Dosimetric Discrepancy Between Whole-Lung and Lobe-Specific Metrics in Lung Stereotactic Body Radiotherapy and Its Implications for Regional Dose Assessment

Introduction: Stereotactic body radiotherapy (SBRT) is a standard treatment for early-stage non-small cell lung cancer (NSCLC). Radiation pneumonitis is a major dose-limiting toxicity, typically assessed using whole-lung metrics like mean lung dose (MLD) and relative lung volume receiving at least 20 Gy (V20Gy). However, the high-dose conformality of SBRT may lead to a volumetric averaging effect, where whole-lung metrics obscure intense local doses within a small fraction of the lung. This study quantitatively evaluates the discrepancy between conventional whole-lung metrics and lobe-specific dosimetry. Methods: We retrospectively analyzed 10 patients with T1-T2N0M0 NSCLC treated with SBRT (42 Gy in four fractions). Lung lobes - right upper, middle, lower, and left upper and lower - were automatically segmented using deep learning-based software (OncoStudio; Oncosoft, Seoul, South Korea) and manually refined. The lobe containing the tumor was defined as the tumor-bearing lobe (TBL), while others were defined as non-tumor-bearing lobes (NTBLs). Dose-volume histogram (DVH) parameters (MLD, relative lung volume receiving at least 5 Gy (V5Gy), relative lung volume receiving at least 10 Gy (V10Gy), and V20Gy) were compared for the whole lungs, TBL, and NTBLs using the Wilcoxon signed-rank test. Spearman’s rank correlation coefficient assessed the relationship between whole-lung and lobe-specific metrics. Results: TBL-specific dose intensities were markedly higher than whole-lung metrics: median V20Gy was 6.3% for TBL versus 1.5% for the whole lungs, and median MLD was 4.61 Gy for TBL versus 1.68 Gy for the whole lungs. Statistically significant differences were observed across all parameters (p < 0.01). Correlation analysis showed that statistically significant correlations were not reached between whole-lung metrics and TBL values, including for V20Gy (p = 0.074). Instead, these whole-lung metrics exhibited strong and consistent correlations with the dose distributed across the combined NTBLs. Conclusion: Conventional whole-lung metrics substantially underestimate the radiation concentration within the TBL due to volumetric dilution. While high-dose whole-lung parameters like V20Gy reflect local lobar intensity, low-dose metrics primarily serve as surrogates for NTBL exposure. Lobe-based analysis provides a refined, anatomically grounded framework for characterizing regional dose distributions in lung SBRT and reveals spatial dose patterns that are obscured when using conventional whole-lung assessment alone.