Cleaning the dose falloff in lung SBRT plan

• Published in: Journal of applied clinical medical physics Vol. 22; no. 1; pp. 100 - 108
• Main Authors: Desai, Dharmin, Narayanasamy, Ganesh, Bimali, Milan, Cordrey, Ivan, Elasmar, Hisham, Srinivasan, Senthamizhchelvan, Johnson, Ellis Lee
• Format: Journal Article
• Language: English
• Published: United States John Wiley & Sons, Inc 01.01.2021; John Wiley and Sons Inc
• Subjects: Algebra; Conformity; dose falloff; dose gradient; dose spill; lung SBRT; normal tissue toxicity; Optimization techniques; Radiation Oncology Physics; Radiation therapy; dose gradient; normal tissue toxicity; dose spill; lung SBRT; dose falloff
• ISSN: 1526-9914; 1526-9914
• DOI: 10.1002/acm2.13113

Purpose To investigate a planning technique that can possibly reduce low‐to‐intermediate dose spillage (measured by R50%, D2cm values) in lung SBRT plans. Materials and Methods Dose falloff outside the target was studied retrospectively in 102 SBRT VMAT plans of lung tumor. Plans having R50% and/or D2cm higher than recommended tolerances in RTOG protocols 0813 and 0915 were replanned with new optimization constraints using novel shell structures and novel constraints. Violations in the RTOG R50% value can be rectified with a dose constraint to a novel shell structure (“OptiForR50”). The construction of structure OptiForR50% and the novel optimization criteria translate the RTOG goals for R50% into direct inputs for the optimizer. Violations in the D2cm can be rectified using constraints on a 0.5 cm thick shell structure with inner surface 2cm from the PTV surface. Wilcoxon signed‐rank test was used to compare differences in dose conformity, volume of hot spots, R50%, D2cm of the target in addition to the OAR doses. A two‐sided P‐value of 0.05 was used to assess statistical significance. Results Among 102 lung SBRT plans with PTV sizes ranging from 5 to 179 cc, 32 plans with violations in R50% or D2cm were reoptimized. The mean reduction in R50% (4.68 vs 3.89) and D2cm (56.49 vs 52.51) was statistically significant both having P < 0.01. Target conformity index, volume of 105% isodose contour outside PTV, normal lung V20, and mean dose to heart and aorta were significantly lowered with P < 0.05. Conclusion The novel planning methodology using multiple shells including the novel OptiForR50 shell with precisely calculated dimensions and optimizer constraints lead to significantly lower values of R50% and D2cm and lower dose spillage in lung SBRT plans. All plans were successfully brought into the zone of no RTOG violations.