Room: Exhibit Hall | Forum 7
Purpose: Managing lung tumor motion through treatment planning is advantageous in minimizing patient involvement and simulation/treatment length. Typically handled with expanded PTV margins, amplified heterogeneity of the lung tumor environment presents a particular challenge in evaluating the actual delivered dose. Creating a treatment plan, â€œrobustâ€? to target position uncertainty may greatly improve the accuracy of the planned vs. realistically delivered doses. We investigate applicability of this approach, typically utilized in particle beam planning, to photon plans.
Methods: Twelve de-identified patient volumes, having lesions up to ~200cc, treated previously with volumetric arc therapy (VMAT), planned on mean intensity projection CT scans, and employing ITV/CTV margin expansions to PTV were re-planned utilizing robust optimization in RayStation Treatment Planning System. All robust plans retained their beam configurations and optimization objectives: at least 95% ITV/CTV coverage and Timmermanâ€™s dose constraints for organs at risk (OARs). Seven scenarios were included in the robust optimization: one nominal, and six with 2mm shifts due to breathing along Â±X,Y,Z directions. For evaluation each plan was perturbed by 2mm in all directions; PTV coverage and surrounding (OARs) doses in these perturbed plans were compared.
Results: Motion perturbation of 2mm in all directions lead to dose calculation conversion in all evaluated plans. Majority of the plans created with robust optimization provided better PTV coverage than those planned with PTV expansion margins, averaging to 91 vs. 85% correspondingly. OARs doses were higher for robust plans, especially for targets in close proximity to critical structures.
Conclusion: While being on a conservative side, motion perturbation of Â±2mm resulted in dose calculation conversion in all evaluated plans. Robust optimization serves as a valuable option for managing lung tumor breathing motion in VMAT plans, especially for targets located away from critical structures. Larger values of motion uncertainty will be evaluated in future studies.
Not Applicable / None Entered.