Answer: Papillary urothelial neoplasm of low malignant potential

Histology: This is a papillary urothelial neoplasm which involves the renal pelvis. The epithelium fuses together the fibrovascular cores. While the epithelial cells are enlarged relative to normal urothelium, they do not sure show significant cytologic atypia, and mitotic activity is low and generally confined to the base of the papillae. These are the typical features of a papillary urothelial neoplasm of low malignant potential (PUNLMP).

Discussion: Papillomas are papillary urothelial neoplasms lined by normal-appearing urothelium. Typically the papillae do not fuse. Low grade papillary urothelial carcinoma features cytologic atypia and greater mitotic activity, including mitotic figures above the basal layer of the epithelium. High grade papillary urothelial carcinomas demonstrate even greater degrees of cytologic atypia, and show disorganized architecture including significant loss of nuclear polarity. Mitotic figures are abundant. In general, the epithelial lining of a high grade noninvasive papillary urothelial carcinoma is similar to that of flat urothelial carcinoma in situ, with the difference being of course that the epithelium is in a papillary architecture in high grade papillary urothelial carcinoma.