Direct aperture optimization for a variable aperture collimator for intensity-modulated radiation therapy

In this note a technique is described for direct aperture optimization of components deliverable by a variable aperture collimator (VAC) for intensity-modulated radiation therapy. The first key result found was that, provided a large number of VAC components were selected for optimization, the resulting fluence profiles and the dose distribution were quite similar, but not identical, to the outcome of a direct inverse-planning algorithm in which the fluence of each bixel was individually adjusted during the iteration process. A second key feature is the ability to be able to construct highly modulated beams from a quite limited number of such components. It was shown that, when the number fell from 300 to 30, a recognizable conformal dose distribution was still obtainable although poorer. The conclusion was that the technique has the flexibility to cope with optimizing any specified number of VAC components and to observe the effect on the dose distribution of reducing this number.