A physically meaningful relationship between R50% and PTV surface area in lung SBRT

PURPOSE: We propose a novel understanding of two characteristics of the planning target volume (PTV) that affect the intermediate‐dose spill in lung stereotactic body radiation therapy (SBRT) as measured by R50%. This phantom model research investigates two characteristics of the PTV that have a marked effect on the value of R50%: the mean dose deposited within the PTV (D(av)) and the surface area of the PTV (SA(PTV)). METHODS: Using a phantom model provided by a CT of the IROC Thorax‐Lung Phantom® (IROC Houston QA Center, Houston, TX) and Eclipse® Treatment Planning System (Varian Medical Systems, Palo Alto, CA), we investigate the two characteristics for spherical and cylindrical PTVs. A total of 135 plans with tightly controlled PTV characteristics are employed. A lower bound for R50% (R50%min(∆r)) is derived and clearly establishes a relationship between R50% and SA(PTV) that has not been fully appreciated previously. RESULTS: The study of PTV D(av) revealed a local minimum for R50% as a function of the PTV D(av) at D(av) ≈ 110% of Rx dose. As PTV D(av) increases above this local minimum, R50% increases; while for PTV D(av) less than this local minimum, the R50% value also increases. The study of PTV surface area (SA(PTV)) demonstrated that as the SA(PTV) increases, the R50% increases if the PTV volume stays the same. The SA(PTV) result is predicted by the theoretical investigation that yields the R50% lower bound, R50%min(∆r). CONCLUSIONS: This research has identified two characteristics of the PTV that have a marked influence on R50%: PTV D(av) and SA(PTV). These characteristics have not been clearly articulated in the vast body of previous research in SBRT. These results could help explain plans that cannot meet the RTOG criteria for R50%. With further development, these concepts could be extended to provide additional guidance for creating acceptable SBRT plans.