Answer: Inverted papilloma

Histology: At low magnification, this lesion appears to have a relatively solid pattern with a focal papillary component at the surface. In areas, the tumor cells show numerous whorls of cells reminiscent of squamous eddies, as seen in irritated seborrheic keratoses. Cytologically, the lesion is comprised of bland urothelial cells with longitudinal grooves, typical of benign urothelium. Mitotic figures are not identified. Focally, the tumor is composed of anastomosing cords of cells with peripheral palisading and spindling of nuclei parallel to the cords within the center of the cords.

Discussion: Areas of the tumor composed of cords of cells with peripheral palisading and spindling of the nuclei in the center of the cords are classic for inverted urothelial papilloma. The presence of focal papillary findings is still consistent with inverted papilloma and represents more of a polypoid projection of the overlying epithelium than true papillary formation. The unusual nature of this case is the solid appearance within the majority of this lesion which mimics urothelial carcinoma. The finding of squamous eddies is something typically seen in low grade urothelial neoplasms. It is not specific for urothelial papilloma but may also be seen, for example, in papillary urothelial neoplasms of low malignant potential. Inverted urothelial papillomas are benign. Although the association of inverted papilloma with urothelial carcinoma is controversial, the most recent evidence suggests an association such that following the removal of inverted papilloma, patients may undergo cystoscopic surveillance.