External beam radiotherapy

External beam radiotherapy treats a tumour using high-energy X-rays produced by a machine outside the body. The X-rays pass into the body and deliver radiation to the tumour.

X-rays are ionising radiation: they can remove electrons from atoms and molecules in cells. This can damage the cells’ DNA. When DNA is damaged, cancer cells become less able to divide and can die. Healthy cells in the path of the radiation can also be damaged.

Limiting damage to healthy tissue

The radiation beam is shaped and directed accurately at the tumour so that as little surrounding healthy tissue as possible is exposed.

Beams can also be aimed at the tumour from several directions. Their doses overlap at the tumour, giving it a higher dose, while each surrounding region lies in the path of fewer beams and receives a smaller dose.

Treatment may be divided into several sessions. The time between exposures allows healthy tissue to repair some of the radiation damage.