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

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 ).

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.

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 .

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.