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 (Dav ) and the surface area of the PTV (SAPTV ).
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 SAPTV that has not been fully appreciated previously.
Results: The study of PTV Dav revealed a local minimum for R50% as a function of the PTV Dav at Dav ≈ 110% of Rx dose. As PTV Dav increases above this local minimum, R50% increases; while for PTV Dav less than this local minimum, the R50% value also increases. The study of PTV surface area (SAPTV ) demonstrated that as the SAPTV increases, the R50% increases if the PTV volume stays the same. The SAPTV 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 Dav and SAPTV . 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.
Keywords: PTV mean dose; PTV surface area; R50%; R50% lower bound; SBRT; lung.
© 2020 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine.