Radiotherapy is a treatment modality that uses ionizing radiation [similar to X-Rays] to destroy cancer cells. The particles in the radiation beam act on cells to produce free radicals, which then destroy the cancerous tissue.
Some 'bystander effect' is also seen on normal tissue around the tumour; this is however minimized using modern radiotherapy machines and techniques such as 'Cyberknife', Intensity Modulated Radiotherapy [IMRT] and Three-dimensional conformal radiotherapy [3D CRT].
Radiotherapy may be used as a curative modality by itself for e.g. for early stage cancers of the cheek and tongue or in combination with chemotherapy as chemoradiotherapy. The treatment may be curative in itself or given to shrink the tumour for surgery - neoadjuvant approach; as is done in rectal cancers.
Radiotherapy is often required after surgery for large tumours or where the tumour could not be completely removed, to prevent it from coming back.
Radiotherapy is also used to relieve symptoms due to a tumour, when it cannot be cured - to stop bleeding from a tumour, to relieve pain due to cancers, which have spread to the bones or relieve a block in the food pipe, intestine or voice box.