Radiation therapy uses high-energy radiation to shrink tumors and kill cancer cells . Photons (x-rays, gamma rays), and subatomic particles (alpha particles, electrons, protons, neutrons) are types of radiation used for cancer treatment.

The radiation may be delivered by a machine outside the body (external-beam radiation therapy), or it may come from radioactive material placed in the body near cancer cells (internal radiation therapy, also called brachytherapy).

Systemic radiation therapy uses radioactive substances, such as radioactive iodine, that travel in the blood to kill cancer cells.

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