Radiotherapy, also known as radiation therapy and radiation oncology, is the medical use of ionising radiation to control or kill malignant cells. Radiotherapy may be curative in some cancers if they are localised to one area of the body. It may also be used as part of the therapy in preventing tumour recurrence after the removal of a primary malignancy, and in some cases may be used in relieving symptoms to improve quality of life. Radiotherapy is often used in a treatment plan with surgery and chemotherapy, and can be used before or after chemotherapy in many cancer treatments.

Radiotherapy is applied to either a cancerous tumour, or the surrounding tissue where cancer has been detected or removed, with the intention to kill or control cancerous cell growth. This process of radiotherapy works by damaging the cell DNA leading to cellular death. In order to conserve the healthy tissue surrounding the area being radiated, very carefully measured radiation beams are aimed from several angles to intersect only at the point of the affected area.