Tissue compensation using dynamic collimation on a linear accelerator

The availability of computer-controlled collimators on some accelerators has led to techniques for dynamic beam modification, mainly to simulate beam wedge filters. This work addresses the practical aspects of dynamic tissue compensation in one dimension using available treatment-planning software.

Data derived from the treatment-planning program is used with an iterative calculational routine to determine the monitor unit settings needed for the collimator-controlling computer. The method was first tested by simulating a 60 degrees physical wedge. Further studies were carried out on a specially fabricated plastic phantom that modeled the sagittal contour of the upper torso, neck, and lower head regions.

Dynamic wedge point doses generated by the planning program agreed within 1% with the values directly measured in a polystyrene phantom. In the patient phantom, dynamic collimation achieved calculated dose uniformity within 0.5% in a reference plane near the phantom midline. A comparison of computer-generated and measured point doses in this case showed agreement within 3%.

Dynamic collimation can provide effective compensation for contours that vary primarily along one direction. A conventional treatment-planning program can be used to plan dynamic collimation and deliver a prescribed dose with reliable accuracy.