Answer: High grade papillary urothelial carcinoma with lamina propria invasion

Histology: The lesion consists of atypical, pleomorphic urothelial cells with exophytic growth. Abundant mitoses and areas of necrosis are also seen. The exophytic component is a high grade papillary urothelial carcinoma. Microscopic foci of urothelial cells can be seen within the lamina propria. These foci consist of small irregular nests and indicate superficial lamina propria invasion

Discussion: Although the diagnosis of high grade papillary urothelial carcinoma may not be a diagnostic challenge, finding small foci of invasion can be. This case illustrates that invasion may be extremely focal within a large papillary tumor, and great care must be given to identify these foci. While papillary tumors in TUR specimens can often show tangential sectioning, the nests seen in this case are too small and irregular to be considered a tangential section off of the base of the tumor. While most papillary urothelial carcinomas show exophytic growth with finger-like projections, some cases may show prominent endophytic growth. In contrast to the current case, papillary tumors with inverted growth shows large rounded nests of tumor cells rather than small irregular nests seen in this case.