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Evaluation of VMAT Dosimetric Effect Caused by Slowing MLC Leaves Using MapCHECK

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Z Xu

Z Xu, IZ Wang, Roswell Park Cancer Institute, Buffalo, NY

SU-E-T-387 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: This study is to report the effects on the quality of volumetric modulated arc therapy (VMAT) dose delivery by simulating multileaf collimator (MLC) leaf speed errors that do not trigger any MLC motion interlocks (i.e. <2 mm leaf position error).
Method and Materials: Four VMAT plans (two one-arc brain cases and two four-arc anal cases) were modified using MATLAB. Within each arc, five consecutive MLC leafs on the same bank were selected for modification. Leaf speed at each control point was calculated. Only those control points with highest speed (average ~4.6% of total control points) were rise to the Varian MLC speed limit (2.5 cm/s), which gave a leaf position difference of less than 2 mm. No changes in MU per control point and gantry speed were introduced. The modified plans were now considered as "standard" plans and the original plans were treated as "wrong" plans with leafs moving at a lower speed. Modified DICOM plans were imported back into Eclipse for dose calculations and DVH comparisons. The recalculated planar dose was compared with original MapCHECK measurement using the standard patient-specific VMAT quality assurance (QA) approach, with a gamma-index of 3%/3mm, 3%/2mm, and 2%/2mm.
Results: All cases passed the >85% criterion for a gamma-index of 3%/3mm. There was only a small passing rate difference (<1%) between the original and the modified plans. The DVH calculations of the modified plans demonstrated very minimal changes in PTV and OAR doses.
Conclusions: By introducing the leaf speed error without triggering the MLC motion interlock, the dose changes cannot be detected by MapCHECK. However, the consequential effects on target and OAR doses are also negligible, which supports the reliability of current patient IMRT QA method for VMAT plan verification.

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