Definition and Overview

External beam therapy (EBT) is a procedure used to deliver one or more x-ray beams with high levels of energy right into a tumour. Also known as external radiation therapy, EBT makes use of beams that target the site of tumours to destroy the cancerous cells, while sparing the normal tissues that surround them. The high-energy x-rays are delivered from a machine called the linear accelerator (linac), and since the beams are generated outside the patient’s body, no radioactive sources are placed inside the patient’s body.

External beam therapy comes in two types, which are the three-dimensional conformal radiation therapy and the intensity-modulated radiation therapy (IMRT). The former works by first mapping out the tumour with the use of an imaging equipment and then treating the affected area using multiple beams of radiation. The intensity-modulated radiation therapy, on the other hand, is a treatment method that allows the intensity of the radiation to be changed while the treatment is ongoing.

Who Should Undergo and Expected Results

External beam therapy is most commonly used as a cancer treatment. Its main goal is to eliminate a tumour or to make sure that a tumour does not return after it has been removed. It can be performed either before or after the surgical removal of an existing cancerous tumour. When used before surgery, its goal is to reduce the tumour’s size prior to its surgical removal. When used after a surgery, its goal is to ensure that the cancer does not grow back.

For cancer cases that are already in the advanced stages, EBT can also be performed as a palliative treatment to lessens the symptoms associated with the disease, although it is assumed that the therapy can no longer treat the condition.

External beam therapy can be used to treat all types of cancer, including:

  • Breast cancer
  • Colorectal (bowel) cancer
  • Esophageal cancer
  • Head and neck cancer
  • Lung cancer
  • Prostate cancer
  • Brain tumour

In any case, the main goal of the procedure is to eliminate the malignant cells effectively without causing any damage to nearby healthy cells, which is typically the main disadvantage of other cancer treatments that are commonly associated with a lot of negative side effects. Due to its more selective and targeted effects, EBT is considered as the less risky option.

How Does the Procedure Work?

An external beam radiotherapy is performed by a radiation therapist, but it is usually requested by the radiation oncologist, a highly trained physician and cancer specialist who is responsible for creating treatment plans for cancer patients. The radiation oncologist typically determines the type of equipment that can best help in the treatment. It could be a linear accelerator or a cobalt machine, with the former being the more common choice.

An external beam therapy is executed in three steps:

  • Simulation
  • Treatment Planning
  • Treatment Delivery

The simulation is the part of the procedure that prepares the patient for the actual treatment. Its goal is to determine the treatment position that the patient is most comfortable with and to find out if there are any special needs.

The treatment planning stage aims to determine the dose of radiation that is needed to achieve the goals of the treatment. The radiation oncologist, radiation physicist, and dosimetrist use a special computer program that helps calculate the required dosage of radiation best suited for the patient’s condition. After the treatment planning, the actual treatment delivery can be initiated.

The patient is asked to change into a gown before the treatment begins. The radiation therapist then brings the patient into the treatment room where he or she is placed on the linear accelerator in the position determined in the simulation stage. The patient is then positioned properly using alignment lasers and marks, as it is important that the patient be positioned accurately. Imaging methods such as x-rays, ultrasound, and cone beam CT are used to document the patient’s position. Once the patient’s position has been verified, the therapist begins the treatment by turning on the linear accelerator from outside the room. Radiation beams from one or more directions then begin to directly target the areas of the tumour.

The overall treatment procedure typically takes an hour or less for each session. A big part is used in positioning and taking images of the patient, while the actual treatment only lasts a few minutes. The duration of the actual treatment depends on the type of method chosen. However, to complete the entire treatment plan, an EBT usually needs to be done several times. Depending on the type of cancer and its stage, the number of treatment sessions that one patient needs to undergo varies. Typically, however, a patient will need five sessions per week for 1 to 10 weeks.

After the completion of the treatment, patients are normally asked to return for follow-ups. In these visits, the patient undergoes an evaluation to know if the cancer has been completely eliminated or if more treatment is needed. The evaluation after the EBT is done using imaging exams and blood tests.

Possible Risks & Complications

External beam therapy is non-invasive and painless. Despite the radiation used, the treatment’s expected benefits often outweigh the risks. One of its advantages is that it is a low-risk procedure, as compared to other cancer treatments. The most common side effects of the treatment include fatigue and a feeling of irritation in the targeted area.


  • Zemen EM, Schreiber EC, Tepper JE. Basics of radiation therapy. In: Niederhuber JE, Armitage JO, Doroshow JH, et al., eds. Abeloff's Clinical Oncology. 5th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2013:chap 27

  • National Cancer Institute. Radiation therapy and you: support for people who have cancer. Available at: Accessed May 29, 2014

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