Dynamic Conformal Arc Informed Volumetric Modulated Arc Therapy for Stereotactic Radiosurgery
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Purpose: Linear accelerator-based Stereotactic Radiosurgery (SRS) is often performed using either dynamic conformal arcs or VMAT. For multifocal disease, multifocal conformal arc techniques can struggle to deliver the desired dose with high conformity for all targets simultaneously. While VMAT may improve coverage and conformity, and can offer the planner more flexibility, it can result in highly modulated treatment plans with non-intuitive MLC trajectories. The complex MLC modulation trajectories can often struggle to shield healthy areas between targets, thus leaving open gaps being irradiated between targets. The purpose of this research is to overcome these limitations by developing a technique for SRS of multifocal targets that combines the intuitive MLC trajectories of dynamic conformal arcs with the flexibility of VMAT.
Methods: A Conformal Arc Informed VMAT (CAVMAT) planning technique was developed in which arcs are assigned subgroups of targets, for which the MLCs are able to effectively conform to all targets in the subgroup. Arc weights are optimized to achieve desired dose per target while minimizing the variation in MU per arc. The optimized conformal arc plan then serves as the starting point in a VMAT inverse optimization to fine tune the dose to each target, optimize conformity, and meet any plan specific objectives. To demonstrate feasibility, ten multifocal VMAT cases were re-planned using the CAVMAT technique. The following metrics of plan quality were used to compare VMAT with CAVMAT: volume of healthy brain receiving 6Gy, 12Gy, and 16Gy, conformity index, and total number of monitor units.
Results: The V6Gy of the healthy brain was 10±13% lower in CAVMAT than in VMAT (range 25% lower to 15% higher for CAVMAT plans than VMAT plans). V12Gy of healthy brain tissue showed 5±14% lower in CAVMAT than in VMAT (range 16% lower to 24% higher in CAVMAT plans than VMAT plans). The V16Gy of healthy brain tissue was 3±16% lower in CAVMAT than in VMAT (range 16% to 4% lower and 41% higher in CAVMAT plans than VMAT plans in one case). The MU (Monitor Units) for the CAVMAT plans were 6156.4MU with a standard deviation of 878.41MU compared to 7031.3MU with a deviation of 1788.89MUs for VMAT. The CI (Conformity Index) for CAVMAT are 1.31 with a standard deviation of 0.13, the VMAT plan has a mean conformity of 1.28 with a standard deviation of 0.18. The mean maximum dose of the CAVMAT plan is 2445.37cGy with a standard deviation of 107.22cGy compared to 2309.28cGy with a standard deviation of 114.72cGy for VMAT.
Conclusion: CAVMAT plans succeeded in lessening low dose spill with lower MUs on average compared to VMAT plans. The conformity indexes are comparable to VMAT plans and maximum doses to patients are higher in the CAVMAT plans than in the VMAT plans.
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