Nested and microcystic variants of urothelial carcinoma displaying immunohistochemical features of basal-like urothelial cells: an immunohistochemical and histopathogenetic study.

Nested/microcystic (NV/MV) urothelial carcinoma (UC) variants are associated with mild cytologic atypia and commonly present at high-stage disease. The histopathogenesis is investigated using urothelial basal cell markers. Archival 14 NV/MV and three inverted papilloma (IP) were immunostained for CD44, cytokeratin 5 (CK5), CK34bE12 and p63. Twenty consecutive cases of invasive high-grade UC including 14 superficial and 6 muscle-invasive UC cases were used as control. Immunostaining was scored as high for staining of full or more than 50% thickness of the epithelial nest or epithelium and low for lesser immunoreactivity and negative reactivity. All 14 NV/MV, 3 IP and 6 control cases showed a high score of immunoreactivity for CK5, CD44, CK34bE12 and focally for p63. The remaining control cases showed a high score of immunoreactivity for CK34bE12, while negative or low for CK5, CD44 and p63. In conclusion, immunoreactivity CK5 and CD44 commonly immunostained NV/MV and some invasive high grade UC. Other basal cell markers (CK34bE12 and p63) appear to be non specific or non sensitive. NV and MV and some UC likely represent a subset of UC displaying immunohistochemical features of urothelial basal cells. They had tendency of endophytic growth and early invasion despite the innocuous cytologic appearance.