Radiotherapy is the use of high-energy ionising radiation such as X-rays, gamma rays, electrons, neutrons, protons and other sources to cure or relieve symptoms of cancer. Radiotherapy is also indicated for use in the treatment of some non-cancerous conditions. Approximately 50% of cancer patients will have some form of radiotherapy. This may be given as a stand-alone treatment or in conjunction with other modalities such as surgery and chemotherapy.

Radiotherapy can be administered externally from outside of the body or internally from within the body. External radiotherapy can be delivered by several different types of radiotherapy machines that work in different ways. Internal radiotherapy can be administered by placing solid radioactive material (the source) close to or inside the tumour for a limited period of time called brachytherapy. Other methods of internal treatment are discussed within the nuclear medicine section.

Radiotherapy works by depositing energy within the cells of the treated area. This causes damage to the cell's DNA. This damage is irreparable in cancer cells and renders them incapable of replication. Normal cells can also be damaged but are usually able to repair themselves. Different tumour types respond to radiotherapy in different ways, some being more or less sensitive. A range of doses and schedules are used, from a single visit up to several weeks, with each treatment uniquely planned for each individual patient.

The aim of radiotherapy is always to maximise the effectiveness of treatment while minimising short and long-term side-effects. Technological advances in tumour visualisation and radiotherapy treatment delivery have, and continue to, reduce the side effects of radiotherapy treatments.