Benign Brain Tumour

 

Such tumours include meningioma and acoustic neur(in)oma. Incidence is around 1 per 100,000 population for each type and each represent 10-15% of all tumours. Occur in about 5:1 ratio for women to men. These benign tumours take many years to mature.

 

Symptoms and Signs

 

General manifestations result from increased intracranial pressure. This may be due to the space-occupying tumour mass itself or to associated cerebral oedema, obstructed flow of CSF (posterior fossa tumours), obstructed dural venous, or obstructed CSF absorption mechanisms (as in leukemic or carcinomatous involvement of the meninges). Headache and vomiting result, as may mental symptoms. Papilloedema (swelling and congestion of the disk or head of the optic nerve within the globe of the eye as a result of increase of pressure within the skull or of severe hypertension) develops in about 25% of patients with brain tumour and may not be an early sign. Its absence does not rule out a tumour or elevated intracranial pressure. In young children, elevated intracranial pressure may enlarge the head. Intracranial pressure is usually normal in patients with small tumours of the cerebral hemispheres, pituitary adenomas, or brainstem tumours that do not obstruct the aqueduct of Sylvius (cerebral sulcus).

 

This is any of numerous deep grooves in the surface of the cerebrum that delineate its various anatomic divisions, or lobes. The fissures originate in the extensive folding of the brain's surface, which greatly increases the area of the cerebral cortex, the major functional section of the human brain. The main fissures:

 

          lateral fissure, or fissure of Sylvius, between the frontal and temporal lobes

          central fissure, or fissure of Rolando, between the frontal and parietal lobes, which separates the chief motor and sensory regions of the brain

          calcarine fissure on the occipital lobe, which contains the visual cortex

          parietooccipital fissure, which separates the parietal and occipital lobes

          longitudinal fissure, which separates the cerebral hemispheres almost completely, leaving only the corpus callosum at the bottom of the fissure to connect them

          transverse fissure, dividing the cerebrum from the cerebellum.

 

Changes in temperature, pulse, respiratory rate or BP are unusual except terminally. Convulsive seizures, either focal or generalised, occur with cerebral hemisphere tumours and may precede other symptoms by months or years. They are more frequent with meningiomas and slowly growing astrocytomas than with malignant gliomas. Focal seizures help to locate the tumour. Mental symptoms (drowsiness, lethargy, obtuseness, personality changes, disordered conduct, impaired mental faculties, psychotic episodes) may appear at any time. They are the initial symptoms in 25% of malignant brain tumours. Special (focal) manifestations are due to localized destruction or compression of nerve tissue or to altered endocrine function, and depend on the tumour's location.

 

Tumours of the cerebral hemispheres

 

Frontal lobe tumours (commonly meningiomas or gliomas) involving the frontal convexity are characterized by progressive hemiplegia (paralysis of one side of the body), focal or generalized seizures, and mental changes. Expressive aphasia may accompany a tumour of the dominant hemisphere. A tumour at the base of the frontal lobes (particularly meningioma of the olfactory groove) produces ipsilateral (same side of the body) anosmia (inability to detect odours). A tumour on the medial surface of a frontal lobe may cause precipitate urination. Mental changes (especially inattention and loss of motivation) and ataxic gait are common when the tumour spreads across the corpus callosum to both frontal lobes.

 

Meningioma of the tuberculum sellae may compress the optic chiasm, producing a visual field defect similar to that of a pituitary adenoma (see discussion of tumours of the pituitary and suprasellar region, below). Meningioma of the inner third of the sphenoid ridge may cause exophthalmos (protruding eye) and unilateral amblyopia (one-sided vision dimness). Meningioma of the outer part of the sphenoid ridge may invade the temporal lobe (see discussion of temporal lobe tumours, below). Parietal lobe tumours may produce either generalised convulsions or sensory focal seizures.

 

False localising signs may accompany prolonged elevated intracranial pressure. They include uni- or bilateral lateral rectus palsy from 6th nerve compression, hemiplegia on the same side as the tumour from compression of the opposite cerebral peduncle (tract of white fibres in the brain) against the tentorium, and visual field defect on the same side as the tumour from compromise of the opposite posterior cerebral artery.

 

Tumours of the brainstem

 

Gliomas of the brainstem are usually astrocytomas. Common symptoms, resulting from destruction of nucleur masses, are unilateral or bilateral paralysis of the 5th, 6th, 7th, and 10th cranial nerves, and paralysis of lateral gaze. Damage to the motor or sensory pathways causes hemiplegia, hemianesthesia, or cerebellar disturbances (ataxia, nystagmus, intention tremor). Increased intracranial pressure appears late in brainstem tumours.

 

Posterior fossa tumours. Tumours of the 4th ventricle and cerebellum (usually medulloblastomas, gliomas, ependymomas, or metastoses) interfere with CSF circulation, and symptoms of increased intracranial pressure appear early. Ataxic gait, intention tremor, and other signs of cerebellar dysfunction follow.

 

Cerebellopontine angle tumours, particularly neurilemomas (acoustic neurinomas, schwannomas), are characterized by tinnitus, unilateral hearing impairment, and sometimes vertigo. Pressure on the adjacent cranial nerves, brainstem, and cerebellum produces loss of corneal reflex, facial palsy and anesthesia, palatal weakness, signs of cerebellar dysfunction, and, rarely, contralateral hemiplegla or hemianesthesia. Loss of vestibular response to caloric stimulation, enlargement of the porus acusticus as shown by x-ray, and high CSF protein content suggest an acoustic neurilemoma (see also acoustic neurinoma).

 

Meningiomas. Benign tumours that appeor to arise from arachnoidal cells and therefore can occur wherever there is dura. The most common locations are over the convexities near the venous sinuses, along the base of the skull, and in the posterior fossa. Rarely, meningiomas may arise within the ventricles, presumably from cells migrating in along with the choroid plexus. Meningiomas < 2 cm in diameter are among the most common tumours found at biopsy. While a variety of pathologic types have been described, they are not usually associated with different clinical courses. Meningiomas occur more commonly in women and are the only intracranial neoplasms to do so. They tend to occur between ages 40 and 60 yr, but can present in childhood.

 

Meningiomas may be multiple and may be become malignant, especially the hemangiopericytoma variant. Meningiomas are one of the few tumours that present characteristic changes in plain skull x-rays. The CT scan is the most specific diagnostic tool, but most meningiomas can be visualized on MRI. Convexity meningiomas often give rise to focal seizures and ultimately to signs of mass effect in the hemispheres. Radiographically they may produce skull atrophy, dilated blood vessels, and occasionally hyperostosis (bone swelling). Parasagittal or falx (sickle-shaped) meningiomas may produce a progressive spastic weakness or numbness, usually beginning in the leg opposite the lesion, but occasionally extending to both legs. The resulting paraparesis may be confusing and lead the physician incorrectly to believe that the patient has a spinal cord lesion.

 

Differing from supratentorial tumours in their presentation, tumours along the base of the skull primarily produce visual disturbances and exophthalmos, while tumours in the posterior fossa usually produce hydrocephalus. Olfactory groove meningiomas impair the  sense of smell and may produce papilloedema and visual loss. Tuberculum sellae meningiomas produce visual loss and are characterized by bony changes. Sphenoid wing meningiomas may arise in the medial, middle, or lateral aspect of the sphenoid wing. If medial, they tend to grow into the cavernous sinus; if in the middle portion, they may grow anteriorly into the orbit, and in the lateral portion, may grow into the temporal bone, producing either a globular mass or a meningioma en plaque (ie, spread into the dura, with dural thickening and invasion of adjacent bone).

 

Posterior fossa meningiomas may occur as a tentorial tumour that grows above and below the tentorium and principally produces hydrocephalus. Clivus and apical petrous bone meningiomas produce gait instability and limb ataxia as well as abnormalities of cranial nerves 5, 7, and 8. Meningiomas around the foramen magnum produce suboccipital pain on the ipsilateral side along with a characteristic weakness that begins in the ipsilateral arm, progresses to the ipsilateral leg, and then to the opposite leg and arm. Cranial nerve involvement may produce dysphagia (swallowing difficulty), difficulty speaking, nystagmus, diplopia, and sensory changes in the face.

 

The treatment of meningiomas is surgical removal if at all possible. When tumours are small or when they occur in the elderly, surgery may be deferred because of the risk to life. Generally, tumours of medium-to-large size can be removed safely and completely, but very large tumours may encroach on vascular elements, especially the surrounding veins, making them especially difficult to remove. There may be value in irradiation for residual or recurrent meningioma. Schwannomas should be removed surgically.

 

In national studies the median survival after surgery, irradiation, and chemotherapy is about 1 yr and 25% of patients survive 2 yr. Favourable prognostic variables include age (<45 yr), pathology of anaplastic astrocytoma rather than glioblastoma multiforme, and better clinical status and little or no residual tumour after initial resection. Low-grade gliomas (astrocytoma, oligodendroglioma) should be resected if possible, or undergo needle biopsy if resection is too dangerous, and then receive radiotherapy.

 

There is some debate about timing of the radiation. MRI permits early diagnosis, when radiation might offer better treatment, but might also expose the brain to damage (from irradiation) earlier than necessary. The overall prognosis is considerably better than with malignant gliomas. Patients might be expected to live 3 to 5 yr before the tumour recurs. In recent years, chemotherapy has been added both as adjunctive therapy and for recurrent disease. Whereas several agents have been reported to be effective in treating recurrent medulloblastoma, an adjunctive chemotherapy regimen has yet to be shown consistently effective, although high-risk patients may benefit. Prognostically, at least 50% of these patients will survive 5 yr, and perhaps 40% for 10 yr. The common intracranial ependymoma arises in the 4th ventricle, usually in children, and obstructs CSF flow, producing headache, vomiting, and ultimately cranial nerve palsies and ataxia. While most of these tumours are benign, malignant degeneration does occur and the tumour may seed along CSF pathways in a manner similar to medulloblastoma. Ependymomas are usually approached surgically with the same goals as for medulloblastoma, ie, to remove as much tumour as is neurosurgically safe and to open the CSF pathways. The incidence of seeding varies with different reports, and thus treatment protocols have differed among treatment centres. One recommendation is to use whole-brain irradiation for all patients with supratentorial low-grade ependymomas, whole brain plus cervical cord extensions for low-grade infratentorial ependymomas without evidence of CSF seeding, and craniospinal irradiation for all high-grade ependymomas or low-grade tumours with evidence of seeding.

 

Pathology and Incidence

 

Generally, CNS changes result from invasion and destruction by the tumour and from its secondary effects (increased intracranial pressure, cerebral oedema, and compression of brain tissue, cranial nerves, and cerebral vessels). Rarely, remote neurologic effects occur as "parancoplastic syndromes". Brain tumours are found in about 2% of routine autopsies. They may occur at any age but are most common in early adult or middle life. Common primary childhood tumours are cerebellar astrocytomas and medulloblastomas, ependymomas, gliomas of the brainstem and optic nerve, germinomas (pineclomas), and congenital tumours. The most common metastatic invaders in childhood are neuroblastoma (usually epidural) and leukemia (meningeal). Primary adult tumours include meningiomas, schwannomas, gliomas of the cerebral hemispheres (particularly the malignant glioblastoma multiforme, anaplastic astrocytoma, and the more benign astrocytoma and oligodendroglioma).

 

The relative frequency of various types of intracranial tumours is gliomas 45%, pituitary adenomas 15%, meningiomas 15%, schwannomas 7%, congenital tumours 3%, metastatic and other types 15%. Overall incidence in males and females is about equal, but cerebellar mebulloblastoma and glioblastoma multiforme are more common in males; meningioma and schwannoma, in females.

 

How lethal a brain tumour is depends on its size, location, rate of growth, and histologic grade of malignancy. Benign intracranial tumours grow slowly, with few mitoses, no necrosis, and no vascular proliferation. They may achieve considerable size before producing symptoms, in part because there is often no associated cerebral oedema. Malignant tumours are characterized by more rapid growth, invasiveness, frequent mitotic figures, necrosis, vascular proliferation, and endothelial hyperplasia. However, "benign" brain tumours that cannot be entirely excised because of size or location are usually lethal. "Malignant" brain tumours rarely metastasise out of the CNS and cause death by inexorable local growth. Thus, the distinction between "benign" and "malignant" is less important for intracranial neoplasms than for systemic cancer.

 

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