Radiotherapy is the treatment of cancer with ionising radiation, and can be used as the main form of treatment instead of surgery or in conjunction with the surgical removal of tumours by either decreasing tumour size before removal, or targeting any sections of tumour that may remain after surgery.  It is also used in combination with chemotherapy in an attempt to maximise the effect on the tumour tissue.  

There are various types of radiotherapy. Brachytherapy, where radioactive wires or seeds are implanted inside the body close to or directly within a tumour. Alternatively, in targetted radionuclide therapy a radioactive liquid that cancerous cells absorb is swallowed or injected. External beam therapy is the most commonly used technique and involves directing a beam of radiation in the form of X-rays, electrons or protons at a tumour from outside the body.    

Who needs it? 
Radiotherapy is a treatment for disease which is localised. Patients with cancerous tumours in organs or near body cavities, such as the prostate, may be treated with brachytherapy. Many other forms of cancer can be treated by external beam therapy. Some thyroid and blood disorders can also be treated.   

Radiotherapy uses the energy from radiation to kill cancerous cells. For brachytherapy treatments, commonly caesium-137 or iridium-192 are used and left in place for up to a few days. Alternatively sources emitting a particularly high dose of radiation are inserted for just a few minutes. Small sealed radioactive sources can also be permanently implanted. For example, iodine-125 seeds can be used to treat prostate cancer in its early stages.    
 
In external beam radiotherapy, different radiation types and numbers of beams are used depending on the cancer requiring treatment. The therapy is usually delivered over a period of weeks, so that the total radiation dose is given in small fractions to keep unwanted side effects - that can range in severity from nausea to new cancers - to a minimum. These side effects occur because the energy from the radiation that kills the cancerous cells also damages the healthy tissue that it passes through. Treatment plans are designed to minimise the dose to the healthy tissues. 
 
One way of doing this is to match the shape of the radiation beam to the tumour by using shielding materials to stop unwanted radiation. This also allows a greater dose to be used, increasing its effectiveness. Intensity Modulated Radiotherapy reduces such damage by using a combination of detailed imaging of the tumour prior to treatment and computer controlled delivery of the radiation beam so different radiation doses can be delivered to specific parts of the tumour.