Assessment of basal cell status and proliferative patterns in flat and papillary urothelial lesions: A contribution to the new WHO classification of urothelial tumors of the urinary bladder

In 1999, the World Health Organization (WHO) published a new classification of papillary urothelial tumors of the urinary bladder Intended to represen t a reproducible, easy-to-use classification system that better separates p atients with true malignancies (bladder cancer) from those patients who are at an increased risk for developing bladder cancer, problems in the differ ential diagnosis of various lesions remained. Probably the most critical di stinction is between papillomas, papillary urothelial neoplasms of low mali gnant potential (lmp), and grade I papillary carcinomas. Conversely, proble ms in the distinction between reactive atypia, atypia of unknown significan ce, and dysplasia, as well as the distinction of dysplasia from carcinoma i n situ (CIS), are unresolved. Whether urothelial basal cell status assessme nt on hematoxylin and eosin-stained slides completed by cytokeratin immunoh istochemistry with anticytokeratin clone 34 beta E12 may help to improve so me of the previously mentioned diagnostic dilemmas was investigated. Basal cell status assessment was helpful in the differentiation between dysplasia and CIS. In dysplasia, CK IHC showed a predominantly basal labeling patter n, whereas in CIS, labeling of all urothelial layers was seen. Basal cell s tatus assessment could separate 2 groups of pTa GIb papillary carcinoma. Gr oup 1 with a continuous basal Cg labeling and a low MIB-1 labeling index (L I) was compared with group 2, with a diffuse labeling pattern and a signifi cantly higher MIB-1 LI. Whether group 1 carcinomas should better be assigne d to the group of papillary urothelial neoplasms of Imp is discussed. HUM P ATHOL 31:745-750. Copyright (C) 2000 by W.B. Saunders Company.