The majority of Radiation Oncology patients have at least one simulation procedure prior to initiating a course of radiation therapy. At NewYork-Presbyterian we use a specialized type of Computerized Tomography (CT) scanner called a CT Simulator. This piece of equipment produces 3-D images of each patient's internal anatomy which helps our doctors to precisely locate the region of interest as well as the surrounding critical structures. Highly accurate measuring devices are then employed to record the treatment site in relation to external markers. This critical first step is important in defining the area to be treated, as well as the fashioning of any necessary immobilization devices to assure that the patient is in the same position each day throughout the entire course of treatment.