AbstractID: 9106 Title: A new algorithm in determination of collimator angles favoring IMRT delivery efficiency When determining collimator angles in IMRT treatment planning with a MLC delivery system, the most common practice is to select the collimator angles so that the MLC can 1 be best conformed to target shape in beam’s eye view . No consideration is given to delivery efficiency. A new algorithm for determining collimator angles to favor delivery efficiency in IMRT was developed. It was found that the number of segments and MUs were largely reduced using the set of collimator angles determined with the new algorithm without compromising the plan quality. In the algorithm the collimator angles are chosen so that the amount of movement of individual MLC leaf pair is minimized by maximizing the number of MLC pairs to shape the target. As a result, the delivery efficiency improvement using the new algorithm relies on the shape and size of the target(s), and the number of the modulation levels. In a typical prostate case, the number of segments is reduced 42%, 29% and 26% and the number of MU is reduced 41%, 35% and 34% when Varian 120 leaf, 80 leaf and 52 leaf MLC are used, respectively. For Siemens MLC, the segment reduction is 32%, and the MU reduction is 14% with IMFAST sequence algorithm. Comparison of the plans using the new and old algorithms in terms of target conformity, inhomogeneity index, and DVH of the OARs showed that the quality of the plans using the new algorithm was uncompromised.  Brahme, Radiother. Oncol. 12(1988)129-140.