Radiation therapy works by damaging the DNA in the cancer cell, thereby preventing the cancer cells from reproducing and growing. The cancer cells then die and the cancer shrinks. The objective of radiation therapy is to kill enough cancer cells to maximize the probability of cure and minimize the side effects. Under some circumstances, radiation therapy may also be used as palliation, or palliative care, which is aimed at reducing symptoms but not curing the underlying disease.

Radiation is usually administered in the form of high-energy beams that deposit the radiation dose in the body where cancer cells are located. Radiation therapy, unlike chemotherapy, is considered a local treatment. This means that cancer cells are killed only at the location in the body where the radiation is delivered, called the radiation field. If cancer exists outside the radiation field, those cancer cells are not destroyed by the radiation.