Room: ePoster Forums
Purpose: Faster delivery of SBRT with FFFB is attractive due to various reasons including reduced patient motion/movement. However, small target size and sharp dose falloff increase the potential of misalignment and consequently treatment failure. This study investigates the robustness of lung SBRTs planned with FFFB and FB.
Methods: Ten lung SBRT patients treated in 2018 were randomly selected. Each patient had a clinical SBRT-VMAT plan with two FFFB (or FB) 6MV arcs (Rx=10Gyx5fx, or 12.5Gyx4fx). A set of comparison plans was created with 6MV FB (or FFFB) and optimized to comparable PTV coverages. To simulate tumor misalignment, beam isocenters were moved by 3mm, 3mm, and 5mm in the anterior-posterior, medial-lateral and superior-inferior directions, respectively. Shifted dose distributions were recalculated and compared against the initial plans, and then between the FFFB and FB plans. Dose coverage to the target and OARs were analyzed.
Results: Average volumes of GTVs and PTVs were 13.2Â±11.7cc (range:4.7-44.1cc) and 42.5Â±25.5cc (range:22.4-110.5cc), respectively. GTV and PTV D99 differences between FFFB plans and FB plans were 0.4Â±1.4% and -1.4Â±2.4%, respectively. Mean heart dose and normal lung dose were higher in FB plans than in FFFB plans by 17.2Â±98.7cGy and 20.6Â±9.9cGy. In simulated misalignment, FFFB plans lost 27.9Â±12.8% PTV D99 coverage while mean heart dose increased by 33.1Â±36.8cGy. FB plans lost 24.4Â±13.2% PTV D99 coverage while mean heart dose increased by 34.4Â±36.6cGy. FB plans maintained slightly better PTV and GTV coverage, while slight increase in mean OAR doses.
Conclusion: Dosimetric effects of misalignment is similar between the FFFB and FB plans, however, FB plan preserves the target coverage better. These misalignment induced dosimetric effects highly depend on target size, anatomy of the treatment site as well as the spread of the beam angles. Further study with larger patient number having tumors at different locations in lungs is warranted.