Evaluating Optimizer and Geometric-Driven Gains In Rapidarc Dynamic Breast Plan Quality
Abstract
Purpose
RapidArc Dynamic (RAD) introduces static-angle modulated ports (STAMPs) and a rotating collimator. It also incorporates a deterministic optimizer. This study aims to evaluate the independent effects of the optimizer versus these additional geometric features on plan quality.
Methods
Thirty retrospective cases treated to the breast/chest wall with regional nodal irradiation to 4005 cGy in 15 fractions were identified. Each case was re-planned with a (1) standard VMAT, (2) RAD-optimized VMAT plan with no geometric features added, and (3) an unconstrained RAD plan utilizing four STAMPS, rotating collimator, and static dominant weighting. All plans utilized two arcs and standardized objectives. Plans were normalized to PTV D95% at 95% of the prescribed dose. Seven organ-at-risk (OAR) DVH metrics and complexity metrics were evaluated.
Results
Hotelling's T2 test was performed and showed statistically significant differences between plan types. Post-hoc analysis shows that all OAR DVH metrics for VMAT were statistically different from both the RAD-optimized and unconstrained RAD plans. Relative to VMAT, RAD-optimized VMAT showed improved OAR sparing including contralateral breast mean dose (216 cGy versus 380 cGy, –43.1%), ipsilateral lung mean dose (1013 cGy versus 1085 cGy, –6.6%), and heart mean dose (236 cGy versus 270 cGy, –12.5%) (p<0.002). The unconstrained RAD plans showed further reductions relative to RAD-optimized, particularly in ipsilateral lung mean dose (962 cGy versus 1013 cGy, -5.0%) and heart mean dose (194 cGy versus 236 cGy, -17.9%) (p<0.002). Equivalent square length suggested higher aperture complexity in RAD plans when compared to VMAT, while aperture area indicated a decrease in aperture size.
Conclusion
RAD breast reduces OAR doses when compared to standard VMAT. A substantial portion of the dose reductions is attributable to the RAD optimizer. However, addition of STAMPS and rotating collimator provides further clinically significant OAR sparing, most notably for the ipsilateral lung and heart.