Room: Exhibit Hall
Purpose: Robust multicriteria optimization (MCO) has recently become available in RayStation v7.0. While this innovative tool holds great potential to reduce normal tissue dose in lung SBRT by creating an IMRT or VMAT plan robust to each phase in the breathing cycle, the optimization is very time-consuming and dosimetric advantage may not exist for every tumor location and motion pattern. This study designs a practical method to assess the necessity of robust optimization, via 4D analysis of the conventional plan created using ITV on average intensity map (AIP).
Methods: This study consists of a retrospective and a prospective phase. Retrospectively, we find ten prior conventional lung SBRT IMRT and VMAT plans with a variety of tumor location and motion pattern. The plan is load to each phase, and dose is accumulated to 50% phase (end-inhale) via deformable registration. The coverage of the PTV expanded from ITV (PTV_ITV) on AIP is compared to the PTV expanded from the GTV in the 50% phase (PTV_GTV50). The two PTVs were created with the same margin. The conformity index (CI) is compared following the RTOG-0813 protocol. The distance between ITV (or GTV50) and the Rx isodose line (target-Rx margin) is compared in the superior and inferior directions on the isocentric plane. Lower CI and larger PTV-Rx are indicators for potential robust MCO. Prospectively, such analysis can be done on new patients using a coplanar dynamic conformal arc plan, which only takes a few minutes in standard plan optimization.
Results: In the first four patients, the 4D analysis shows increased PTV V100 (23.8%, 10.1%, 7.0% and 6.5%), degraded CI passing with larger deviation, and increased target-Rx margin (superior/inferior: 2.6/1.8, 0.0/2.4, 0.6/0.1, 1,1/8.0 mm).
Conclusion: The 4D analysis reveals three metrics that can be used to help determine the necessity of robust MCO in lung SBRT.