Although more than 12 years have passed since publication of the first
WHO histological classification of central nervous system (CNS) tumours,
the changes in this revised edition are not radical. Only one formerly
recognized entity has been deleted: the monstrocellular sarcoma, because
there is immuno- cytochemical evidence of its astrocytic nature. Several
new tumour types have been added: the pleomorphic xanthoastro- cytoma
has been generally acknowledged for almost a decade; the neurocytoma has
gradually evolved as a clinical-pathological entity; and two new
entries, the dysembryoplastic neuroepi- thelial tumour and the
desmoplastic infantile ganglioglioma, have only been characterized
morphologically during the past few years. We regard the classification
as an international standard to facilitate communication and have tried
to avoid current con- ceptual controversies. The majority of
partiCipants supported inclusion of the term "primitive neuroectodermal
tumour" (pNET). However, because of our limited knowledge of the biol-
ogy of embryonal CNS tumours, preference was given to use PNET
selectively, rather than applying it to all small cell embryo- nal
childhood tumours, irrespective of their histological pheno- type.
Ependymomas and meningiomas now have new histological SUbtypes. Most of
these are not associated with biological behaviour different from the
parent tumour type, but their description will aid the practising
pathologist to identify and classify these lesions. 2 Introduction
Histological Typing Following the philosophy of this WHO series,
classification is based primarily on histological assessment of cell
types and tis- sue patterns recognized by conventional light microscopy.
