The World Health Organization's histologic classification of bone tumors. A commentary on the second edition.

BACKGROUND: The World Health Organization's histologic classification of bone tumors was revised in 1993. METHODS: The first edition was reviewed by an international panel of pathologists from nine countries and modified to incorporate advances made in the twenty years since it appeared. RESULTS: The framework and concept of the classification remain the same (i.e., based on histologic criteria in terms of differentiation shown by the tumor cells via conventional light microscopy supplemented by immunohistochemistry). New entities not described in the first edition include, for example, benign and malignant fibrous histiocytomas, well differentiated osteosarcoma, round-cell osteosarcoma, clear cell chondrosarcoma, primitive neuroectodermal tumor of bone, osteofibrous dysplasia, and giant cell reparative granuloma. CONCLUSION: The revised and expanded classification reflects advances in our knowledge, but is similar in concept and framework to the original version to allow comparisons between data collected in the past and future.