Simulation helps your radiation oncologist define your treatment area - the exact place in your body where the treatments will be aimed. Many cancers are located deep within the body. Your radiation oncologist must know the exact location of your cancer in relation to your body surface and surrounding normal tissues. This allows him or her to focus treatment on cancerous cells while sparing normal cells from damage. In addition to examining you and reviewing your medical history, the doctor may request CT scans, MRI studies, ultrasound exams or nuclear medicine tests to help the doctor map out your treatment area.

During the simulation process, you will be asked to lie very still while the radiation therapist uses a special X-ray machine, called a simulator or a CT scanner, to set up the treatment area(s) according to the doctor's direction. Once your treatment area (portal) is determined, the skin over the area is marked with colored ink. This allows the radiation therapist to position you precisely on the treatment machine each day.

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