肺定位体幹放射線治療における全肺および肺葉別指標間の線量測定の不一致とその領域線量評価への影響

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 (V_20Gy). 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.

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 (V_5Gy), relative lung volume receiving at least 10 Gy (V_10Gy), and V_20Gy) 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.

TBL-specific dose intensities were markedly higher than whole-lung metrics: median V_20Gy 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 V_20Gy (p = 0.074). Instead, these whole-lung metrics exhibited strong and consistent correlations with the dose distributed across the combined NTBLs.

Conventional whole-lung metrics substantially underestimate the radiation concentration within the TBL due to volumetric dilution. While high-dose whole-lung parameters like V_20Gy 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.