NEUROPATHOLOGY FOR MEDICAL STUDENTS
Presented by William I. Rosenblum, MD
CHAPTER 9--TUMORS OF THE NERVOUS SYSTEM
This chapter contains four interrelated sections. The other three sections are:
PRETEST: Answers will be found in the text of this section or click on link at end of questions
These tumors account for approximately 15% of all intracranial neoplasms. They have their greatest incidence in middle age and in women. However, they occur at all ages and are common in men. The meningioma is almost always benign, though aggressive examples are sometimes encountered. Malignant transformation occurs very rarely.
The majority of these tumors arise from arachnoidal cells in the dura mater.Others may sometimes arise from leptomeninges and within the ventricles.
In keeping with their benign nature, the meningiomas, on gross inspection, are well encapsulated. They tend to push into, but not invade the brain (image below, arrow) and frequently, they can be excised.
Histologically, sheets of plump, uniform meningothelial cells are seen, which have a marked tendency to form whorls (image below).
These whorls may degenerate and mineralize to form psammoma bodies (image below, arrow).
The appearance of the tumor cells is almost precisely that of the arachnoid "rests" that are seen in the dura and are thought to be the source of many meningiomas (image below, arrow).
Those arising in the ventricles are thought to come from rests in the choroid plexus.There are numerous histologic variants of meningiomas, but there is no prognostic significance to them except for the angioblastic variant which behaves more aggressively.
Schwannomas, or neurilemomas, make up about 7% of all intracranial neoplasms. Like meningiomas, they have their highest incidence in middle age and in women. They are rarely malignant.
The gross appearance is also much like that of a meningioma except that the cut surface often shows small areas of hemorrhage and discoloration. These nerve sheath tumors are seen most frequently on the eighth nerve (acoustic neuromas), in which case, they occupy the cerebello- pontine angle (image below, arrows). If large, they greatly distort the cerebellum and brain stem. Acoustic tumors can be removed, but usually not without damaging the eighth nerve and sometimes the facial nerve and brain stem. Clinically, the patients may present with deafness and tinnitus on the affected side as well as ataxia. Identical nerve sheath tumors are also seen on spinal roots and the peripheral nerve.
Histologically, the tumor is made up of sheets of uniform spindle cells, some of which are forming palisades called Verocay bodies (image below). These are Verocay bodies.
These cells resemble normal Schwann cells. In addition, a foamy, reticulated tissue is sometimes seen in these tumors, which may represent degeneration of the cells just illustrated. The dense spindled areas are known as Antoni A areas while the looser areas are Antoni B. Both types of tissue are seen in the image below.
NEUROFIBROMASThese peripheral nerve tumors resemble Schwannomas and are differentiated from the latter by histological criteria that are not always distinct. As a rule, however, neurofibromas are less cellular, lack striking palisading, are more loosely packed than Schwannomas, and may have a much more striking collagenous component (red bundles, image below).
In some tumors, irregular nerve bundles are also seen. In about 10% of cases, the neurofibromas may become malignant. These tumors may represent a developmental error in the nerve which allows overgrowth of perineurial connective tissue, axons and Schwann cells. This would be in keeping with the fact that multiple neurofibromas are seen in von Recklinghausen's disease, a familial disorder also characterized by pigmented (cafe au lait) spots on the skin. Von Recklinghausen's disease is often associated with multiple meningiomas, Schwannomas, and glial tumors within the cranial cavity.
About 1% of intracranial tumors are hemangioblastomas and they are often associated with von Hippel-Lindau disease, a familial condition characterized by retinal angiomas, pancreatic cysts, and kidney tumors. They are typically found in the cerebellum as a mural nodule within a cystic cavity whose appearance is much like that of the cystic cerebellar astrocytoma and can often be totally excised. Many hemangioblastomas appear to cause polycythemia because of the presence of erythropoietin-like material in the cyst fluid. Microscopically, numerous capillaries are seen coursing through a matrix of cells with foamy cytoplasm. This picture may closely resemble a renal cell carcinoma.LYMPHOMASThese tumors of the CNS are identical with lymphomas elsewhere. These tumors include large- and small-celled varieties (principally of B cell type), Hodgkin's disease, and multiple myeloma, etc. The latter is largely confined to the dura mater and bony coverings of the CNS. Newer classifications and nomenclature for lymphomas are constantly emerging. Lymphomas may arise as primary CNS tumors, but are more frequently seen in association with lymphomas elsewhere in the body, unless the patient is immunosuppressed. Primary (and secondary) CNS lymphomas are being seen with increasing frequency as a result of immunosuppression for organ transplantation and the current AIDS epidemic. If primary, they tend to be within the substance of the brain, and if secondary, the spinal canal is more frequently involved. Here, the tumors usually present as epidural masses which arise from the vertebrae. However, intracranial vascular and meningeal involvement is also common in secondary lymphomas and this may be accompanied by mild infiltration of the brain parenchyma.