Transplant-Associated Decline in Pectoral Muscle Density Following Autologous Hematopoietic Stem Cell Transplantation: A Longitudinal CT-Based Study

Background and Objectives: Autologous hematopoietic stem cell transplantation (AHSCT) can lead to skeletal muscle loss owing to the catabolic state and inflammatory burden it induces. Pectoral muscle density (PMD), measured on thoracic CT, is a valuable indicator for assessing muscle composition and quality. However, its time-dependent change during AHSCT and the early post-transplant period, as well as its prognostic value, remain insufficiently defined. This study evaluated the dynamic changes in PMD following AHSCT and the clinical relevance of pre-transplant PMD. Materials and Methods: This single-center retrospective study included patients who underwent AHSCT between January 2022 and December 2024 and had a pre-transplant thoracic CT scan available. Patients with both pre- and post-transplant thoracic CT images constituted the longitudinal subgroup in which the dynamic change in PMD was assessed. PMD was measured in Hounsfield units (HU) on non-contrast CT, on a single axial slice immediately above the aortic arch using a 1-cm2 region of interest, taking the average of three measurements. The primary endpoint was the change in PMD between the pre- and post-transplant scans; secondary endpoints were length of hospital stay, neutrophil and platelet engraftment, and overall survival. Paired t-tests, Wilcoxon signed-rank and Mann–Whitney U tests, Spearman correlation, multivariable linear regression, and Cox proportional hazards and Kaplan–Meier analyses were used. Results: 142 patients (57.0% male; mean age 53.5 ± 14.9 years; multiple myeloma 43.7%, non-Hodgkin lymphoma 38.0%, Hodgkin lymphoma 14.1%) were analyzed; mean pre-transplant PMD was 52.95 ± 9.66 HU. In the longitudinal subgroup (n = 79; post-transplant CT at a median of day +12), PMD declined in 96.2% of patients, from 52.80 ± 9.22 to 41.99 ± 9.21 HU (mean reduction 10.81 HU; 95% CI 9.19–12.43; p < 0.001; Cohen’s dz = 1.50; mean change −20.27 ± 13.36%). The decline was independent of the inter-scan interval, sex, age, and diagnosis. Lower PMD correlated with older age (ρ = −0.281, p < 0.001) and differed by underlying diagnosis (p = 0.022). Patients with pre-transplant PMD ≤ 60 HU had a longer mean hospital stay than those with PMD > 60 HU (19.6 vs. 16.4 days); however, this difference was not significant in non-parametric (p = 0.33) or multivariable analyses (p = 0.25). Baseline PMD was not significantly associated with engraftment or overall survival (HR per 10-HU decrease 1.35; 95% CI 0.91–2.00; p = 0.14). Conclusions: A marked and consistent decline in pectoral muscle density occurs in the early period following AHSCT, irrespective of scan timing, sex, age, or diagnosis. Baseline PMD was not significantly associated with engraftment, hospitalization, or survival, although numerical trends toward longer hospitalization and poorer survival were observed with lower PMD. PMD, obtainable from routine non-contrast thoracic CT, may serve as a marker of transplant-related changes in muscle quality, and its prognostic value warrants prospective evaluation.