Room: AAPM ePoster Library
Purpose: Treating multiple lung lesions synchronously by single-isocenter volumetric modulated arc (VMAT) stereotactic body radiation therapy (SBRT) improves treatment efficiency and patient compliance. However, aligning multiple lung tumors on single pre-treatment cone beam CTs can be problematic. Potential target coverage loss due to small, but clinically realistic setup uncertainties was evaluated.
Methods: Twenty-two patients with multiple lung lesions were treated with single-isocenter VMAT-SBRT (2 lesions n=20, 3 lesions n=1, 5 lesions n=1) to 54Gy in 3 fractions (n=8) or 50Gy in 5 fractions (n=14) prescribed to 70-80% isodose line. An in-house script which allows rigid-transformations in six-degrees was plugged in with a treatment planning system to simulate setup uncertainties. Random setup errors of ±5mm and ±2° were simulated for each fraction and dose recalculated. Plan summation of transformed fractions was compared to the clinical plan. Target coverage (target volume covered by prescription dose), maximum target dose, normal lung V20, mean lung dose (MLD) and maximum dose to ribs were compared. Target coverage loss was evaluated as a function of target size and distance to isocenter.
Results: Simulated plans demonstrated average target coverage losses of 24.0±12.2% (range: 4.0-59.3%) compared to original clinical plans. Average isocenter to tumor distance and tumor sizes were 5.4±2.5cm (range: 0.1-11.5cm) and 18.5±13.4cc (range: 2.4-80.9cc) respectively. Target sizes = 10cc or > 10cc exhibited average coverage losses of 31.4±13.9% and 21.4±9.7%, respectively. Statistically insignificant differences were shown for maximum target dose, V20, MLD, and maximum dose to ribs.
Conclusion: Small, but clinically realistic setup errors may result in large deviations (up to 60% loss) from planned target coverage when treating multiple lung lesions using single-isocenter VMAT-SBRT plan. Smaller targets had largest deviations from planned coverage. Further multivariate analysis of loss of target coverage due to distance to isocenter, sharp dose-gradient and lung heterogeneity will be investigated.