Ratio of PTV to normal lung volumes as a dosimetric predictor for lung metrics in peripheral lung SBRT

To identify a dosimetric predictor for lung doses in stereotactic body radiotherapy (SBRT) for peripheral lung cancer and demonstrate the utilities of the predictor in aiding plan evaluation and treatment modality selection. We performed a retrospective review of 108 plans that were previously treated on a TrueBeam in our institute. The cohort was further stratified into three subgroups based on the level of the chest wall (CW) involvement. The achieved lung metrics including the lung V20, V5 and mean lung dose (MLD) were evaluated against an anatomy-based parameter R_PTV/Lungs-ratio of the planning target volume (PTV) to lung volume. Linear regression and prediction interval were used to identify outlier plans that had "suboptimal" lung doses. Re-optimization using alternative strategies to improve the lung doses were carried out for these outlier cases. We demonstrated the utility in treatment platform selection through a comparison with a magnetic resonance (MR) guided system, MRIdian. Strong correlation (R² ∼ 0.9) existed between the lung doses and R_PTV/Lungs for each subgroup. Increasing CW involvement progressively reduced lung doses as is evident by the regression coefficient, which dropped from 414.68 when the PTV-CW distance was > 5 mm, to 274.34 when > 10% of PTV was within CW. Three outlier plans with high lung doses were identified for replanning. Alternative optimization approaches were successful in reducing the lung V20, V5 and MLD by 0.7%, 1.3% and 0.4 Gy on average, respectively. The correlation showed different characteristics between a TrueBeam and a MRIdian, enabling treatment modality selection. The achieved lung metrics in peripheral lung SBRT were highly correlated to R_PTV/Lungs, which can be used as a simple and practical tool to guide plan evaluation and treatment modality selection.