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.
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 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.
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.
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.
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.
Thousands of Canadian Cancer Society volunteers work in regional cancer centres, lodges and community hospitals to support people receiving treatment.