Regression models for predicting physical and EQD₂ plan parameters of two methods of hybrid planning for stage III...

To establish regression models of physical and equivalent dose in 2 Gy per fraction (EQD₂) plan parameters of two kinds of hybrid planning for stage III NSCLC.

Two kinds of hybrid plans named conventional fraction radiotherapy & stereotactic body radiotherapy (C&S) and conventional fraction radiotherapy & simultaneous integrated boost (C&SIB) were retrospectively made for 20 patients with stage III NSCLC. Prescription dose of C&S plans was 2 Gy × 30f for planning target volume of lymph node (PTV_LN) and 12.5 Gy × 4f for planning target volume of primary tumor (PTV_PT), while prescription dose of C&SIB plans was 2 Gy × 26f for PTV_LN and sequential 2 Gy × 4f for PTV_LN combined with 12.5 Gy × 4f for PTV_PT. Regression models of physical and EQD₂ plan parameters were established based on anatomical geometry features for two kinds of hybrid plans. The features were mainly characterized by volume ratio, min distance and overlapping slices thickness of two structures. The possibilities of regression models of EQD₂ plan parameters were verified by spearman's correlation coefficients between physical and EQD₂ plan parameters, and the influence on the consistence of fitting goodness between physical and EQD₂ models was investigated by the correlations between physical and EQD₂ plan parameters. Finally, physical and EQD₂ models predictions were compared with plan parameters for two new patients.

Physical and EQD₂ plan parameters of PTV_LN CI_60Gy have shown strong positive correlations with PTV_LN volume and min distance_{(PT to LN)}, and strong negative correlations with PTV_PT volume for two kinds of hybrid plans. PTV_(PT+LN) CI_60Gy is not only correlated with above three geometry features, but also negatively correlated with overlapping slices thickness_{(PT and LN)}. When neck lymph node metastasis was excluded from PTV_LN volume, physical and EQD₂ total lung V₂₀ showed a high linear correlation with corrected volume ratio_{(LN to total lung).} Meanwhile, physical total lung mean dose (MLD) had a high linear correlation with corrected volume ratio_{(LN to total lung)}, while EQD₂ total lung MLD was not only affected by corrected volume ratio_{(LN to total lung)} but also volume ratio_{(PT to total lung).} Heart D₅, D₃₀ and mean dose (MHD) would be more susceptible to overlapping structure_{(heart and LN)}. Min distance_{(PT to ESO)} may be an important feature for predicting EQD₂ esophageal max dose for hybrid plans. It's feasible for regression models of EQD₂ plan parameters, and the consistence of the fitting goodness of physical and EQD₂ models had a positive correlation with spearman's correlation coefficients between physical and EQD₂ plan parameters. For total lung V₂₀, ipsilateral lung V₂₀, and ipsilateral lung MLD, the models could predict that C&SIB plans were higher than C&S plans for two new patients.

The regression models of physical and EQD₂ plan parameters were established with at least moderate fitting goodness in this work, and the models have a potential to predict physical and EQD₂ plan parameters for two kinds of hybrid planning.