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.
Methods: Twenty treated lung SBRT plans with high modulation factors were replanned in Varian Eclipse TPS ver 15.5 utilizing 2 mm craniocaudal and 1 mm axial block margins followed by optimizing with low-to-moderate modulation. A Desai et al.¹ style optimization which utilizes a novel shell structure (OptiForR50) for R50% optimization and five 5 mm shells was employed to control dose falloff according to RTOG 0813 and 0915 recommendations. The prescription varied from 34-54 Gy in 1-4 fractions, and dose objectives were PTV D95%=Rx, PTV Dmax<140% of Rx, and minimizing the modulation factor. Plan evaluation metrics included modulation factor, CIRTOG, IPCI, homogeneity index (HI), R50%, D2cm, high dose spillage (HDS) V105%, and Lung V8-12.8 Gy (Timmerman Constraint). A paired two-tailed t-test was used with p≤0.05 testing for statistical significance.
Results: Statistically, the plans had significantly lower modulation factors (3.65±0.35 vs 4.59±0.54; p<0.001), lower CIRTOG (0.97±0.02 vs 1.02±0.06; p<0.001), higher IPCI (0.93±0.02 vs 0.89±0.05; p<0.001), higher HI (1.35±0.06 vs 1.14±0.04; p<0.001), lower R50% (4.09±0.45 vs 4.56±0.56; p<0.001), lower HDS (0.44%±0.49% vs 1.10%±1.64%; p=0.051), and lower lung V8-12.8 Gy (Timmerman) (4.61%±3.18% vs 4.92%±3.37%; p<0.001). The D2cm was not statistically different (46.06%±4.01% vs 46.19%±2.80%; p=0.83).
Conclusion: Lung SBRT plans with low modulation factors can be generated, and RTOG constraints can be improved using our planning strategy. ¹Desai et al. J Appl Clin Med Phys 2020; 22: pp. 100-108.