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Simulation for external beam radiation therapy

Simulation is a planning session done before the first external beam radiation treatment is given. It is done to make sure the radiation is aimed at exactly the same area each time treatment is given. Simulation is usually done in one session and may take anywhere from 15 minutes to an hour or more.

A machine called a simulator is used to set up the treatment plan and choose the treatment area. A simulator is not a treatment machine. The most common type of simulator used is a CT (computed tomography) simulator. It can take images or scans, which give the healthcare team a picture of the part of the body to be treated. A conventional x-ray simulator may also be used in some situations. These images help the radiation therapy team decide where and how to direct the radiation.

Calculating the dose

The dose of radiation is precisely calculated and prescribed in units called grays (Gy). The total dose is usually divided into several smaller daily doses (called fractions). The dose of radiation and the number and length of treatments depend on:

  • the type of cancer
  • how sensitive the tumour is to radiation therapy (radiosensitivity)
  • tumour size
  • tumour location
  • the stage of the cancer
  • the amount of tissue to be radiated
  • the ability of surrounding normal tissue to tolerate radiation
  • whether other cancer treatments have been or will also be given


The position that the person should be in to receive the treatment is chosen during the simulation. The person is positioned the same way each time a treatment is given.

Skin markings

The skin may be marked to act as a map and help ensure that each radiation treatment is targeted to the same area. The skin may be marked with a pen or ink, and the person must be careful not to wash off these temporary markings. Tiny permanent tattoos – about the size of a freckle – may also be used.

Immobilization devices

Sometimes immobilization devices are made to help the person stay in the same position and lie still for treatment. Forms, moulds, pads or other devices are made from plastic, foam, plaster or other materials. Immobilization devices are specially made to fit the person’s shape. Marks to pinpoint the treatment area may be made on the mould rather than the skin.

Shields and blocks

Special shields or lead blocks may be used to protect or guard normal tissue or organs from radiation. Blocks are usually made specifically for each person. Just before treatment is given, the shields or blocks are placed between the radiation therapy machine and the areas of the body that need protection.

The healthcare team enters the data collected during simulation into a sophisticated computer to develop the treatment plan. The first external beam radiation treatment is usually scheduled shortly after the planning session.


A description of the extent of cancer in the body, including the size of the tumour, whether there are cancer cells in the lymph nodes and whether the disease has spread from its original site to other parts of the body.

Stages are based on specific criteria for each type of cancer.

The process of determining the extent of cancer in the body based on exams and tests is called staging.


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