Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Radiation Therapy

What Is It?

Radiation therapy is a form of cancer treatment that uses an intense form of energy, called ionizing radiation, to damage or destroy cancer cells. Ionizing radiation attacks cancer cells by harming their genetic material, which kills them or interferes with their ability to grow and multiply. During radiation therapy, normal cells located near the cancer cells can be damaged as well, but these usually are able to recover and survive.

Radiation therapy can be given externally in the form of X-ray beams, gamma rays or beams of subatomic particles. Treatment with external radiation is usually painless and takes 5 to 15 minutes per session. The number of treatments varies for each person. In some cases, therapy can take several weeks of closely scheduled sessions.

Radiation also can be delivered internally. Radioactive substances either are placed inside a body cavity or implanted inside the tumor itself.

Some newer methods that can increase the effectiveness of radiation therapy include:

• Conformal beam techniques — Radiation is delivered from many beams at the same time. This allows the radiation to be concentrated on the tumor with less damage to nearby normal tissues.

• Intraoperative radiation therapy — Radiation is delivered to a tumor during surgery.

• Radiosensitizers — These drugs increase radiation's damaging effect on cancer cells.

• Radioprotectors — These drugs protect normal cells from radiation damage, while nearby cancer cells are destroyed.

• Radioimmunotherapy — Radioactive substances are attached to antibodies, which are defensive chemicals made by the body's immune system. These antibodies specifically target cancer cells and deliver damaging radioactivity only to them. Because the antibodies do not attack noncancerous, healthy cells, this reduces the potential for radiation damage outside the tumor.