Clinical experience with the Jefferson Hospital Radiotherapy Simulator.

Abstract

As radiotherapeutic technics become more sophisticated, the need is obvious for accurate field placement and for means of checking the field placement. Portal films are being widely used for this purpose, taken either by the therapy beam or by a diagnostic x-ray machine placed co-axially or isocentrically with the therapy beam. Such portal films have limitations. Details tend to be poor on supervoltage portal radiographs, and organs such as the esophagus are difficult to visualize. Corrections can be made only after the set-up has been completed and the film developed, so that considerable therapy machine time may be wasted. Moreover, only a static impression is obtained, whereas in moving-field therapy visualization of the irradiated volume throughout the whole sweep of the treatment beam might be desirable. In the past, machines have been constructed which duplicated the pertinent dimensions of therapy machines and were capable of the same range of motions (1, 2). Such simulators were employed principally for trial set-up and static field check. The improvement of image intensifiers and the use of closed-circuit television in diagnostic radiology now make it possible to obtain fluoroscopic visualization of the chosen volume in any part of the patient to be treated by either fixed or moving fields under precisely simulated treatment geometry. A simulator incorporating these features was designed with the help of and constructed by the Picker Special Products Division and has now been in clinical use in our department (Jefferson Medical Center, Philadelphia, Penna.) for almost two years. The system (Fig. 1) consists of a diagnostic x-ray tube mounted at one end of a C-arm and an image amplifier with an Orthicon television camera mounted at the other end of the arm. The image is displayed on a television monitor and can be recorded simultaneously on a video tape-recorder. Adjustment of the whole system is managed from a remote control mobile console. The patient lies on a Siemens therapy couch identical to that used for treatment. One ceiling mounted and two wall lights define the intersect of axis of rotation and central ray of the x-ray beam. The same system exists in the therapy room, and these lights are used for setting-up purposes for both simulation and therapy. The design details are as follows: The C-arm supporting the system is mounted on a bearing which is fixed in position on the wall of the simulator room. The size of the bearing and the rigid construction throughout assure stability of the beam at the center of rotation (±3 mm during 360° rotation). The x-ray tube is attached to one end of the C-arm so that the focus-to-axis of rotation distance can be varied from 75 to 100 em to accommodate various source-axis distances of the therapy units with which the simulator is used. This movement is provided by a hand-cranked lead-screw system.