External Beam Therapy

These x-rays can destroy the cancer cells and careful treatment planning allows the surrounding normal tissues to be spared.

External beam therapy (EBT) is a method for delivering a beam of high-energy x-rays to the location of the patient’s tumor. The beam is generated outside the patient (usually by a linear accelerator, see below) and is targeted at the tumor site. These x-rays can destroy the cancer cells and careful treatment planning allows the surrounding normal tissues to be spared. No radioactive sources are placed inside the patient’s body. Delivery of external beam therapy requires a treatment team, including a radiation oncologist, radiation physicist, dosimetrist, and radiation therapist. The radiation oncologist is a physician who evaluates the patient and determines the appropriate therapy. He or she determines what area to treat and how much radiation to deliver. Together with the radiation physicist and the dosimetrist, the radiation oncologist determines what techniques to use to deliver the prescribed dose. The physicist and the dosimetrist then make detailed treatment calculations. The radiation therapists are specially trained technologists who deliver the daily treatments.

The process of external beam therapy can be divided into three parts:

  • Simulation
  • Treatment Planning
  • Treatment Delivery

During simulation, the radiation therapist places the patient in the treatment position on a special x-ray machine or CT scanner and takes simulation x-rays. Masks, pads, or other devices may be used to help the patient to hold still during the simulation and treatment processes. The radiation oncologist then locates the tumor volume and the region to be treated on these images. The dosimetrist and the radiation oncologist determine the best arrangement of radiation beams needed to treat the patient, and the radiation therapist places small marks on the patients to help guide the daily treatments.

For treatment planning the dosimetrist, radiation physicist, and radiation oncologist use a special computer to calculate the radiation dose that will be delivered to the patient’s tumor and the surrounding normal tissue. They calculate how long the treatment beam must be left on to deliver the prescribed dose. In certain cases, this process may employ such techniques as three-dimensional conformal therapy or intensity-modulated radiation therapy.

After the simulation and treatment planning have been completed, the treatment itself can begin.


The radiation therapist brings the patient into the treatment room and places him/her on the treatment couch of the linear accelerator in exactly the same position that was used for simulation using the same immobilization devices. The therapist carefully positions the patient using the alignment lasers and the marks that had been placed on the patient during simulation. The therapist goes outside the room and turns on the linear accelerator from outside. Beams from one or more directions may be used and the beam may be on for as long as several minutes for each field.

The treatment process can take from 10 to 30 minutes each day and most of the time is often spent positioning the patient.Patients usually receive radiation treatments once a day, five days a week, for a total time ranging from two to nine weeks. The patient’s diagnosis determines the total duration of treatment. Occasionally, treatments are given twice a day. External beam therapy is painless, but you will hear a buzzing noise during treatment.