Radiation Therapy for Cancer – National Cancer Institute

What is radiation therapy?

Radiation therapy uses high-energy radiation to shrink tumors and kill cancer cells (1). X-rays, gamma rays, and charged particles 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.

About half of all cancer patients receive some type of radiation therapy sometime during the course of their treatment.

How does radiation therapy kill cancer cells?

Radiation therapy kills cancer cells by damaging their DNA (the molecules inside cells that carry genetic information and pass it from one generation to the next) (1). Radiation therapy can either damage DNA directly or create charged particles (free radicals) within the cells that can in turn damage the DNA.

Cancer cells whose DNA is damaged beyond repair stop dividing or die. When the damaged cells die, they are broken down and eliminated by the bodys natural processes.

Does radiation therapy kill only cancer cells?

No, radiation therapy can also damage normal cells, leading to side effects.

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Radiation Therapy for Cancer - National Cancer Institute