Grading of human urothelial carcinoma based on nuclear atypia and mitotic frequency. I. Histological description.

The World Health Organization (WHO) grade II bladder tumors constitute a clinically and histologically heterogeneous group due to imprecise histological definitions. In an attempt to improve this problem 124 transitional cell carcinomas of the bladder were histologically reviewed using a modified Bergkvist system. The grade II tumors were divided into 2 groups, grades IIa and IIb, mainly with reference to the degree of nuclear atypia and number of mitoses. Using definitions possible to express by numerical means, the separation was easy and reproducible. Due to great histological resemblance between certain close grades we propose a new grading system with 3 grades combining I and IIa (grade A), IIb and III (grade B), and a third group (grade C) for the anaplastic tumors.