Computer-assisted selection of coplanar beam orientations inintensity-modulated radiation therapy*

Abstract

In intensity-modulated radiation therapy (IMRT), the incident beam orientations are often determined by a trial anderror search. The conventional beam's-eye view (BEV) tool becomes less helpfulin IMRT because it is frequently required that beams go through organs at risk(OARs) in order to achieve a compromise between the dosimetric objectives ofthe planning target volume (PTV) and the OARs. In this paper, we reporta beam's-eye view dosimetrics (BEVD) technique to assist in the selection ofbeam orientations in IMRT. In our method, each beam portal is divided into agrid of beamlets. A score function is introduced to measure the `goodness' ofeach beamlet at a given gantry angle. The score is determined by the maximumPTV dose deliverable by the beamlet without exceeding the tolerance doses ofthe OARs and normal tissue located in the path of the beamlet. The overallscore of the gantry angle is given by a sum of the scores of all beamlets. Fora given patient, the score function is evaluated for each possible beamorientation. The directions with the highest scores are then selected as thecandidates for beam placement. This procedure is similar to the BEV approachused in conventional radiation therapy, except that the evaluation by a humanis replaced by a score function to take into account the intensity modulation.This technique allows one to select beam orientations without the excessivecomputing overhead of computer optimization of beam orientation. It alsoprovides useful insight into the problem of selection of beam orientation andis especially valuable for complicated cases where the PTV is surrounded byseveral sensitive structures and where it is difficult to select a set of`good' beam orientations. Several two-dimensional (2D) model cases were usedto test the proposed technique. The plans obtained using the BEVD-selectedbeam orientations were compared with the plans obtained using equiangularspaced beams. For all the model cases investigated, the use of BEVD-selected beamorientations improved the dose distributionssignificantly. These examples indicate that the technique has considerablepotential for simplifying the IMRT treatment planning process and allows forbetter utilization of the technical capacity of IMRT.