Urothelial carcinoma in situ: diagnostic update.

Based on the current WHO 2016 classification, the spectrum of flat urothelial lesions with atypia includes reactive urothelial atypia, urothelial atypia of unknown significance, urothelial dysplasia, and urothelial carcinoma in situ (CIS). Classification along this continuous spectrum of atypia can be one of the most challenging areas in all genitourinary pathology. This review will focus on the diagnosis of urothelial CIS in routine clinical practice, emphasising histological criteria, histological spectrum (i.e., 'variant' types), adjunctive immunohistochemistry, intraoperative evaluation, and distinction from its morphological mimics.