Effect of PTV Margins and Lung Density Variations on Target Coverage for Small‑Volume Lung Lesions In SBRT
Abstract
Purpose
Target coverage in lung SBRT is influenced by tumor size, PTV margin, and local lung density. This study investigates target dose coverage in low‑density lung as a function of PTV margins and lung density.
Methods
A small‑volume central lung patient was selected for an in silico lung SBRT study. Lung densities ranging from 0.1–0.4 g/cm³ (−900 to −600 HU) were used. Spherical GTVs with diameters of 1, 2, and 3 cm were generated, each expanded by 2, 3, or 5 mm to form the PTV. Using the Eclipse treatment planning system (TPS), 48 plans were created with identical optimization criteria using 6 MV beams. For each plan, D100, D98, D50, D2, maximum dose, monitor units (MU), and conformity, homogeneity, and gradient indices (CI, HI, GI) were evaluated.
Results
Across all 48 plans, target coverage was adequately maintained, though at the expense of increased MU. For the 1‑cm lesion, HI remained below 0.04 regardless of PTV margin or lung density. For the 2‑cm lesion, HI was <0.12 and decreased with decreasing density; larger PTV expansions resulted in lower HI as expected. For the 3‑cm lesion, HI decreased with density, but margin size had minimal influence. MU/Gy ranged from 400–500, 500–600, and 450–600 for the 1‑, 2‑, and 3‑cm lesions, respectively, across densities of 0.1–0.4 g/cm³. Smaller margins produced higher MU in low‑density lung by up to 12%.
Conclusion
Despite the buildup effect in lung, adequate target coverage (CI ≈ 1.0) can be achieved, though at the cost of increased MU/Gy, which strongly depends on lung density. HI is also density‑dependent. PTV margins have relatively limited dosimetric impact and should be selected primarily based on clinical considerations and OAR constraints.