Histopathological Classification of Brain Tumors According to the Revised WHO Classification: Current State and Perspectives

Abstract

The first edition of the World Health Organization (WHO) international histological classification of tumors of the central nervous system was the result of a collaborative effort coordinated by Professor Klaus-Joachim Zulch in cooperation with 10 experts from 9 countries. This edition of the so-called Blue Book was published under WHO auspices in 1979. To keep the classification up to date, meetings were convened in Houston, Texas, USA (1988), and in Zurich, Switzerland (1990). At the latter meeting an expanded board of 26 experts from 13 countries formulated a consensus report for a revised classification. This revised classification, consisting of definitions, explanatory notes, and an extended panel of representative photomicrographs, was published in 1993 by P. Kleihues, P.C. Burger, and B.W. Scheithauer. Like the first edition, the revised classification is based on histopathological tumor typing and tumor grading. However, a number of changes have been included in the new edition. The monstrocellular sarcoma has been deleted because immunohistochemistry for glial fibrillary acidic protein (GFAP) has clearly documented the astrocytic nature of this neoplasm. Several new tumor types have been added, including most notably the pleomorphic xanthoastrocytoma, the central neurocytoma, the dysembryoplastic neuroepithelial tumor, and the desmoplastic infantile ganglioglioma. Ependymomas and meningiomas appeared with new histological subtypes. The term primitive neuroectodermal tumor (PNET) has been introduced to designate the small-cell malignant tumors of childhood, the most frequent example of which is the cerebellar medulloblastoma. Based on immunohistochemical and molecular genetic data, several tumor types have been moved from one tumor group to another; for example, the glioblastoma is no longer listed under the poorly differentiated and embryonal tumors but is regarded as the most malignant type among the astrocytic neoplasms. The essentials of the new WHO classification are discussed in this chapter. Particular emphasis will be placed on the impact of immunohistochemistry and molecular genetics for the differential diagnosis and grading of brain tumors. It is hoped that further advances in the field of molecular genetics will not only provide new tools for the improvement of diagnostic accuracy but also rational guidelines for better therapeutic strategies.