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What is External Beam Radiation Therapy?

By Toni Henthorn
Updated May 17, 2024
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External beam radiation therapy, or teletherapy, is a technique for delivering high-voltage x-rays from an external radiation source to a cancer by using either a linear accelerator or a cobalt machine. Physicians precisely target the radiation dose to destroy the tumor cells while sparing the normal tissues near the cells. Patients typically undergo treatments once daily, five days per week, for approximately two to nine weeks. External beam radiation therapy does not incorporate the use of implanted radioactive plaques or other internal radiation sources. This form of radiotherapy is efficacious in the management of a variety of cancers, including breast cancer, brain tumor, prostate cancer, and lung cancer.

Therapeutic x-rays are generated by acceleration of electrons to high speeds. The electrons come from radioactive isotopes, such as cobalt-60 or iridium-192. Moving at high speeds, the electrons bombard tungsten alloy, resulting in rapid deceleration. This generates high-energy monochromatic photon beams, called gamma rays. Energy levels achieved vary with the isotope that is used.

The procedure for external beam radiation therapy transpires in three stages, comprised of simulation, planning, and treatment delivery. Simulation involves determining the patient’s best position for treatment, obtaining computerized axial tomography (CT) scans of the target organ with the patient in that position, and creating pads and devices that maintain the patient in that position during each treatment. Therapists place marks on the patient to guide placement and insert small marker seeds into the target organ or tumor. Planning by a team of dosimetrists, oncologists, and radiation physicists determines the exact zones to be irradiated and avoided, the tumor volume calculation, and the correct dose of radiation to be delivered. Treatment can begin after the simulation and planning stages are completed.

During external beam radiation therapy, the patient reclines on a treatment couch in the position prescribed during the simulation. Alignment marks and lasers aid the radiation therapist in precisely placing the patient. Once the correct arrangement is verified with x-rays or scans, the linear accelerator is activated. Treatments take between 10 and 30 minutes per day, with most of the time being spent on achieving accurate alignment. The exact duration of a therapy session is dependent on the dose and delivery method.

Patients do not experience pain during external beam radiotherapy. Many patients will hear a slight humming noise during the treatment. Ozone released by the linear accelerator may produce an electrical smell during the session. Occasionally, some patients, especially those who are receiving brain treatments, will notice colored lights.

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