Impact of four-dimensional computed tomography phase number on target coverage of four-dimensional robustly optimized lung stereotactic body radiotherapy

This study evaluated the dosimetric benefits of various four-dimensional (4D) robust optimization strategies for enhancing plan robustness in free-breathing stereotactic body radiotherapy (SBRT). For ten early-stage lung cancer patients, we generated four helical tomotherapy-SBRT plans (42 Gy/4 fractions) using 4D robust optimization based on 4D-computed tomography (CT) images from all ten phases (4D_10), six phases (4D_6), four phases (4D_4), and two phases (4D_2). Conventional internal target volume (ITV)-based plans served as a reference. Setup uncertainty robustness was assessed across all CT phases. We compared dose and target coverage (D_98%, V_100%) for the gross tumor volume, doses to relevant organs at risk (OARs), and reductions in plan robustness for both nominal and perturbed scenarios. Optimization durations were recorded. All nominal plans met clinical criteria for targets and OARs. The 4D_10 plans exhibited the highest robustness in target coverage, with D_98% significantly different from ITV plans (p < 0.05) and V_100% significantly different from 4D_2 plans (p < 0.05). Optimization time increased linearly with the number of CT phases. Four-dimensional robust optimization enables free-breathing SBRT plans resilient to respiratory and setup uncertainties; however, limiting phase number may compromise robustness for certain tumors despite shorter optimization times.