The pectoralis major index: a novel computed tomography biomarker for low thoracic muscle mass and eastern cooperative oncology group performance status in lung cancer patients.

[BACKGROUND] Chest computed tomography (CT) seldom covers the third lumbar (L3) vertebral level, the standard landmark for assessing total body muscle mass. As muscle measurements at the fourth thoracic (T4) level show high concordance with those at L3, the T4 level may serve as a viable alternative. We evaluated the discriminatory performance of T4 musculature for lung cancer-related low thoracic muscle mass (LTMM) and its association with Eastern Cooperative Oncology Group (ECOG) performance status.

[METHODS] We retrospectively included 289 inpatients with newly diagnosed lung cancer who underwent chest CT within 3 months. At T4, the pectoralis major, pectoralis minor, and chest-wall muscle group were segmented to derive cross-sectional area (CSA), density, and height-normalized indices (cm/m). Low thoracic muscle mass (LTMM) was defined by sex-specific 25th-percentile thoracic 4th vertebra level muscle index (T4MI) cutoffs (40.78 cm/m for men, 33.15 cm/m for women). Patients were stratified by sex and clinical stage (I-II early-stage vs. III-IV advanced-stage). ROC analyses compared discriminatory performance; logistic regression tested associations with poor ECOG performance status.

[RESULTS] Compared with non-low thoracic muscle mass (non-LTMM), low thoracic muscle mass (LTMM) group showed smaller CSA, lower density, and reduced indices. Across muscle groups, indices outperformed CSA and density. The pectoralis major index achieved the highest AUC in the overall cohort (AUC = 0.833) and reached excellent discrimination in the overall male cohort (AUC = 0.922). Notably, stage-stratified analyses showed consistently superior discriminatory performance for the pectoralis major index in the male population (early-stage AUC = 0.917, advanced-stage AUC = 0.912). In addition, the pectoralis major index was independently associated with poor ECOG performance status (odds ratio = 0.932,  = 0.009).

[CONCLUSION] T4-level muscle metrics showed good internal discrimination for lung cancer-related low thoracic muscle mass across sex and disease stage, with the pectoralis major index achieving the highest overall performance. Moreover, the pectoralis major index may be a potential imaging biomarker associated with ECOG performance status.