Author: William N. Duggar, John C. White 👨🔬
Affiliation: UMMC, University of Mississippi Medical Center 🌍
Purpose:
To evaluate the impact of patient motion on PTV coverage for 95% and 90% target volumes in VMAT breast treatment plans and to determine the appropriate flash margins required to maintain robust coverage across varying PTV sizes.
Methods:
Eight VMAT breast patients were analyzed using RayStation 2023b. Initial plans were robustly evaluated with simulated clinical motion using shifts of 0.5 cm superior/inferiorly, 0.7 cm anteriorly/posteriorly, and 0.2 cm left/right. Robust optimization was performed to address these uncertainties, and plans were re-evaluated with the same shifts combined with a 1 cm uniform uncertainty robust analysis. The effects of motion and flash margins on PTV coverage were analyzed across varying volumes.
Results:
For the original plans, nominal 95% PTV coverage averaged 98.02%, decreasing to 90.79% in worst-case scenarios. For 90% coverage, nominal and worst-case values were 98.80% and 94.76%, respectively. Robustly optimized plans improved stability, achieving nominal 95% coverage of 97.92% and worst-case of 91.10%. For 90% coverage, nominal and worst-case values were 99.46% and 97.95%, respectively. The 1 cm uniform uncertainty analysis showed worst-case 95% and 90% coverage dropped to 75.24% and 91.28%, respectively. Larger PTVs were less sensitive to motion, requiring smaller flash margins, while smaller PTVs exhibited greater variability, needing larger margins or motion management strategies.
Conclusion:
Robust optimization improves PTV coverage stability compared to regular plans, particularly in worst-case scenarios. Regular plans showed significant decreases in coverage under motion, with worst-case 95% PTV coverage dropping to 90.79% and 90% PTV coverage to 94.76%. In contrast, robustly optimized plans improved worst-case coverage to 91.10% for 95% and 97.95% for 90%. However, under 1 cm uniform uncertainty evaluation, even robust plans showed a significant decrease in coverage, emphasizing the need for larger flash margins or additional motion management strategies, especially for smaller PTVs.