Morphologic expressions of urothelial carcinoma in situ - A detailed evaluation of its histologic patterns with emphasis on carcinoma in situ with microinvasion

The recently proposed World Health Organization/ International Society of U rological Pathology (WHO/ISUP) consensus classification of flat urothelial lesions expands the definition traditionally used for urothelial (transitio nal cell) carcinoma in situ (CIS), basing its diagnosis predominantly an th e severity of cytologic changes. Lesions now encompassed within the diagnos is of CIS exhibit an array of cytologic and architectural features, which h ave not been documented in detail. In this study, cases were examined with respect to histologic patterns and microinvasion (invasion into the lamina propria to a depth of less than 2 mm). Five major patterns of CIS, often oc curring in the same specimen (160 patterns in 77 cases), were noted. Common to each pattern was the presence of high-grade cytologic atypia, the defin itional feature. The patterns found include 1) large cell CIS with pleomorp hism (57%), in which the cells had abundant cytoplasm and nuclear pleomorph ism; 2) large cell CIS without nuclear pleomorphism (48%); 3) small cell CI S (14%), in which the cytoplasm was relatively scant and pleomorphism was u sually minimal; 4) clinging CIS (40%), in which the urothelium was denuded with a patchy, usually single layer of atypical cells; and 5) cancerization of urothelium (16%) with either pagetoid spread (clusters or isolated sing le cells) or undermining or overriding of the normal urothelium. Carcinoma in situ with microinvasion into the lamina propria (13 cases: 3 of 77 CIS c ases studied above and 10 additional cases) was evident as invasive cells w ith retraction artifact mimicking vascular invasion (77%, 10 cases); nests, irregular cords, and strands, or isolated single cells with desmoplasia(8% , 1 case); or absent stromal response (15%, 2 cases). Although the diagnost ic terminology for all of these patterns, for the purposes of the surgical pathology report, should be simply urothelial CIS with no specific mention of the morphologic pattern, awareness of the histologic diversity of CIS wi ll facilitate the diagnosis of this therapeutically and biologically critic al flat lesion of the urothelium. These lesions may be associated with micr oinvasion, which may be clinically unsuspected and histologically subtle.