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Optimized 2D Motion Tracking Using Dynamic Multi-Leaf Collimator (DMLC) for a Clinical Volumetric Modulated Arc Therapy (VMAT) Plan

Z Xu

Z Xu*1,2, IZ Wang1,2, (1) Roswell Park Cancer Institute, (2) The State Univerity of New York at Buffalo, Buffalo, NY


SU-E-T-400 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: This study is to develop a 2D motion tracking algorithm combined with controlled shallow breathing and collimator angle optimization to improve MLC tracking accuracy.
Method and Materials: One SBRT VMAT plan for lung cancer (2 Arcs; 8 Gy/fx in 4 fx) was selected. A non-motion 3D VMAT plan was generated based on GTV from one single selected phase of the 4DCT. A motion tracking algorithm using MATLAB was developed to modify MLC positions of the 3D VMAT DICOM file based on an ideal breathing motion model. We derived the correlations between gantry angle, collimator angle and motion amplitudes in SI and AP directions that projected onto MLC aperture. All delivery constraints followed mechanical limits of Varian TrueBeam. To avoid large time interval between adjacent control points, motion amplitudes restriction and collimator angle optimization were incorporated into the algorithm. Each arc was then divided into 6 sub-arcs corresponding to the 6 breathing phases. Sub-arc doses were individually calculated and merged to acquire the 4D dose distribution. DVH comparison and 3D gamma were used for dosimetric evaluation.
Results: Total delivery time of static 3D VMAT plan (collimator=45°), plan with MLC tracking under free breathing (collimator=45°, SI=10mm and AP=8mm), plan with collimator angle optimized only (collimator=77.8°, SI=10mm and AP=8mm) and collimator angle optimized under shallow breathing (collimator=80.1°, SI=8.3mm, AP=6.6mm) were 135.7s, 235.3s, 213.9s and 188.5s, respectively. Compared with the non-motion 3D VMAT, optimized MLC tracking demonstrated small changes in target dose (%ΔD90=1%, 3D gamma passing rate=97.5%).
Conclusions: Total delivery time of VMAT with tracking tends to decrease as collimator angle approaches to 90°. At selected angle, plan with shorter breathing cycle period has longer delivery time when using MLC for motion compensation. MLC tracking combined with shallow breathing and optimized collimator angle cannot only reduce delivery time but increase the tracking accuracy.

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