6 - Astrocytic and Oligodendroglial Tumors

The infiltrative or "diffuse" gliomas include astrocytomas and oligodendrogliomas. Taken together, these are the most frequent primary neoplasms of the central nervous system parenchyma. These tumors are difficult to manage clinically, in large part because of their widespread invasiveness, their strong tendency toward biologic progression, and the resistance of many of these neoplasms to conventional adjuvant therapies. The WHO classification currently divides diffuse gliomas into astrocytomas and oligodendrogliomas and includes criteria for their grading. The category of "oligoastrocytoma" is no longer supported, since these lesions have been demonstrated to represent either astrocytomas or oligodendrogliomas at the molecular genetic level. Diffuse astrocytoma (WHO grade II), anaplastic astrocytoma (WHO grade III), and glioblastoma (GBM; WHO grade IV) form a malignancy continuum for the diffusely infiltrating astrocytomas. The diffusely infiltrative astrocytomas of adults are now subdivided based on the mutational status of isocitrate dehydrogenase-1 or -2 (IDH1/2) genes into either IDH-mutant or IDH-wildtype for each grade. Another form of diffuse high-grade astrocytoma that most often occurs in the pediatric population is characterized by histone H3F3 mutations and occurs either in the midline (K27 codon mutations) or in the cerebral hemispheres (G34 codon mutations). The oligodendrogliomas include WHO grade II and III designations and are defined histologically and by the presence of IDH mutations and the codeletion of the entire 1p and 19q chromosomal arms.