Present-day treatment of intracranial tumours with radiotherapy utilises one of the following:
- megavoltage X-rays
- y rays from cobalt60
- electron beam from a linear accelerator.
- accelerated particles from a cyclotron, e.g. neutrons, nuclei of helium, protons (awaits full evaluation).
In contrast to older methods of 'deep X-ray therapy', these modern techniques produce greater tissue penetration and avoid radiation damage to the skin surface. The effect of radiotherapy depends on the total dose - usually up to 60 Gy, and the treatment duration. This must be balanced against the risk to normal structures. Treatment aims to provide the highest possible dose to a specified region whilst minimising irradiation to adjacent normal brain. Various methods have been developed to achieve this -
• Stereotactic irradiation where multiple converging beams from a linear accelerator or from multiple cobalt60 sources are focused on a selected target (see page 371).
• Interstitial techniques where the tumour is treated from within (brachytherapy) by the implantation of multiple radioactive seeds, e.g. iodine125.
• Conformal therapy where standard radiotherapy is administered, but the beams are shaped by the use of variable collimators or blocks which conform with the shape of the tumour, thereby eliminating normal brain.
Radiotherapy is of particular value in the management of malignant tumours - malignant astrocytoma, metastasis, medulloblastoma and germinoma, but also plays an important part in the management of some benign tumours - pituitary adenoma, craniopharyngioma. With some tumours that seed throughout the CSF pathways, e.g. medulloblastoma, whole neural axis irradiation minimises the risk of a distant recurrence.
Complications of radiotherapy: following treatment, deterioration in a patient's condition may occur for a variety of reasons:
• Increased oedema - during treatment - reversible.
• Demyelination - after weeks, months - usually reversible.
• Radionecrosis - in usually 1-2 years (range 6 months-10 years) irreversible. Similar complications may involve the spinal cord after irradiation of spinal tumours.
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