Radiotherapy can be used to treat cancer in one of two ways. It can either be given to try and cure the cancer or to relieve symptoms caused by the cancer.

When used to try and cure the condition high doses of radiation are needed. This means that the area of the body that has been treated will usually have received close to the maximum safe dose of irradiation. As a consequence it would not be safe to give further courses of radiotherapy to that particular site, although treatment could be given to other parts of the body if it was necessary.

Generally much lower doses, and shorter courses of treatment, are needed when radiotherapy is used to relieve unpleasant symptoms. Often for bone pain a single dose of radiotherapy, given as an out-patient, is sufficient to ease the discomfort (although it is usually a week to 10 days after treatment before the benefit is apparent). The use of lower doses of radiation means that treatment can be repeated at the same site, sometimes on several occasions, if it is necessary.