Proceedings of the British Institute of Radiology Megavoltage portal imaging

A prototype Theraview™ system was installed on a Varian Clinac 6/100 6MV linear accelerator at Cookridge Hospital during one weekend in April 1991. Minor modifications were needed to the gantry counterweights etc. The system is used with the lower accelerator cover removed. The detector head is a copper plate/thick phosphor (450 mg m−2) fluorescent screen measuring 40 × 40 cm (detection resolution 1 lp mm−1). Contrast detectability of the system is 0.5%. Light is reflected applicathrough 90° by a high reflectance mirror into a 1000 lines high resolution video camera. The detector head can be retracted (manual — 125 cm from beam axis towards the gantry), lowered (motorized — 3O cm travel), or detached to facilitate patient set-up. Images (512 × 512 matrix) are acquired in three ways: single exposure of the treatment area, double exposure to demonstrate surrounding anatomy, and continuous (movie-loop).