Cleaning the dose falloff with low modulation in SBRT lung plans

Purpose.This dosimetric study is intended to lower the modulation factor in lung SBRT plans generated in the Eclipse TPS that could replace highly modulated plans that are prone to the interplay effect.Materials and methods.Twenty clinical lung SBRT plans with high modulation factors (≥4) were replanned in Varian Eclipse TPS version 15.5 utilizing 2 mm craniocaudal and 1 mm axial block margins followed by light optimization in order to reduce modulation. A unique plan optimization methodology, which utilizes a novel shell structure (OptiForR₅₀) for R_50%optimization in addition to five consecutive concentric 5 mm shells, was utilized to control dose falloff according to RTOG 0813 and 0915 recommendations. The prescription varied from 34-54 Gy in 1-4 fractions, and the dose objectives were PTV D_95%= Rx, PTV D_max< 140% of Rx, and minimizing the modulation factor. Plan evaluation metrics included modulation factor, CI_RTOG, homogeneity index (HI), R_50%, D_2cm, V_105%, and lung V_8-12.8Gy(Timmerman Constraint). A random-intercept linear mixed effects model was used with a p ≤ 0.05 threshold to test for statistical significance.Results.The retrospectively generated plans had significantly lower modulation factors (3.65 ± 0.35 versus 4.59 ± 0.54; p < 0.001), lower CI_RTOG(0.97 ± 0.02 versus 1.02 ± 0.06; p = 0.001), higher HI (1.35 ± 0.06 versus 1.14 ± 0.04; p < 0.001), lower R_50%(4.09 ± 0.45 versus 4.56 ± 0.56; p < 0.001), and lower lungs V_8-12.8Gy(Timmerman) (4.61% ± 3.18% versus 4.92% ± 3.37%; p < 0.001). The high dose spillage V_105%was borderline significantly lower (0.44% ± 0.49% versus 1.10% ± 1.64%; p = 0.051). The D_2cmwas not statistically different (46.06% ± 4.01% versus 46.19% ± 2.80%; p = 0.835).Conclusion.Lung SBRT plans with significantly lower modulation factors can be generated that meet the RTOG constraints, using our planning strategy.