Exhibit Hall | Forum 4
Purpose: This study is a dosimetric comparison between different low modulation factor lung SBRT plans generated in the Eclipse TPS that could replace highly modulated plans that are subject to the interplay effect.
Methods: Four types of SBRT lung plans were planned in Varian Eclipse TPS ver 15.5: block margin based DCA (dynamic conformal arc) plans, block margin based lightly modulated VMAT plans, iteratively adjusted margin based lightly modulated VMAT plans using Desai et al.¹ style optimization, and block margin based lightly modulated VMAT plans using Desai et al.¹ style optimization. Desai et al.¹ style optimization utilizes a novel shell structure (OptiForR50) and a shell 2 cm from the PTV to optimize R50% & D2cm respectively. Several plans of each type were made with their metrics compared to the original highly modulated plan. The prescription was 54 Gy in 3 fractions, and the dose objectives were PTV D95%=54 Gy and PTV Dmax<140%. Plan evaluation metrics include CIRTOG, R50%, D2cm, Lung V11.4Gy (Timmerman), and the homogeneity index.
Results: DCA plans with block margins had unacceptably high R50% (>8) and D2cm (>60%). All three of the lightly modulated plans had comparable metrics to the original clinically delivered plan with smaller modulation factors (3.79-3.88 vs 5.03). Both Desai et al.¹ style optimization plans had superior R50% values than standard optimized plans (48.9% & 49.2% vs 52%) with the block margin Desai et al.¹ style optimization requiring the least amount of effort to implement.
Conclusion: The optimization scheme proposed by Desai et al.¹ produces dosimetrically superior plans that can be generated quickly compared to other methods. A block margin Desai et al.¹ style optimization is suggested to achieve optimal evaluation metrics with minimal effort.¹Desai et al. J Appl Clin Med Phys 2020; 22: pp. 100-108.