Poster - Thurs Eve-01: Comparison of clinical IMRT plan quality and delivery accuracy: Few large segments vs many small segments.

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Commercial radiation treatment planning systems for intensity modulation use optimization algorithms that can vary multi-leaf collimator (MLC) segment sizes, segment number and the minimum number of monitor units (MU) per segment. These parameters are varied according to the treatment site, size, location, and proximity to the organs at risk. This study compares the utility of optimization using (Case A) few large segments and a higher minimum MU per segment to that of (case B) using many smaller segments with a lower minimum MU per segment. For Case A, the patient benefits from a reduced treatment time associated with fewer MUs and fewer MLC movements and an increased accuracy in dose delivery. Also, shorter treatment times may lead to fewer patient movement uncertainties. The accumulated MLC leakage dose is reduced, the patient specific quality assurance (QA) is more manageable and small field modeling inaccuracies are reduced. Pinnacle-3 (v8) plans are generated with direct machine parameter optimization (DMPO) for both scenarios. Three dimensional dose distributions and dose volume histograms are used to compare plan quality. We compare plans using few large MLC segments with those using many small MLC segments for some clinical cases. Improved plan quality is demonstrated using fewer MLC segments. Dose QAs are performed and compared for each scenario using MapCheck and film. When comparing dose delivery accuracy between different MU per segment settings, a decrease in delivery errors with minimum MU size is observed. In conclusion, few large MLC segments with larger area should be used when possible.