Radiotherapy involves the use of ionising radiation to kill cancer cells. Ionising radiation is radiation that is able to affect material it passes through by disrupting its atomic and molecular structure causing instability. As a cancer treatment, the target of ionising radiation is cellular DNA.

Radiotherapy works more effectively for particular types and sites of cancers. It tends to be most effective when cells are dividing, particularly in phases of the cell cycle when DNA synthesis can be disrupted or replication disturbed (i.e. growth phases and mitosis). Radiotherapy can potentially affect all cells in the treatment area, but it has a greater effect on cancer cells, as they replicate more frequently and are therefore more likely to be in phases of the cell cycle when they are prone to its action. Successful treatment depends on maximising the dose delivered to the cancer while minimising the damage to normal cells. This is achieved by:

Control of the treatment field: Detailed plans are developed of the location and depth of the treatment area, often using CT or MRI simulation, to ensure the radiotherapy is focused on the cancer. Before each treatment, the patient is positioned carefully so that the radiotherapy is accurately and consistently delivered according to this plan. This is why it can take longer to position the patient prior to treatment than it takes to deliver it.

Fractionation: