Revised WHO histological classification of urothelial bladder tumors and flat urothelial lesions

Recently the World Health Organization published a new classification of ur inary bladder tumors which is intended to take into account better the biol ogy of the various lesions and to better distinguish between clearly benign and malignant lesions. We examine the possible diagnostic and clinical imp act of the new classification, including recent immunohistochemical finding s. Papillary urothelial lesions include papillomas, papillary neoplasms of low malignant potential, and papillary carcinomas. Flat urothelial lesions include hyperplasia, reactive atypia/atypia of unknown significance, dyspla sia, and carcinoma in situ. invasive patterns of papillary carcinomas are d iscussed, with special emphasis on lamina muscularis mucosae substaging. Th e most important feature of the new classification is its differentiation o f two types of low-grade, noninvasive papillary urothelial lesions: papilla ry neoplasm of low malignant potential vs. papillary carcinoma. Long-term f ollow-up studies are needed to determine the clinical significance of this differentiation.