External beam radiation therapy is typically delivered as a series of outpatient treatments using a linear accelerator, or LINAC. Treatment X-rays, like chest X-rays, cannot be seen or felt, and the machine does not touch you. Treatments are usually given daily, five days a week, for three to seven weeks. You will be scheduled for a planning session before beginning treatment to map out your treatment area. This is referred to as a simulation. A CT scan is performed while lying on a table, usually with the assistance of a form-fitting mask, to ensure that treatment is delivered consistently. Based on the results of the simulation scan and other imaging studies you’ve had, including MRIs, your doctor will create an individualized treatment plan. Marks are made on the mask to assist the radiation therapist in positioning you precisely for daily treatment.