Morphological classification and definition of benign, preneoplastic and non-invasive neoplastic lesions of the urinary bladder.

The morphological classification used in this essay has been based on the most recent World Health Organization (WHO) classification of tumours of the urinary system (i.e. 2004 WHO classification). It includes epithelial abnormalities and metaplasias as well as dysplasias and carcinomas in situ. The lesions are broadly subdivided into two major groups: benign, preneoplastic and non-invasive neoplastic lesions of the urothelium; and benign, preneoplastic and non-invasive neoplastic bladder lesions other than urothelial. Each of these lesions is defined with strict morphological criteria to provide more accurate information to urologists and oncologists in managing patients. There is still debate in the literature as to whether the 2004 WHO system should be the only one to be used and whether the 1973 WHO system should be abandoned.