OC-0249: A Multi-Institutional Study on VMAT Delivery Accuracy and Efficiency for a Benchmark Plan

3rd ESTRO Forum 2015 S125 was investigated for both sliding window and VMAT techniques (RapidArc TM technology).Results: Dynalog files premit to predict the number of tolerance faults as a function of the tolerance value, with accurate predictions for low incidences of tolerance faults.All MLCs presented a very similar behavior, with no significant difference between the Millennium120 and the HD120 models.In sliding window techniques the number of beams with an incidence of hold-offs >1% rapidly decreases for a tolerance ~1.5 mm.It is to remark that most of the beam hold-offs (>97%) produced in sliding window beams with the 2 mm tolerance were caused by the maximum leaf speed specification not being properly taken into account in the treatment planning process.In VMAT techniques the number of tolerance faults sharply drops for tolerances around 2 mm.For a tolerance of 2.5 mm less than 0.1% of the VMAT arcs presented tolerance faults.Conclusions: Dynalog analysis provides a feasible method to investigate the optimal tolerance for MLC positioning in dynamic fields.In sliding window treatments the tolerance of 2 mm was found to be adequate.However, to further guarantee the accuracy of treatment delivery, this tolerance could be reduced to 1.5 mm.In VMAT treatments the typically used 5 mm tolerance is excessively high.Instead, a tolerance of 2.5 mm is recommended.