Exhibit Hall | Forum 7
Purpose: Volumetric modulated arc therapy (VMAT) is increasingly used for breast and chest wall treatments. In conventional breast treatments, tangential fields incorporate flash to account for motion and setup uncertainty. The addition of flash to VMAT plans currently requires complicated optimization structures and goals. The removal of these optimization structures for final dose calculation can result in unanticipated, large hot spots in final dose distribution. We investigated the feasibility of adding flash to breast VMAT plans outside of the optimization process through post-optimization MLC modification, and compare this approach to incorporating flash during optimization.
Methods: 10 patients were chosen for retrospective analysis. Plans were re-optimized without consideration for flash. After optimization, MLC sequences were analyzed and modified using in-house software to incorporate flash when possible, while respecting deliverability parameters. The modified plans were analyzed to quantify what fraction of control points (CPs) have added flash, the mean MLC extension added for flash, and the robustness to 0.5 cm posterior-anterior setup error in terms of target V95%. The two approaches (flash during optimization and post-optimization flash) were compared in terms of change in hot spot relative to original optimized plan.
Results: Plans with post-optimization flash tended to have comparable hot spots (mean V107% increase of 2.1c vs 5.6cc) compared to plans with flash during optimization. Plans with flash added post-optimization showed that flash would be incorporated in 61% of CPs, with a mean of 19.1 mm of MLC extension. Both approaches shown robustness to setup error, with target V95% of 94.3% in the optimized-for-flash group and 94.0% in the retrospective-flash group.
Conclusion: Post-optimization addition of flash to breast/chestwall VMAT plans improves plan robustness with minimal impact on plan quality metrics. Incorporating flash in post-processing rather than in the optimization process could simplify the planning process without compromising quality.