TISSUE COMPENSATION USING DYNAMIC COLLIMATION ON A LINEAR-ACCELERATOR

Purpose: The availability of computer-controlled collinators on some a ccelerators has led to techniques for dynamic beam modification, mainl y to simulate beam wedge filters. This work addresses the practical as pects of dynamic tissue compensation in one dimension using available treatment-planning software. Methods and Materials: Data derived from the treatment-planning program is used with an iterative calculational routine to determine the monitor unit settings needed for the collima tor-controlling computer. The method was first tested by simulating a 60 degrees physical wedge. Further studies were carried out on a speci ally fabricated plastic phantom that modeled the sagittal contour of t he upper torso, neck, and lower head regions. Results: Dynamic wedge p oint doses generated by the planning program agreed within 1% with the values directly measured in a polystyrene phantom. In the patient pha ntom, dynamic collimation achieved calculated dose uniformity within 0 .5% in a reference plane near the phantom midline. A comparison of com puter-generated and measured point doses in this case showed agreement within 3%. Conclusions: Dynamic collimation can provide effective com pensation for contours that vary primarily along one direction. A conv entional treatment-planning program can be used to plan dynamic collim ation and deliver a prescribed dose with reliable accuracy.