Before beginning radiation treatment, the radiation oncologist must plan a customized treatment program so that all the areas "at risk" (i.e., tumor or areas potentially harboring cancerous cells) are included in the radiation treatment field while sparing as much normal tissue as possible.

This usually requires a treatment planning session using a special x-ray machine called a simulator. This session is called a simulation. A radiation therapist will work with the radiation oncologist during the simulation.

More than one simulation may be required. Additional imaging procedures, such as CT scans and MRIs, may also be necessary to outline the affected areas and/or to spare normal tissues.

During the simulation, the exact location of the affected area is determined and treatment fields are designed. X-ray pictures are typically taken at that time. Patients do not receive any radiation therapy during this time. 

Once the area to be treated has been delineated, the skin over this area may be marked with small ink dots and pinpricks to serve as permanent "landmarks." These assist the radiation therapists in directing the radiation beam precisely to the treatment area. Depending on individual requirements, the simulation may last from 30 minutes to two hours.

Following the simulation, further treatment planning and calculations are made to determine the best method of delivering the exact dose to the affected area.

Additionally, special shielding devices or blocks may be custom made for use in the treatment. All this may take up to a week to complete before treatment begins.