University, 6565 Fannin St, Houston, TX 77030. E-mail: [email protected]. Funding for this work was provided by the Houston Methodist Hospital Department of Pathology and Genomic Medicine. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors have no funding or conflicts to declare. AJSP: Reviews & Reports: 11/12 2020 - Volume 25 - Issue 6 - p 270-275 doi: 10.1097/PCR.0000000000000413 Buy Metrics Abstract 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 review of 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 area with atrophic tubules within 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. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.