Development of video frame store and distortion correction facilities for an external-beam radiotherapy treatment simulator

Abstract

During the simulation of an external-beam X-ray radiotherapy treatment, a low-energy X-ray source (tube voltage ≈ 50–100 kVp) is used to irradiate the patient and the transmitted intensities are recorded either by X-ray film or an X-ray image intensifier (XRII). The XRII enables the clinician to observe immediately the result of moving the patient within the X-ray beam in order to select an appropriate treatment field. Since the image is produced by the XRII in real time, the image is lost from the screen whenever the X-ray screening button is released. Therefore, the screening button is normally kept depressed during the time in which the clinician reaches a decision about the size and position of the correct treatment field. Since the displayed image is not altered during the decision period, a higher than necessary dose is delivered to the patient, and also to the clinical staff present, during the simulation session. To reduce this dose and simplify the simulation procedure, a device has been constructed which maintains the image on the television monitors in the simulator area whenever the screening button is released. This enables the clinician to view the most recent XRII simulation image during any decision period without delivering any further X-radiation to the patient. The true XRII image is displayed as soon as the screening button is next depressed. This mode of operation is termed the “active hold” mode.