ABSTRACT
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Gliomas are primary tumors of the central nervous system (CNS) that have predominantly
glial histopathological and genetic features. The broad category of gliomas can be subdivided
into different histological subtypes, which includes the astrocytic tumors, oligodendroglial
tumors, and mixed oligoastrocytomas. The 2000 World Health Organization (WHO) classi-
fication of tumors of the nervous system further grades these tumors into varying degrees of
malignancy based on the presence or absence of morphological features such as pleomorph-
ism (or nuclear atypia), high mitotic activity, microvascular proliferation (MVP), and necro-
sis. Although the WHO grading system is currently the accepted gold standard criteria for
grading gliomas, it is simply an estimate of malignancy for most tumors of the nervous system
including some gliomas. For the diffuse glial tumors, however, it is a true grading system since
a spectrum of progression from low-(WHO Grade II) to high-grade (WHO Grades III and
IV) actually exists.1 Glial tumors designated as WHO Grade II have only pleomorphism
present, whereas WHO Grade III gliomas have both pleomorphism and high mitotic activity
present. Finally, WHO Grade IV gliomas have any three of the four morphological features
present. WHO Grade I is usually reserved for circumscribed gliomas, most often found in
children, such as pilocytic astrocytoma.
