The 1973 WHO classification of bladder tumours anticipated a probable need for eventual revision of the criteria for diagnosing papillary and flat bladder neoplasia. A workshop sponsored by the WHO consisting of pathologists, urologists, cytologists, oncologists and basic scientists interested in bladder tumours addressed this subject, and after a follow- -up meeting sponsored by the International Society of Urological Pathology, the classification and terminology used in this text were agreed upon. A major change is in the introduction of a new category: papillary urothelial neoplasm of low malignant potential. Many of the tumours previously designated as papillary transitional cell carcinoma, grade I now fall into that category. Another major change is in the designation of flat lesions, and this includes a definition of carcinoma in situ. Furthermore, a number of variant forms of urothelials carcinomas are included as well as new entities not recognized when the 1st edition was issued.

This classification is based primarily on the microscopic charac- teristics of tumours and, therefore, is concerned with morpho- logically identifiable cell types and histological patterns, as seen with conventional light microscopy. The term tumours is used synonymously with neoplasm. The phrase tumour-like is applied to lesions which resemble neo- plasms, clinically or morphologically, but do not behave biologi- cally in a neoplastic manner. They are included in this classifica- tion because they give rise to problems in differential diagnosis and because of the unclear borderline between neoplasms and certain non-neoplastic lesions. Synonyms are listed only if they have been used widely, or if they are considered to be helpful to the understanding of the lesion. In such cases, the preferred term is given first, followed by the synonym. Although the emphasis of this classification is on histological typing, in the examination of kidney tumours, consideration should be given to the degree of cellular anaplasia, the extent of local spread, vascular and lymphatic invasion, and the occur- rence of metastasis. The scheme of histological grading suggested here is as fol- lows: Grade I applies to the tumours that have the least degree of cellular anaplasia compatible with a diagnosis of malignancy; . grade II! applies to tumours with the most severe degrees of cel- lular anaplasia; and grade I! applies to those tumours in be- tween. This scheme is applicable to the carcinomas of the renal parenchyma and pelvis.

This revision of the book originally published in 1980 is the result of a collaboration among scientists from 10 countries. The authors include not only pathologists but also a urologist and a basic scientist. The second edition - containing 146 colour photographs - is considerably more extensive than its predecessor. A number of new entities, unrecognized in 1980, are included: prostatic intraepithelial neoplasia, which is commonly associated with carcinoma and may also be seen in a biopsy for elevated PSA; basal cell carcinoma; small cell carcinoma; paracrine-endocrine elements; variants of carcinoma; and stromal sarcoma. The criteria for the diagnosis of carcinoma have been expanded, including the minimal criteria. The Gleason grading system, based on growth pattern, is presented in detail. The WHO grading system, based on nuclear anaplasia and glandular differentiation, is more clearly defined.