Mean dose constraint in optimization shells of a lung SBRT plan helps further reduce normal lung dose

Abstract

Purpose: This study aims to explore the effect of mean dose constraint in optimization shells on the reduction of normal lung dose in lung SBRT plans.

Materials and Methods: This study investigated 28 VMAT-based lung SBRT plans optimized with three artificial shells, which were re-generated with same setup and an additional mean dose constraint besides the maximum dose limit.  Dosimetric measurements of target volume and organs at risk (OARs) were compared between the original plans and re-generated ones using Wilcoxon signed-rank test at 5% level significance (two-tailed).

Results: Replanning resulted in slight improvements in some parameters, such as R50% and Gradient measure (GM) respectively reduced by 1.3% and 1.0% with p<0.05, but slight increases in others, such as D2cm and Maximum target dose. However, those increases were not statistically significant. The Conformity Index (CI) and V105% values remained largely unchanged after replanning. The parameters for dose deposited in normal lung tissue showed statistically significant reductions ranging from 1.0% to 1.7%. In addition, the mean dose to the spinal cord, esophagus, and skin were slightly reduced, but the mean dose to the heart showed a slight increase.

Conclusion: The study found that adding mean dose constraints to optimization shells in lung SBRT plans can reduce normal lung dose while maintaining dose conformity to the target. However, there may be slight changes in some OARs such as the spinal cord, esophagus, and skin. These changes were not statistically significant.