Astrocytoma classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

There are two broad classes of classification identified within astrocytomas, based on histology, as to whether the zones of infiltration of cancer cells are narrow or diffuse.

Classification

Within the astrocytomas, there are two broad classes recognized in literature, those with:

• Narrow zones of infiltration (mostly invasive tumors; e.g., pilocytic astrocytoma, subependymal giant cell astrocytoma, pleomorphic xanthoastrocytoma), that often are clearly outlined on diagnostic images
• Diffuse zones of infiltration (e.g., low-grade astrocytoma, anaplastic astrocytoma, glioblastoma), that share various features, including the ability to arise at any location in the CNS, but with a preference for the cerebral hemispheres; they occur usually in adults; and an intrinsic tendency to progress to more advanced grades.

Grading

Astrocytomas have great variation in their presentation. The World Health Organization acknowledges the following grading system for astrocytomas:

• Grade 1 — pilocytic astrocytoma - primarily pediatric tumor, with median age at diagnosis of 12
• Grade 2 — diffuse astrocytoma
• Grade 3 — anaplastic (malignant) astrocytoma
• Grade 4 — glioblastoma multiforme (most common)

• The WHO-grading scheme is based on the appearance of certain characteristics: atypia, mitosis, endothelial proliferation, and necrosis. These features reflect the malignant potential of the tumor in terms of invasion and growth rate. Tumors without any of these features are grade I, and those with one of these features (usually atypia) are grade II. Tumors with 2 criteria and tumors with 3 or 4 criteria are WHO grades III and IV, respectively. Thus, the low-grade group of astrocytomas are grades I and II. According to the WHO data the lowest grade astrocytomas (grade I) make up only 2% of recorded astrocytomas, grade II 8%, and the higher grade anaplastic astrocytomas (grade III) 20%. The highest graded astrocytoma (grade IV GBM) is the most common primary nervous system cancer and second most frequent brain tumor after brain metastasis. Despite the low incidence of astrocytomas compared to other human cancers, mortality is significant, as the higher grades (III & IV) present high mortality rates (mainly due to late detection of the neoplasm).

• Various types of astrocytomas are given these WHO grades:

WHO Grade	Astrocytomas	Description
I	Pilocytic astrocytoma Pleomorphic xanthoastrocytoma Subependymal giant cell astrocytoma Subependymoma	Consist of slow growing astrocytomas, benign, and associated with long-term survival. Individuals with very slow growing tumors where complete surgical removal by stereotactic surgery is possible may experience total remission. Even if the surgeon is not able to remove the entire tumor, it may remain inactive or be successfully treated with radiation.
II	Low-grade (fibrillary astrocytoma) astrocytoma Mixed oligoastrocytoma	Consist of relatively slow-growing astrocytomas, usually considered benign that sometimes evolve into more malignant or as higher grade tumors.
III	Anaplastic astrocytoma
IV	Glioblastoma multiforme (GBM)	Consists of Glioblastoma multiforme (GBM), which is the most common and most malignant primary brain tumor. Less than 10% form more slowly following degeneration of low-grade astrocytoma or anaplastic astrocytoma. These are called secondary GBM.

See Also

Main articles: pilocytic astrocytoma and glioblastoma multiforme