Novel prognostic significance of the pectoralis muscle index in newly diagnosed multiple myeloma: A retrospective cohort study

Background Sarcopenia is common in multiple myeloma (MM) and may impact survival outcomes. The pectoralis muscle index (PMI), derived from chest CT, has shown prognostic relevance in various types of malignancies but has not been studied in MM. We aimed to evaluate the clinical significance of the PMI in newly diagnosed MM and compared it to the PMI in a cohort with diffuse large B-cell lymphoma (DLBCL). Methods This retrospective cohort study included 115 patients with newly diagnosed MM who underwent baseline PET-CT. PMI was calculated from non-contrast CT images at the T4 level obtained from PET-CT and stratified into low and high PMI groups using sex-specific cutoffs. Overall survival (OS) and time to next treatment (TTNT) were analyzed. PMI values were also compared with those from a DLBCL cohort. Results PMI was significantly lower in MM than in DLBCL ( p  < 0.001), suggesting more severe sarcopenia in MM. Low PMI was associated with older age, higher frequency of high-risk cytogenetics, and reduced ASCT rates. Low PMI correlated with shorter OS (2-year OS: 58% vs. 85%) and shorter TTNT (2-year TTNT: 26% vs. 60%). While low PMI was not an independent prognostic factor in the overall cohort, it remained independently associated with poor OS in patients with standard-risk cytogenetics (HR: 4.16; p  = 0.014). Conclusion Low PMI is associated with inferior clinical outcomes in MM, particularly in standard-risk patients. PMI may serve as a practical prognostic marker using routine PET-CT scans and complement existing sarcopenia measures.