AbstractBackgroundWe investigated the prognostic significance of body mass index in small‐cell lung cancer and explored whether skeletal muscle status affects the body mass index–survival relationship.MethodsThis retrospective study evaluated data from patients who underwent platinum‐etoposide chemotherapy for small‐cell lung cancer between March 2010 and December 2021. Skeletal muscle status was assessed using non‐contrast computed tomography images of baseline positron‐emission tomography‐computed tomography, with the skeletal muscle index defined as the cross‐sectional area of skeletal muscle divided by height squared, and the average attenuation values of skeletal muscle. Cox proportional hazards regression analysis was used to determine the correlations of body mass index, skeletal muscle metrics, and overall survival.ResultsWe analysed the data of 1146 Asian patients (1006 men and 140 women, with a median age of 67 years [interquartile range: 61–72 years]), including 507 and 639 patients with limited and extensive disease, respectively. Being underweight, defined as a body mass index <18.5 kg/m², was associated with shorter overall survival, independent of clinical covariates in both the limited‐disease (hazard ratio, 1.77; 95% confidence interval, 1.01–3.09) and extensive‐disease (hazard ratio, 1.71; 95% confidence interval, 1.18–2.48) groups. The prognostic value of being underweight remained significant after additional adjustment for skeletal muscle index and attenuation in both limited‐disease (hazard ratio, 1.96; 95% confidence interval, 1.09–3.51) and extensive‐disease (hazard ratio, 1.75; 95% confidence interval, 1.17–2.61) groups.ConclusionsBeing underweight is an independent poor prognostic factor for shorter overall survival in Asian patients with small‐cell lung cancer, regardless of skeletal muscle status.