SU-E-T-540: Verification of Dosimetric Accuracy on the TrueBeam STx: Rounded Leaf Effect of the High Definition MLC

Abstract

Purpose: The Dosimetric Leaf Gap (DLG) in the Eclipse treatment planning system is determined during commissioning and is used to model the rounded leaf effect of the multi‐leaf collimator (MLC). This parameter attempts to model the physical difference between the radiation and light field and accounts for inherent leakage between adjoining leavess. With the increased use of single fraction high dose treatments requiring larger monitor units comes an enhanced concern in the accuracy of leakage calculations, as it accounts for much of the patient dose. This study serves to verify the dosimetric accuracy of the algorithm used to model the rounded leaf effect for the TrueBeam STx, given the novel capabilities of the TrueBeam STx such as Flattening Filter Free (FFF) treatments and a High Definition MLC (HDMLC). Methods: During commissioning, the nominal MLC position was verified and the DLG was determined using MLC defined field sizes and moving slit tests, as commonly used in clinical commissioning. Clinical treatment plans were created, and the DLG was optimized to achieve less than 1% difference between measured and calculated dose. The DLG value found was compared for all energies and modalities available on the TrueBeam STx. Results: The DLG parameter found during the initial MLC testing did not match the leaf gap modeling parameter that provided the most accurate dose delivery in clinical treatment plans. Using the physical leaf gap size as the DLG for the HDMLC can lead to 5% differences in measured and calculated doses for RapidArc delivery. Conclusions: Separate optimization of the DLG parameter using end‐to‐end tests must be performed to ensure dosimetric accuracy in the modeling of the rounded leaf ends for the Eclipse treatment planning system. The difference in leaf gap modeling versus physical leaf gap dimensions is more pronounced for the HDMLC than the Millennium MLC.