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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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