Radiation is similar to chemotherapy in that both are designed to attack cells that are dividing .
Because cancer cells are diving rapidly and out of control, they are more susceptible to damage from radiation. During external beam therapy, radiation is generally delivered with a linear accelerator with a focused beam that affects only tissue within the radiation field. Radiation is only present when the linear accelerator is on. Occasionally, radioactive sources will be placed within the tumor bed area to treat the cancer internally. These can be permanently or temporarily placed, depending on the treatment needed, and is called brachytherapy , or internal radiation therapy.