Brain: Pathology-Based Diagnoses: Neoplasms,

Cysts, and Disorders

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Neoplasms Overview

Introduction

The most widely accepted classification of brain neoplasms is

sponsored by the World Health Organization (WHO). A

working group of world-renowned neuropathologists

periodically convenes for a consensus conference on brain

tumor classification and grading. The results are then

published. An update to the 2007 edition of the so-called

"Blue Book" is scheduled for early 2017.

Brain tumors are both classified and graded. Although this is

rapidly changing with the advent of molecular profiling,

histological grading has been the primary means of predicting

the biological behavior of tumors. Although many different

grading schemas have been proposed, the WHO classification

and grading of CNS neoplasms is the most widely accepted

and is utilized in this text.

Classification/Grading of CNS Neoplasms

General Considerations

CNS neoplasms are divided into primary and metastatic

tumors. Primary neoplasms are divided into 6 major

categories. The largest by far is tumors of neuroepithelial

tissue, followed by tumors of the meninges. Tumors of cranial

and spinal nerves, lymphomas and hematopoietic neoplasms,

and germ cell tumors are less common but important

groupings. The final category of primary neoplasms, tumors of

the sellar region, is identified by geographic region rather than

histologic type.

Tumors of Neuroepithelial Tissue

This category is huge, and therefore it is divided into several

discrete tumor subtypes. Once thought to arise from

dedifferentiation of mature neurons or glial cells (e.g.,

astrocytes, oligodendrocytes, ependymal cells, etc.), it is now

recognized that most brain tumors arise from neural stem

cells.

Astrocytomas: There are many histologic types and subtypes

of astrocytomas, ranging in biological behavior from benign,

relatively circumscribed tumors, such as pilocytic astrocytoma

(PA) and subependymal giant cell astrocytoma (SGCA), to the

highly malignant, diffusely infiltrating glioblastoma (GBM).

Two of the localized astrocytomas (PA and SGCA) are

designated as WHO grade I neoplasms. Neither displays a

tendency to malignant progression, although a variant of PA

called pilomyxoid astrocytoma may behave more aggressively

and is classified as a WHO grade II tumor.

Diffusely infiltrating astrocytomas have no distinct border

between tumor and normal brain, even though the tumor

may look relatively discrete on imaging. The lowest grade is

simply called "diffuse astrocytoma" and is designated as WHO

grade II. Anaplastic astrocytoma is WHO grade III, and GBM is a

grade IV neoplasm.

Patient age has a significant effect on astrocytoma type and

location. For example, diffusely infiltrating astrocytomas are

most common in the cerebral hemispheres of adults and the

pons in children. PAs are tumors of children and young adults.

They are common in the cerebellum and around the 3rd

ventricle but only rarely occur in the hemispheres.

Oligodendroglial tumors: These vary from a diffusely

infiltrating, but relatively well-differentiated, WHO grade II

neoplasm (oligodendroglioma) to anaplastic

oligodendroglioma (WHO grade III).

Low-grade gliomas (LGGs): LGGs have been distilled into 3