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Radiotherapy
Radiotherapy, exposure of a defined portion of the body to a source of ionizing radiation, usually for the treatment of cancer. The radiation is either from naturally occurring radioactive isotopes, or from artificially produced X-rays. Treatment involves accurate localization of the tumour and the prescription of multiple daily, or periodic, fractions of irradiation for a specified time period. The unit of absorbed dose is the gray (Gy) and is equivalent to 1 joule per kilogram of body tissue.
Ionizing radiation causes damage to cells by interaction with nuclear DNA, thereby preventing normal cell division. As with the cytotoxic agents used in chemotherapy, there is only a limited selectivity in the effects of this form of cancer treatment, with normal (non-malignant) cells also invariably damaged. For this precise reason, radiotherapy must take into account the exact location of the particular tumour that is to be irradiated to minimize the exposure of normal tissues, while at the same time ensuring that the cancerous tissues are properly treated. It is extremely important that the patient can be accurately and reproducibly positioned during radiotherapy.
Patients are usually treated in the supine position (lying on their backs). A beam of radiation is shone through the body part to be irradiated. The precise anatomical location is first drawn out on the skin by use of a special marking pen. Alternatively, a radioactive source is placed near or within a body cavity and subsequently removed (see below). Increasingly, a method called computerized tomography (CT) is being used to assist in planning radiotherapy, especially as CT can provide information about the position of tumour margins (the outside edges of a tumour).
Radiotherapy localization is usually carried out on specialized equipment known as a simulator, which is designed to allow isocentric rotation. In simulating the exact axis distance of the treatment machine, the simulator ensures the least possible dose of radiation will be given to normal tissues in order that a homogeneous dose level will be applied to the tumour.
Most radiotherapy is given with teletherapy techniques, which use a beam of photons to irradiate the tumour from outside the patient's body. Alternatively, for specific defined sites, brachytherapy is used where a source of radiation is implanted in a body cavity or within the tumour itself.
Teletherapy requires the use of low energy orthovoltage or kilovoltage sources and more widely used megavoltage sources. Low-energy radiation (50-100kVp) is useful for treating carcinomas (malignant tumours of epithelia) of the skin and lip and orthovoltage (250-300kVp) is sometimes used for palliative treatment of bone metastases (spreading of a tumour to bone from a primary tumour site, conveyed by the bloodstream or the lymphatic system) and tumours of the chest wall. However, orthovoltage machines are unsuitable for treatment of deep-seated tumours.
Cobalt machines and linear accelerators are the most widely used teletherapy machines. These can be used in an isocentric fashion: the radiation source is mounted in a gantry which is rotated around the axis of the patient, thus allowing the direction of multiple beams to the centre of the target volume of the tumour, with great accuracy. In general, cobalt and linear accelerators are used for the treatment of carcinomas of the head, neck, and breast. Higher-energy accelerators are used to treat lymphomas (tumours of lymphoid tissues) and some deep-seated abdominal malignancies. In addition to producing X-rays, linear accelerators can also be made to produce accelerated electrons. These are charged particles which are reabsorbed within a finite distance inside the tissues and are extremely useful for the treatment of superficial cancers, such as those on the skin. This electron therapy can also be employed for the treatment of cancers of the head and neck, spinal cord, and lesions in the breast.
Brachytherapy is performed by introducing sealed sources of radioactivity, for example, caesium-137, into a body cavity or tumour for a few days. This is used particularly for the treatment of cervical cancer. The advantage of such therapy is that a high dose of radioactivity is achieved locally in the tumour, thereby sparing normal tissues. However, this type of treatment can only be applied when the tumour is readily accessible and when its size can be accurately assessed.
As well as treating cancer, radiotherapy can also induce cancer. In particular, children exposed to high levels of irradiation therapy during their early years were noted to subsequently develop thyroid malignancies. Indeed, most of us are familiar with the terrible consequences of atomic irradiation following the atomic bombing of Hiroshima and the Chernobyl disaster. Many childhood leukaemias may also be attributable to the effects of radiotherapy for other cancers.
As with chemotherapy, most patients on radiotherapy suffer side effects. These include lethargy and loss of appetite. Nausea and vomiting can also occur. In the skin, erythema (redness), dry desquamation, and itching are common problems. Occasionally, radiotherapy can cause a radiation enteritis (inflammation of the gastrointestinal tract). It can also lead to ablation of the bone marrow. Profound immunosuppression ensues, and patients commonly develop opportunistic infections such as TB, fungal infections, and parasitic diseases.
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