Prognostic value of the baseline triglyceride-glucose-body mass index in small cell lung cancer undergoing chemoimmunotherapy: A multicenter retrospective study

• Our findings demonstrate that the baseline TyG-BMI index, dichotomized using the median value, holds significant and independent prognostic value for outcomes in ES-SCLC patients treated with immune checkpoint inhibitors. • This binary metabolic marker exhibits a treatment-dependent prognostic pattern: low TyG-BMI is robustly associated with superior overall survival in the immunotherapy cohort, whereas high TyG-BMI is associated with inferior progression-free survival in the chemotherapy-only cohort. • The TyG-BMI index, categorized as high or low, retains independent prognostic significance in multivariate analysis. • These results implicate host metabolic status, categorized by the TyG-BMI index, as a critical and differential modulator of treatment response, underscoring its potential as a biomarker for tailoring therapy in SCLC. The prognostic role of the triglyceride-glucose-body mass index (TyG-BMI), a surrogate marker for insulin resistance, in small cell lung cancer (SCLC) patients treated with immune checkpoint inhibitors remains undefined. In this multicenter retrospective study, SCLC patients who initiated first-line chemoimmunotherapy or chemotherapy alone between March 2015 and December 2023 were enrolled. The association between baseline TyG-BMI index and survival was analyzed using Kaplan-Meier and Cox regression methods. A prognostic nomogram was developed and validated via time-dependent ROC and calibration analyses. Among 437 patients, a low TyG-BMI index was significantly associated with improved overall survival (OS) in the chemoimmunotherapy cohort ( P < 0.001). In the chemotherapy-alone cohort, a high index predicted worse progression-free survival (PFS, P = 0.031). The TyG-BMI index was an independent prognostic factor for both PFS (HR = 1.44, P = 0.005) and OS (HR = 1.42, P = 0.002). The nomogram demonstrated good predictive accuracy for OS (9–18 month AUCs: 0.72–0.73; C-index=0.66, 95% CI: 0.63–0.69) and was well-calibrated. The baseline TyG-BMI index is a treatment-specific prognostic biomarker. A low index identifies SCLC patients most likely to achieve long-term survival benefit from chemoimmunotherapy, offering a practical tool for risk stratification and therapeutic guidance.