Histological classification of human gliomas: state of art and controversies

The histological classification of human gliomas remains in 2005 a challenge. The aim is to define the histological type of glioma (astrocytic, oligodendrocytic or mixed) and the grade in order to classify the patients and give them an accurate treatment. Although the standard remains the WHO classification, this classification suffered from lack of reproducibility among pathologists. In particular this classification does not take into account the intrinsic morphological heterogeneity of infiltrative gliomas and does not discriminate the tumour cells from the residual brain parenchyma. According to the WHO classification, infiltrative gliomas encompass astrocytic gliomas (diffuse astrocytomas grade II, anaplastic astrocytomas grade III and glioblastomas grade IV), oligodendroglial tumours (oligodendrogliomas grade II, anaplastic oligodendrogliomas grade III) and mixed gliomas (oligoastrocytomas grade II and anaplastic oligoastrocytomas grade III). Circumscribed gliomas mainly corresponds to pilocytic astrocytomas (grade I). In contrast, the Sainte Anne classification takes into account the macroscopic informations provided by imaging techniques and the tumour growth patterns. Three distinct tumour growth patterns may be seen in gliomas, type I: tumor tissue only, type II: tumour tissue and isolated tumor cells permeating the brain parenchyma (ITC) and type III: ITCs only and no tumor tissue. According to the Sainte Anne classification, gliomas are divided into astrocytic gliomas (pilocytic astrocytomas, structure type I, glioblastomas structure type II) and oligodendrogliomas and mixed oligoastrocytomas (grade A: lack of contrast enhancement and lack of endothelial hyperplasia, structure type III; and grade B: contrast enhancement or endothelial hyperplasia, structure type II and III). In the future the glioma classification has to be unique and should take into account clinical data, neuroradiological and histological features and results of molecular biology.