Introduction
Although either benign or malignant, almost
all brain tumours are malignant in the sense that they may lead eventually to
death if not treated. Brain tumours are responsible for 2 per cent of all cancer
deaths. The annual incidence of newly diagnosed brain tumours in the USA is
approximately
Aetiology
The aetiology of brain tumours is still not
clearly understood. Although there is no genetic predisposition, chromosome
abnormalities have been noted in many CNS tumours (e.g. von Recklinghausen’s).
Some chemicals show carcinogenic activity in animals producing CNS tumours.
Immunosuppression can markedly increase the incidence of primary CNS lymphoma.
The current molecular understanding of oncogenesis involves both the addition
of oncogenes to the genome and the loss of normally occurring tumour
sup-presser genes. Mutation in the p53 tumour
suppresser gene is the most common gene alteration and found in both
astrocytomas and meningiomas.
Clinical
features
The clinical presentation of intracranial
neoplasms will be as a result of one or a combination of raised ICP focal
neurological signs, organic mental changes and seizures.
The
ability of the intracranial contents to adapt to an increase in volume is
limited and ultimately an increase in pressure will occur. This will manifest
itself initially as a result of headache due to stretching of the dura. In the
initial phase this headache is characteristically worse first thing in the
morning and progressive. Nausea and vomiting may occur, followed by disturbance
of vision as a result of papilloedema. Finally, a deterioration will occur in
the level of consciousness as a serious and late consequence of raised ICP (Fig.
35.28).
Late-onset
epilepsy, particularly over the age of 30, should -prompt investigations to
exclude an intracranial neoplasm.
Forty per cent of patients with supratentorial
glial tumours will have experienced a seizure by the time they come to surgery.
Benign tumours may also be an underlying cause of long-standing epilepsy and
may render the seizures surgically remediable. Focal neurological signs
associated with intracranial neoplasms are related to the area of cortex
involved. Contralateral signs are associated with lesions in the posterior
frontal area (motor) or anterior parietal (sensory)
Gliomas
Gliomas account for 50 pet cent of adult
intracranial tumours and ate usually supratentorial. They are of
neuroectodetmal
origin arising from glial cells of which there are four types: astrocytes,
oligodendroglioma, ependymal cells and neuroglial precursors. The most frequent
is the astrocytoma. They rarely metastasise (except for medulloblastoma and
ependymoma) and spread along axonal pathways.
CT
scanning before and after contrast is able to localise and confirm the diagnosis
in the majority of cases. The enhancement is often irregular around a centre of
low density which may represent necrosis. Calcification may be present. MRI
is more accurate in defining low-grade lesions. Tissue definition and anatomical
localisation of lesions in and adjacent to eloquent areas ate also superior,
facilitating surgical planning and resection (Fig. 35.30).
The
aim of treatment is to obtain a pathological diagnosis and grading and to debulk
the tumour, both to alleviate symptoms of raised pressure and as a precursor to
adjuvant treatments. This usually involves a combination of surgery and
radiotherapy and will depend on the size and location of the tumour, as well as
the patient’s clinical state and age. Patients are started on dexamethasone in
order to diminish cerebral oedema prior to surgery. Spread along axons makes
total resection impossible unless the lesion is low grade, small and polar.
Radiotherapy is usually as an external beam (50—60 Gy fractionated over 30
days), stereotactic focal irradiation using a gamma knife or linear accelerator
or brachytherapy (implantation of a radioactive source). Chemotherapy has little
role for lesions other than oligodendrogliomas. Photodynamic therapy,
immunotherapy and gene therapy are all currently undergoing clinical trials to
determine advantages over conventional treatment. In view of the poor prognosis
with high-grade lesions, conservative treatment is often appropriate for
selected patients.
Metastatic
tumours
Metastatic tumours comprise 15 per cent of
intracranial tumours. Approximately 30 per cent of deaths are due to cancer and
up to 25 per cent of these have intracranial metastatic deposits at autopsy.
The common sites of origin are illustrated in Table
35.4. In 15 per cent a
primary source is never found. Up to 60 per cent are multiple, with 85 per cent
situated above the tentorium. The interval between diagnosis of a primary and
cerebral metastasis varies considerably. Melanoma and renal carcinoma may
present with haernorrhage.
Metastatic
tumours appear isodense on unenhanced CT but enhance vividly after intravenous
contrast. Melanoma may appear hyperdense prior to contrast. MRI will frequently
reveal lesions not visible on CT. A chest radiograph is essential to exclude a
source or other metastic deposits. Steroids should be commenced to reduce
peritumour oedema and if surgery is planned an anticonvulsant (phenytoin 300 mg
nocte) started. Surgery is appropriate if there is a solitary surgically
accessible lesion and no systemic spread, particularly if the primary site is
unknown and the histological diagnosis in doubt. This may take the form of
either resection or biopsy. Stereotactic or image-guided techniques ate often
employed. Radiotherapy is used to treat multiple rnetastases and following
resection. Mote recently focused stereotactic tadiosurgery is being used in the
treatment of metastases smaller than 2 cm with results comparable to surgery.
About 30—50 per cent of patients will survive a year following resection.
Meningiomas
These account for 15 per cent of intracranial
neoplasms and are the most common benign neoplasms. They are uncommon in
children, occur more frequently in women than men and their incidence peaks in
middle age. They originate from meningothelial cells that occur in the greatest
abundance in the arachnoid villi, correlating with their site of occurrence.
They are most commonly found along the superior sagittal sinus (patasagittal),
over the free convexity and falx, along the sphenoid wing, beneath the frontal
lobes (olfactory groove and tuberculum sellae), within the posterior fossa (cerebellopontine
angle and fotamen magnum), the optic nerve and in the ventricles. They
classically arise from a broad base along the duta, may invade bone, and derive
their blood supply from the external carotid circulation (Fig.
35.31).
There
is a spectrum of histological appearances but malignant meningiomas ate
relatively rare. Surgical treatment should be, wherever possible, total
excision. If this includes the site of dural attachment, the recurrence rate is
less than 10 per cent. Incomplete excision of the dural attachment will result
in a recurrence rate of between 20 and 30 per cent. Subtotal excision will
result in 40 pet cent showing progressive growth. The role of radiotherapy is
controversial and is usually reserved for
tumours with aggressive histological
features or recurrent
tumours, or where a subtotal resection has been
performed.
Nerve sheath
tumours
These benign tumours originate from Schwann
cells and have a predilection for sensory nerves, especially the eighth nerve