AbstractID: 9106 Title: A new algorithm in determination of collimator... IMRT delivery efficiency

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
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.
[1] Brahme, Radiother. Oncol. 12(1988)129-140.
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