Low pectoralis muscle index is associated with inferior survival in standard-risk newly diagnosed multiple myeloma

Background The severity and prognostic significance of sarcopenia in hematologic malignancies, particularly newly diagnosed multiple myeloma (NDMM), remain insufficiently characterized. Objectives We aimed to compare the pectoralis muscle index (PMI) distribution in NDMM with that in an unmatched diffuse large B-cell lymphoma (DLBCL) reference cohort and to assess the prognostic value of PMI in NDMM. Design A retrospective cohort study with an independent reference cohort. Methods The study included 179 patients with NDMM. PMI was measured at the T4 level using the non-contrast CT component of baseline PET-CT. PMI values were compared with those from an independent cohort of 305 patients with DLBCL. Within the NDMM cohort, patients were stratified into low and high PMI groups using study-derived sex-specific cutoffs. Associations between PMI and clinical outcomes, including overall survival (OS) and time to next treatment (TTNT), were analyzed with subgroup analyses according to Revised International Staging System (R-ISS). Results Median PMI was significantly lower in the NDMM cohort than in the DLBCL reference cohort for both males (4.19 vs. 5.58 cm 2 /m 2 , p < 0.001) and females (3.25 vs. 4.14 cm 2 /m 2 , p < 0.001). In multivariable analysis adjusting for age, sex, body mass index, and inflammatory and nutritional markers, NDMM remained independently associated with lower PMI (β = −0.88). Within the NDMM cohort, low PMI was independently associated with inferior OS (adjusted HR, 2.79; 95% CI, 1.42-5.47; p = 0.003; 3-year OS, 48% vs. 78%). Patients with low PMI also had shorter TTNT (3-year TTNT, 14% vs. 45%). These associations appeared more evident among patients with R-ISS stage I-II disease, whereas no significant differences were observed in those with R-ISS stage III disease. Conclusion PMI may serve as a practical imaging-based measure of sarcopenia with potential prognostic relevance in NDMM, particularly among patients with R-ISS stage I-II disease.