Tumours of the CNS

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Primary neoplasms of the CNS are important as they commonly affect young patients; in incidence they are second only to the leukaemias, accounting for about 10% of cancer deaths in those aged between 15 and 35 years. Overall, they account for only about 2% of all deaths from cancer and, in general, are uncommon. Tumours are derived from the various tissues that make up the CNS.

Metastases. Overall, the most common cause of a neoplasm affecting the brain.

Meningeal. Tumours derived from epithelial cells of the meninges, termed meningiomas.

Neuroepithelial. Tumours loosely termed gliomas, which are derived from astrocytes, oligodendrocytes, ependyma, neurons or primitive embryonal cells.

Non-neuroepithelial. Tumours including cerebral lymphomas, germ-cell tumours, cysts and tumours extending from local growth in the skull and pituitary.

Tumours of the brain commonly affect young patients. In this case the tumour is a primary neuroepithelial (glial) tumour. 

METASTASES TO CNS 

Metastatic tumours are commonly seen in the brain and vertebral column

Metastases to the CNS are very common; outside specialist clinical neuroscience units they are encountered more often than primary brain tumours.

As well as focal neurological signs, metastases to brain cause signs of raised intracranial pressure . The main primary sites of metastatic tumour to brain are lung, breast and skin (melanoma). Macroscopically, metastases are often multiple and commonly begin as lesions at the junction of the cortex and white matter. Cerebral oedema is often extensive around metastases, accounting for the main problem of raised intracranial pressure. 

Metastatic tumour commonly affects the spinal cord as extradural deposits, causing compression. The most common are metastases from carcinomas of the prostate, kidney, breast and lung, together with lymphoma and myeloma.


TUMOURS OF MENINGEAL ORIGIN 

Meningiomas are benign tumours derived from epithelial cells of the meninges

Meningiomas are tumours derived from meningothelial cells, the epithelial cells of the meninges. They are common intracranial tumours, and have a female preponderance. 

Meningiomas are typically rounded lesions that arise from the dura and grow slowly to compress underlying brain. Most are fleshy and rubbery in consistency, but a minority are tough and fibrous. Tumours can vary in diameter from 1 or 2 cm up to 7 cm. Although most lesions are solitary, they may be multiple. Infiltration of the skull by tumour may occur, causing local bony thickening. Meningiomas typically present with either focal neurological signs or with features of raised intracranial pressure. 

Tumours arise anywhere in the meninges, the most frequent sites being next to the falx, over the cerebral convexities, or over the sphenoid wings. Less commonly, meningiomas arise from the spinal dura and compress the spinal cord. 

Histologically, tumours are composed of meningothelial cells, which may have a wide variety of histological patterns. A characteristic feature is the presence of small foci of calcification (psammoma bodies).

TUMOURS OF NEUROEPITHELIAL ORIGIN

Tumours of neuroepithelial origin are common primary brain tumours, broadly grouped under the term gliomas. 

 Astrocytomas are diffuse lesions that range from benign to highly malignant

Astrocytomas may arise in the cerebral hemispheres, brain stem, spinal cord or cerebellum, and are derived from astrocytic cells. They vary from tumours with no histological features of atypia and a slow pace of growth (astrocytoma) to lesions with high cellularity, mitoses, pleomorphism and a rapid pace of growth (anaplastic astrocytoma).

Macroscopically, astrocytomas appear as ill-defined, pale areas of softening in the tissue of the nervous system, which blend into adjacent normal brain.

Glioblastomas are highly malignant tumours derived from glial cells

The most common form of glioma, glioblastomas are highly malignant astrocytic glial tumours with a rapid pace of growth. The incidence is greatest around the age of 65 years, but lesions also occur less commonly in childhood and adolescence.

Macroscopically, tumours are necrotic haemorrhagic masses, arising principally in the cerebral hemispheres, less frequently in the brain stem, and only rarely in the cerebellum or spinal cord. Tumours are composed of a mixture of astroglial cell types with many mitoses and nuclear pleomorphism. Necrosis is always present, as this is the feature that delineates this type of lesion from the anaplastic astrocytoma.

Tumours may present as glioblastomas or may have evolved into glioblastoma from a previously diagnosed lower-grade astroglial tumour. They usually cause death by rapid local growth, but may also spread within the neuraxis. They have a median survival of around ten months from diagnosis.
A large glioblastoma arises from one cerebral hemisphere and has grown to fill the ventricular system. Such malignant tumours are associated with necrosis and haemorrhage.

Glioblastomas are composed of pleomorphic cells. A characteristic feature is necrosis, with cell nuclei pallisaded around the necrotic material. Growth factors secreted by tumour cause proliferation of endothelium in vessels.

Oligodendrogliomas usually occur in the cerebral hemispheres of adults

Oligodendrogliomas and anaplastic oligodendrogliomas are glial tumours composed of cells resembling oligodendrocytes. They arise in the cerebral hemispheres and have only rarely been described in the brain stem, cerebellum or cord. Macroscopically, tumours are very similar to astrocytomas, arising as ill-defined greyish white lesions that merge with adjacent brain. Histologically, tumours are composed of cells with rounded nuclei and pale vacuolated or pink-staining cytoplasm resembling oligodendrocytes. These lesions may be divided into low-grade and anaplastic oligodendrogliomas on the basis of cellularity, mitoses, pleomorphism, and vascular proliferation. It is not uncommon to find mixed glial lesions with both astrocytic and oligodendroglial features (oligoastrocytomas).

Low-grade tumours in the temporal lobe have a favourable prognosis, whereas high-grade tumours recur locally and can also invade the meninges and spread in the CSF pathways. Oligodendrogliomas may progress to form tumours identical to glioblastomas.

Ependymomas, most commonly seen in childhood, often occur in the spinal cord and ventricles

Ependymomas and anaplastic ependymomas are tumours derived from ependymal cells. They are most common in the first two decades of life, accounting for around 10% of all intracranial tumours in childhood. The most common sites are the spinal cord and the region of the fourth ventricle.

Histologically, ependymomas form tubules resembling the central canal of the spinal. Although most ependymomas demonstrate no cellular atypia, anaplastic ependymomas show cells with mitoses, pleomorphism, and vascular endothelial proliferation, and are associated with a worse prognosis.

Myxopapillary ependymomas are a variant of ependymoma seen in the filum terminale of the spinal cord. They behave as locally infiltrative lesions.

Embryonal tumours of the CNS are rapidly growing malignant lesions

The embryonal tumours of the CNS are common in childhood, forming a large proportion of primary tumours. They are composed of primitive small cells, which resemble the multipotential cells of the developing fetal brain, and are also called primitive neuroectodermal tumours (PNETs). As a group, they are rapidly growing lesions composed of small cells with many mitoses. As well as being prone to local spread, they have a tendency to spread via CSF pathways. The main tumour in this group is the medulloblastoma, which arises in the cerebellum and is composed of primitive small cells with multiple lines of differentiation. As a result of modern treatment with surgery, radiotherapy and chemotherapy, the 10-year survival rate is over 50% .

Histologically, tumours are composed of sheets of small anaplastic cells with rod-shaped and rounded nuclei. Evidence of neuronal and glial differentiation may be seen. Less common PNETs are cerebral neuroblastomas (composed of primitive neurons) and ependymoblastomas (composed of primitive ependyma).

Primitive neuroectodermal tumours are composed of small cells with a high mitotic rate. In some, neuroblastic rosettes form, indicating primitive neuronal maturation.

NON-NEUROEPITHELIAL TUMOURS OF THE CNS 

 Lymphomas of the nervous system are increasing in incidence as a complication of immunosuppression

Primary lymphomas of the nervous system are usually high-grade non-Hodgkin's lymphomas of B-cell type. These tumours may arise sporadically, but are increasing in incidence and are associated with immunosuppression, particularly in patients with AIDS. Lesions are ill-defined and multifocal, usually being seen deep in the hemispheric white matter. Histology shows brain infiltrated by atypical lymphoid cells. These tumours have a very poor prognosis, with most patients dead five years after diagnosis.

Craniopharyngiomas are infiltrative epithelial tumours arising from the region of the pituitary fossa

Composed of squamous-cell-like epithelium, craniopharyngiomas are derived from remnants of Rathke's pouch, the embryological source of the anterior pituitary gland. Accounting for 3% of intracranial tumours, they are most common in childhood. Tumours compress the pituitary gland and damage the overlying hypothalamus and optic chiasm, presenting with either hypopituitarism or visual problems. Macroscopically, lesions have cystic and solid areas, frequently growing into adjacent brain and around major blood vessels, often with calcification.

Although craniopharyngiomas are benign, local recurrence is often a problem because of inability to completely excise infiltrative tumours.
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