Answer: Inverted urothelial papilloma with atypia

Histology: The majority of this lesion has the classic histology of a benign inverted urothelial papilloma. The surface lacks true papillary fronds and is covered by normal urothelium with an undulating surface. Projecting down into the underlying lamina propria are anastomosing columns of urothelium. Occasional colloid filled cysts are present. The nests of the urothelium tend to have peripheral palisading with central streaming of the nuclei parallel to the nests. The vast majority of the lesion shows no cytologic atypia with totally uniform nuclei many of which contain nuclear grooves. Mitotic figures are not identified. Focally, there are areas which have greater cytologic atypia approaching the level of carcinoma in-situ (CIS), although some of the atypia appears degenerative in nature.

Discussion: This lesion is controversial. There are some experts who feel that any atypia within a non-invasive lesion with an inverted growth pattern warrants a diagnosis of a non-invasive urothelial carcinoma with an inverted growth pattern. Depending on the atypia this may range from papillary urothelial neoplasm of low malignant potential to low grade urothelial carcinoma to high grade urothelial carcinoma with an inverted growth pattern. In my opinion, these carcinomas with an inverted growth pattern should be distinguished from lesions which are for the most part classic benign inverted urothelial papillomas with focal atypia. In the current case, the majority of the lesion has the typical architecture and cytology of a benign inverted urothelial papilloma. This includes architecturally the lack of papillary fronds, anastomosing columns of urothelium extending down from the surface, and colloid filled cysts. Cytologically, there is central streaming of the nuclei parallel to the anastomosing columns, peripheral palisading, and entirely benign cytology with prominent nuclear grooves. The lack of associated inflammation is also typical of inverted papilloma. Focally, however, there is cytologic atypia which in another context would be equivalent to CIS. A stain for Ki-67 was performed which was not elevated in the area of atypia compared to the surrounding more benign appearing inverted urothelial papilloma. The significance of atypia within an otherwise benign inverted urothelial papilloma is not entirely known. In the one study that we have done to analyze these patients, they did not have associated urothelial carcinomas either in the past, concurrently, or subsequently with relative short follow up. Consequently, I feel that these lesions should not be lumped together with carcinomas with an inverted growth pattern and rather these patients should only be followed closely

Reference(s):
– Hum Pathol 35;1499-504, 2004