Inverted urothelial papilloma of the urinary bladder with focal papillary pattern: a previously undescribed feature

We report 2 examples of inverted urothelial papillomas with a focal papillary pattern. Both patients, a 43-year-old man and a 13-year-old adolescent boy, presented with hematuria. In addition to the characteristic trabecular endophytic growth pattern and bland cytologic features, the tumors showed focal papillary architecture in the endophytic component. Because of this feature, both tumors were misinterpreted as papillary urothelial carcinoma with inverted pattern. However, the papillary fronds were similar to those of exophytic urothelial papilloma. They were lined by 3 to 8 layers of normal-appearing urothelial cells often covered by a continuous or discontinuous layer of superficial (umbrella) cells. Although follow-up is limited, the 2 patients have remained asymptomatic. The recognition of papillary structures in urothelial inverted papilloma broadens the morphological spectrum of this unusual benign urothelial neoplasm and complicates the microscopic interpretation of urothelial lesions with inverted growth patterns. Surgical pathologists should be aware of this unusual feature of inverted urothelial papilloma of the urinary bladder to avoid misinterpretation with urothelial carcinoma with an inverted pattern.