Brain tumors are collections of abnormally proliferating cells arising from the brain or from its coverings. They fall into two broad categories: primary brain tumors and metastatic brain tumors (tumors that have spread from elsewhere in the body).
Overview
Primary brain tumors may arise from glial cells (astrocytes, oligodendrocytes and ependymal cells), from neuronal cells, from meningeal cells or from Schwann cells.
Most primary brain tumors, astrocytomas foremost among them, are classified according to a four-tier grading system established by the criteria of the World Health Organization (WHO), ranging from Grade I to Grade IV.
Why brain tumors develop remains unclear. Despite a great many extensive epidemiological studies, no specific risk factor has been identified. Risk has been found to increase with age. The risk of glioma and meningioma has, however, been shown to be raised by therapeutic radiation.
Metastatic tumors are the most frequently encountered group of brain tumors. All metastatic tumors are regarded as malignant. Their behavior varies considerably with the type of the tumor of origin. In order of frequency, the tumors that most commonly metastasize to the brain are:
- lung cancer;
- breast cancer;
- renal cell carcinoma;
- melanoma;
- colon cancer.
Types of Brain Tumor
There are many types of central nervous system tumor; some of the more common are described below.
Astrocytoma
Astrocytomas arise from the astrocytes that invest and support the central nervous system. They are divided into several grades according to their histological features, the grade being determined by a pathologist examining the cells under the microscope. Low-grade astrocytomas account for 15% of primary tumors and tend to present in patients in their thirties and forties. Treatment is surgical wherever possible, with chemotherapy and radiotherapy where required.
Glioblastoma Multiforme (GBM)
The commonest and most malignant form of astrocytoma is termed grade IV astrocytoma, or glioblastoma multiforme. It accounts for 25% of primary brain tumors and is among the tumors most frequently diagnosed in patients between 55 and 75 years of age. Standard treatment is a combination of radiation and chemotherapy following resection or biopsy.
Oligodendroglioma
These tumors arise from the oligodendrocytes, the cells that produce myelin (the sheath of the nerve cell) within the central nervous system. Oligodendrogliomas account for 15–20% of primary brain tumors. They carry a better prognosis than astrocytomas. Oligodendrogliomas with 1p19q deletion tend to respond better to treatment (radiotherapy or chemotherapy) and to behave more favorably than the others irrespective of treatment.
Ependymoma
Ependymomas arise from the ependymal cells lining the cerebrospinal fluid spaces. They occur in children as well as in adults, and account for 2.5% of the tumors encountered in adults. Standard treatment is a combination of surgery, radiotherapy and/or chemotherapy.
Meningioma
Meningiomas arise from the “cap” cells of the arachnoid, one of the membranes covering the brain and spinal cord. They account for 20–25% of primary brain tumors and their incidence rises with age. The great majority are benign and can be treated by surgical resection. Radiosurgery is also among the successful treatments for meningioma. For high-grade meningiomas, chemotherapy and other medical treatments are further options.
Epidermoid and Dermoid Tumors
Epidermoids are benign, keratin-forming lesions usually situated in the posterior fossa. Dermoid tumors tend to lie in the midline of the central nervous system and may contain such tissues as hair, sweat glands and sebaceous glands. Both may be treated surgically.
Schwannoma
Schwannomas arise from the Schwann cells that produce the myelin investing the peripheral nerves. They account for 8–10% of primary brain tumors. The commonest sites are the eighth cranial nerve (vestibular schwannoma) and the fifth cranial nerve. These tumors are usually benign and can be treated by surgical excision. A small schwannoma may be treated successfully by radiosurgery.
Symptoms
The symptoms of a brain tumor vary with the region in which the lesion lies. The following may be observed:
- headache;
- seizures;
- difficulty with speech;
- alteration of consciousness;
- weakness or numbness of one part of the body;
- disturbance of the cranial nerves (impaired hearing, vision or swallowing, for example).
The symptoms vary with the rate at which the tumor grows and are usually progressive.
Diagnosis
A brain tumor is usually diagnosed by magnetic resonance (MR) imaging with and without contrast. Computed tomography (CT) and digital subtraction angiography may be used in addition where required.
Treatment
The treatment options for a brain tumor are surgery, radiotherapy and chemotherapy. Depending upon the patient and the pathology, one of these may be used alone or several in combination.
At operation an incision is made in the scalp, the bone overlying the tumor is removed (craniotomy), and the tumor is then excised under the microscope. Patients usually remain in hospital for three or four days after surgery.
In such pathologies as astrocytoma, glioblastoma, oligodendroglioma, ependymoma and medulloblastoma the disease persists at a microscopic level even after the tumor has been removed, because of its infiltrative nature.
Radiosurgery is a further option for lesions such as meningioma, schwannoma, craniopharyngioma, pituitary tumors and metastatic tumors. Using a frame, radiation is delivered to the target region while the healthy brain tissue is spared.
Follow-up
All patients treated for a brain tumor require prolonged and close follow-up with neurological examination and imaging, both to assess the effectiveness of treatment and to detect recurrence.