Intracranial tumours

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 18 per 100 000 persons of which 30 per cent are primary. At autopsy the prevalence is 1—2 per cent. The incidence varies with age. In children tumours of the CNS comprise 20 per cent of all childhood malignancies. There is a peak at 2 years followed by a decline for the rest of the first decade. The incidence then slowly increases, peaking at 20 per 100 000 in late adulthood. The mortality rate averages 5 per 100 000. The classification of brain tumours is determined by their cell of origin. Over 50 per cent are neuroepithelial in origin, 15 per cent metastatic, 15 per cent meningiomas and 8 per cent pituitary tumours. The World Health Organization (WHO) classification is outlined in Table 35.3.

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) lobe. Tumours in the dominant hemisphere may cause problems with language (aphasia) and in the nondominant hemisphere apraxia. The optic pathway may be involved producing various visual symptoms, usually a contralateral homonymous hemianopia. Temporal lobe lesions frequently cause focal seizures with auras and visual field defects. Tumours in the frontal lobes can grow to considerable sizes before producing altered cognitive functioning and subtle personality changes. Subfrontal lesions may involve the olfactory nerves (anosmia). Sellar and parasellar tumours present with visual field and acuity problems due to com­pression of the optic chiasm. Tumours in relation to the ventricular system may obstruct CSF drainage and result in hydrocephalus, compounding raised ICR Tumours of the brainstem and cerebellar—pontine angle may result in cranial nerve palsies and long tract signs. Tumours involving the cerebellar vermis cause ataxia, while tumours in the hemispheres produce appendicular signs such as inco ordination and nystagmus.

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. Grading is on the basis of histological features, namely the presence of mitoses, necrosis, endothelial proliferation and nuclear atypia. Low-grade tumours have long median survival whereas high-grade tumours (glioblastoma multiforme) have a 20 per cent 2-year survival (Fig. 35.29).

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 (e.g. acoustic neuroma), followed much less frequently by the fifth nerve. Schwannomas of the eighth nerve arise from the superior or inferior vestibular portion in the internal auditory canal. As the tumour grows, it expands the internal auditory canal and extends into the cerebellopontine angle, compressing the pons, cerebellum and cranial nerves (Fig. 35.32). Bilateral Schwannomas of the eighth cranial nerves are diagnostic of type 2 neurofibromatosis. However, the majority of patients harbouring acoustic Schwannomas has no stigmas of this disease. Surgical intervention is aimed at total excision with preservation of neurological function. Patients frequently sustain seventh nerve palsies. More recently ‘gamma knife’ radiosurgery has been used effectively in the treatment of those tumours with a diameter of less than 30 mm.