Which of the following is the most frequent primary malignant tumor of the CNS?

Glioblastoma multiforme (GBM) is the most frequent primary malignant tumor of the CNS. It usually affects middle-aged or elderly patients and most frequently arises in the cerebral white matter. GBM is a malignant astrocytoma; it is referred to as grade IV astrocytoma in the WHO classification and grade 4 astrocytoma in the St. Anne-Mayo grading system. This tumor is very aggressive; median survival is approximately 12-14 months following optimal treatment, ie, a combination of surgery and radiation therapy. Medulloblastoma, a much less frequent malignant tumor, affects children; it arises in the posterior fossa in this population. The histogenetic origin of this tumor is unceain, although it is thought to develop from immature cell precursors that give rise to glial and neuronal cells. Meningioma and pituitary adenoma are frequent but benign brain tumors that arise from meningothelial cells and endocrine cells of the adenohypophysis, respectively. Oligodendrogliomas constitute 5% of all primary brain tumors. Although most oligodendrogliomas have benign histologic features (e.g., low mitotic rate and mild nuclear atypia), their complete surgical excision is viually impossible owing to a diffuse, poorly circumscribed pattern of growth. This tumor allows a much longer survival (up to 7-10 years) than GBM, but eventually transforms into a malignant glioma indistinguishable from GBM. Ref: Ropper A.H., Samuels M.A. (2009). Chapter 31. Intracranial Neoplasms and Paraneoplastic Disorders. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor's Principles of Neurology, 9e.