Brain and Meninges

Department of Pathology, Sinai Hospital of Baltimore Pathology, Baltimore, MD, USA


AstrocytomaOligodendrogliomaMeningiomaTeratomaCraniopharyngiomaHemangioblastomaSchwannomaGlialMicrovascular proliferationGlioblastomaEpendymoma

The pathology of the central nervous system (CNS) is an intimidating area for pathologists. In part this is because we have virtually no role in the gross examination, just processing shreds of gray pulp on gauze, and in part because of the feeling that “it could be anything at all,” including a long list of exotic zebras that look just like the common things except behave completely differently. The fact that we are often asked to make our diagnoses on frozen section does not help matters. However, even in the brain, the list of likely diagnoses is still reasonably short if you have three pieces of key information: the age of the patient, the location, and the radiographic appearance of the tumor. Table 27.1 lists differential diagnoses that should at least put you in the right ballpark.

Table 27.1.
Differential diagnoses of CNS neoplasms , the most common entities.


Infants and young children

Adolescents and young adults

Adults to elderly

Cerebellum (infratentorial)




Pilocytic astrocytoma



Pilocytic astrocytoma


Cerebellopontine angle (cranial nerves)


Cerebrum (supratentorial)

Neuroblastoma (although rare in this location, more often an abdominal tumor)


Astrocytoma and glioblastoma



Pilocytic astrocytoma


Pleomorphic xanthoastrocytoma







Germ cell tumors

Pituitary adenoma

Pituitary adenoma



Germ cell tumors

Pineal tumors/cysts

Pineal tumors/cysts


Ventricles (in or adjacent to)

Choroid plexus papilloma/carcinoma

Choroid plexus papilloma



Pilocytic astrocytoma


Subependymal giant cell astrocytoma


Solitary fibrous tumor/hemangiopericytoma

The most important first step is to start by asking if your “lesion” could be normal tissue (i.e., the surgeon has missed). To answer this question you have to know a little normal histology (Figures 27.1 and 27.2). Pay attention to autopsy brain specimens to get a sense for the normal cellularity and cellular components of brain tissue. Second, you should ask if your lesion is neoplastic or non-neoplastic . The non-neoplastic lesion that pathologists worry most about is the demyelinating lesion, which can look like a tumor by radiology. Abundant foamy macrophages, and an absence of obvious malignant cells, should make you think of a possible demyelinating lesion or infarct (Figure 27.3). Gliosis, a reactive proliferation of astrocytes, can also simulate a glioma (glial tumor) histologically (Figure 27.4). However, once you have decided you have a neoplasm, the real work begins.


Figure 27.1.
Normal cerebral cortex . (a) In the gray matter, there is a fine fibrillary background of neuropil (arrowhead), with glial nuclei in the background. Neurons are unmistakable, with their plump cell bodies and prominent nucleoli (arrow). (b) The white matter has a coarser texture, as it is made up of myelinated axons, and the glial nuclei are visible as small dense dots (white arrow).


Figure 27.2.
Normal cerebellum . The tiny nuclei of the granule cell layer are seen at the bottom left (black asterisk), underlying a layer of large neurons called Purkinje neurons (arrowheads). The Purkinje cells send their dendrites up into the molecular layer (white asterisk). The arachnoid layer is seen between folds of the cerebellum (arrow).


Figure 27.3.
Infarct . The presence of sheets of histiocytes or macrophages, with their round contours and bubbly cytoplasm, should be a flag to consider an infarct or demyelinating lesion.


Figure 27.4.
Reactive astrocytes in gliosis. Normal resting astrocytes generally do not have visible cytoplasm. When responding to inflammation or injury, they become compact in shape, with dense pink cytoplasm and stubby processes (arrow).

General Principle I

First, as in any organ, there are a finite number of cell types in the brain, and each cell type can give rise to a spectrum of neoplasms. In the brain, the cells and their common neoplasms are summarized in Table 27.2. Neurons, which are thought to be nondividing terminally differentiated cells, are a pretty rare source of tumors, so most CNS tumors arise from the supporting cell types of the brain.

Table 27.2.
Probable cells of origin of CNS tumors.

Native cells



Astrocytoma and variants

} Gliomas, collectively



Ependymal cells

Ependymoma and variants

Neurons and precursors

Neurocytoma and gangliocytoma

Meninges (arachnoid cells)

Meningioma and variants, hemangiopericytoma

Choroid plexus cells

Choroid plexus papilloma/carcinoma

Pituitary cells

Pituitary adenoma

Schwann cells (in nerves)


Stromal or vascular cells


Embryonal (immature) cells

Medulloblastoma, neuroblastoma, others

Pharynx remnantsa

Craniopharyngioma, Rathke’s cleft cyst

Germ cell remnantsa

Germinoma, teratoma, etc.

Notochord remnanta


a Remnants are those cell lines that do not anatomically belong in the brain but sometimes get left behind in a developmental fluke. They create midline tumors.

General Principle II

There is a broad grading system used for CNS neoplasms, the World Health Organization (WHO) tumor grade, which ranges from 1 (most indolent) to 4 (most aggressive). In this system, grade 1 is equivalent to “benign” and curable by resection, but in the CNS something otherwise benign may be clinically devastating depending on where it is growing. Conversely, while grade 4 tumors are often lethal and are considered “malignant,” they do not metastasize throughout the body. For this reason, CNS tumors are not described as benign versus malignant but are graded according to the WHO scale, which predicts prognosis. The grade 1 and 2 tumors are referred to as “low grade,” whereas grade 3 and 4 lesions are considered “high grade.” There is no TNM staging for primary brain tumors; margin status and tumor size are also not usually determined by the pathologist.

Most neoplasms are assigned to a grade by definition, but some tumor families have a spectrum of grades based on certain histologic features. For these tumors , the following features are used to assign a higher grade to the lesion:

  • Cytologic atypia (a subjective observation requiring some experience)

  • Increasing cellularity relative to lowest-grade tumor (again, subjective)

  • Increasing numbers of mitoses (usually quantitative)

  • Microvascular proliferation (objective: either present or absent)

  • Necrosis (objective: either present or absent)

These features need to be assessed in every tumor (Table 27.3). Note that tumors with an “anaplastic” qualifier are grade 3, while tumors with a “blastoma” in the name are grade 4 (hemangioblastoma not included).

Table 27.3.
Morphologic features of glial tumors in increasing order of concern (from left to right).

EGBs or RFs

No atypia

No mitoses


High cellularity

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Jan 30, 2018 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Brain and Meninges

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