THE IMPLEMENTATION OF PATIENT POSITION CORRECTION USING A MEGAVOLTAGE IMAGING DEVICE ON A LINEAR-ACCELERATOR

The problem of using information from the analysis of megavoltage images to adjust patient set-up has been addressed. In the case of rotational corrections it has been assumed that the treatment head is to be adjusted, although for gantry angles of 0° and 180° couch rotation may be used. In the case of translational shifts adjustment of the collimator jaws or of the couch have both been considered for arbitrary combinations of couch and gantry angle. For couch movement the case has been considered where it is desirable to minimize both the number of parameters to be adjusted and also the magnitude of the change in the patient's position. Values obtained for frequently used set-up parameters have been presented. Adjustment of the treatment couch positioning is the most desirable option, as this should bring the patient closer to the correct position for subsequent treatment fields. However, rotational errors are not correctable for all gantry angles and furthermore the collimator settings may be set more accurately than those of the treatment couch. Hence, in some cases, adjustment of the collimation system may be desirable or necessary. The formulae given in Equations (13) to (18) are currently being used in an intervention study to correct patient set-up during the course of a treatment fraction.