Skeletal Muscle Index and Baseline Biomarkers in Atezolizumab-Treated Extensive-Stage Small-Cell Lung Cancer: A Real-World Cohort Study

Body composition and routinely available blood biomarkers may help characterize prognosis in extensive-stage small-cell lung cancer (ES-SCLC). We retrospectively evaluated 81 patients who received first-line carboplatin, etoposide, and atezolizumab. Baseline L3 skeletal muscle index (SMI) was derived from [18F]FDG PET/CT and classified using the Martin criteria. Overall survival (OS) was assessed using Kaplan-Meier and Cox regression. Of 80 classifiable patients, 33 (41.3%) had low SMI. Median OS was 10.51 months (95% CI, 7.98-14.26) and was 8.48 versus 14.26 months in patients with low versus normal SMI (log-rank p = 0.050). In the primary multivariable model, low SMI was associated with higher mortality (HR, 2.03; 95% CI, 1.07-3.86; p = 0.030); lower albumin and higher lactate dehydrogenase were also associated with mortality. In exploratory analyses, the low-SMI association persisted after post hoc adjustment for bone metastasis (HR, 2.16; 95% CI, 1.05-4.42; p = 0.035), although diagnostics suggested time-varying effects. Continuous SMI and alternative thresholds were not associated with OS, and low SMI was not associated with progression-free survival. Martin-defined low SMI may complement routine biomarkers in prognostic assessment, but its cutoff- and time-dependent association requires cautious interpretation and external evaluation.