Gliomas arise from the glial (supporting) cells of the brain. They are not derived from the neurones (nerve cells) themselves. These are the commonest primary brain tumours.
There are three types of glioma: the commonest is the astrocytoma, followed by the ependymoma and the oligodendroglioma, each one gaining its name from the cell type from which it arose.
Gliomas are graded by their histological appearances (appearances down the microscope). Grade 1 and 2 gliomas are relatively slow growing and may have a long natural history, so much so that sometimes they are watched carefully by neurologists without proceeding to active therapy. High grade tumours: grade 3 and grade 4 gliomas (grade 4 also known as glioblastoma multiforme) are much more malevolent and require active management.
The diagnosis of a higher grade glioma may have been suggested on the MR scan as these faster growing tumours may outstrip their blood supply and this leads to a highly typical necrotic (dead) centre to the tumour as seen on scan. The biopsy or decompression specimen confirms the diagnosis. Gliomas are dangerous because of uncontrolled growth within the brain; they do not (or hardly ever) metastasise outside the brain.
Meningiomas arise from the coverings of the brain; the cell of origin is the arachnoid fibroblast. They are usually benign tumours that are cured by a radical surgical excision (or stereotactic radiation therapy e.g. gamma knife technology) in most instances.
Pituitary tumours are of interest in that they often secrete a hormone product (the pituitary being the master controller of the endocrine/hormone system in the body): prolactin by prolactinomas, growth hormone by the acidophil adenoma that causes acromegaly and gigantism, and adrenocorticotrophin by the basophil adenoma that causes Cushing’s disease. Furthermore, they arise just under the visual pathways, where the optic nerves pass backwards from the eyes into the brain , and by pressure exerted by their growth upwards can disturb vision (particularly vision out to the sides), and this can be the presenting symptom, particularly in the cases where there is no hormonal over-secretion.
Like meningiomas, pituitary tumours are almost invariably benign adenomas.
Craniopharyngioma is a benign overgrowth of embryonic remnants that occurs in the pituitary area, usually just above the pituitary itself; it declares itself by pressure effects e.g. on the visual pathways and with visual loss, or, in children particularly with growth disturbances due to pituitary dysfunction.
Acoustic/vestibular neuroma is a benign tumour of the sheath of usually the vestibular nerve. The vestibular nerve subserves the function of balance and runs from the brainstem to the inner ear accompanied by the highly sensitive auditory nerve which subserves hearing. The pressure of the tumour against the acoustic nerve is enough to cause gradual hearing loss and this is the method of presentation in the majority of cases.
Other cranial nerves may develop neuromas but there is a poorly understood predilection for the disease to affect the vestibular nerve and patients who have the condition neurofibromatosis are at high risk. The MR image is usually fairly characteristic of an acoustic/vestibular neuroma, although it may occasionally be mistaken on scan for a meningioma or other growths.
Other brain tumour types
This is a tumour that occurs in the cerebellum of younger patients (children more than adults) and infiltrates rather like a glioma. Additionally, it has a predisposition to shed off cancer cells into the fluid (cerebrospinal fluid or CSF) which surrounds the brain and this may lead to spread within the CSF and seed implantation and growth of metastatic disease at other sites within the brain or spinal cord. Similar tumours called: Primitive NeurEctodermal Tumours (PNET) arise in other sites in the brain and are treated as are medulloblastomas.
The treatment is surgical resection followed by radiotherapy to the whole neuraxis (the brain and all its coverings, together with the spinal cord and all its coverings). Furthermore, this tumour has been shown to be sensitive to chemotherapy and particularly to platinum based therapy. Therefore most current protocols have a chemotherapy component as well.
Primitive neuroectodermal tumour (PNET)
This is a homologous tumour to the cerebellar medulloblastoma down the microscope, but occurs in the cerebral hemispheres. It also has a predisposition to spread via the CSF to other neuraxis sites and it is basically treated as for medulloblastoma viz. surgical resection followed by neuraxis radiotherapy and chemotherapy. It should be noted that ependymomas (one type of glioma vide supra) can also spread via the CSF and neuraxis radiotherapy may also be indicated here in some cases (for example high grade ependymomas of the posterior fossa).
Intracranial germ cell tumours
These are homologous with the germ cell tumours of the testis or ovary, but are malignant and need to be treated seriously (in the modern era they are usually curable). The most common site of origin is in the pineal region followed by the region above the pituitary gland (the supra-sellar region). Others can occur at other midline sites e.g. the fourth ventricle.
These tumours present due to expansive growth and compressive neurological symptoms. The diagnosis is made by biopsy or tumour markers (blood or CSF: HCG or AFP – see testicular teratoma section for explanation of these marker terms).The therapy of intracranial germ cell tumours is by chemotherapy and neuraxis radiotherapy for they too have a high tendency to spread via the CSF to other sites within the neuraxis but rarely further afield.
These tumours are exquisitely sensitive to chemotherapy based on cis-platinum and shrink quickly and dramatically with this therapy. Treatment usually commences with chemotherapy and radiotherapy follows completion of the chemotherapy programme. Surgery is usually restricted to a biopsy (if there is doubt over the diagnosis) or ‘shunt’ to overcome hydrocephalus at presentation; there may be a role for removal of a residual lump at the end of therapy, but this necessity is unusual.
The effects of chemotherapy are so profound that many are now reducing the subsequent radiotherapy dosages, although most have not yet been convinced that the cure of these tumours can be routine without radiotherapy. Tumours which are not secreting markers HCG or AFP tend to have the best outlook for cure, perhaps in excess of 80%, whereas those secreting large quantities of AFP certainly have a lesser chance of cure.
These are high grade B cell lymphomas of brain and occur spontaneously but are predisposed to in immunosuppressed patients (e.g. renal transplant recipients, AID patients etc) more frequently than the rest of the population. They present as do gliomas with which they are often confused on brain MRI scans.
The diagnosis is made by biopsy and therapy is then commenced with chemotherapy followed by radiotherapy - although the role of radiotherapy is being questioned in the age of high dose methotrexate based chemotherapy. The tumour is highly responsive to this therapy and survivals in excess of 50% are achieved in patients without spread beyond the primary site at presentation.
These are vascular malformations within the brain and are not really brain tumours at all. However, they are of some importance as they tend to bleed and are the commonest cause of strokes in young patients. Furthermore the component blood vessels supply no blood to normal brain (i.e. they are redundant) and so surgical removal or obliteration by stereotactic radio surgery (vide supra) effects cure without depriving normal brain of oxygenated blood.
Up to 20% of all ostensibly primary brain tumours turn out to be metastases from a primary cancer outside the central nervous system. In particular primary breast, lung, renal and adrenal carcinoma and melanoma have a predisposition to spread to the brain; other primary cancers may also do so.