Comparison of bioelectrical impedance-derived muscle indices and rectus femoris ultrasound in women with breast cancer and overweight or obesity: a cross-sectional study

In women with breast cancer (BC), treatment-related changes in body composition may complicate muscle assessment, particularly in the presence of excess adiposity. This cross-sectional study compared bioelectrical impedance analysis (BIA)-derived muscle indices and ultrasound-derived rectus femoris thickness (RF-MT) and examined their relationships with muscle function in women with BC and overweight or obesity. Women with BC and BMI ≥ 25 kg/m² undergoing endocrine therapy were enrolled. Participants underwent anthropometric and functional assessments, BIA-derived evaluation of appendicular skeletal muscle index (ASMI) and skeletal muscle mass-to-weight ratio (SMM/W), and ultrasound measurement of RF-MT. Probable sarcopenia was defined according to the European Working Group on Sarcopenia in Older People 2 (EWGSOP2) muscle-strength criteria. Muscle indices were compared according to probable sarcopenia status, and their associations with anthropometric and functional measures were examined using Spearman correlations and multivariable linear regression. Multivariable logistic regression models evaluated factors associated with probable sarcopenia. A total of 157 women were included (mean age 53.88 ± 7.64 years), of whom 56 (35.7%) met the criteria for probable sarcopenia. Women with probable sarcopenia were older and had higher BMI and waist circumference. SMM/W was lower in women with probable sarcopenia (27.50 ± 3.81 vs. 29.85 ± 3.95, p < 0.001), whereas ASMI and RF-MT did not differ significantly between groups. SMM/W was positively associated with handgrip strength (ρ = 0.27, p < 0.001) and inversely associated with 5×STS time (ρ=−0.28, p < 0.001) and SARC-F score (ρ=−0.35, p < 0.001). Among muscle indices, only ASMI and RF-MT showed a weak but significant association (ρ = 0.21, p = 0.010). In multivariable models, waist circumference was positively associated with ASMI and inversely associated with SMM/W, while handgrip strength remained positively associated with SMM/W. In multivariable logistic regression, age was consistently associated with probable sarcopenia across all models, and among muscle indices, only SMM/W was inversely associated with the outcome. In women with BC and overweight or obesity, probable sarcopenia was present in approximately one-third of participants. SMM/W, but not ASMI or RF-MT, differed between women with and without probable sarcopenia and showed the most consistent relationships with functional measures. The quantitative indices showed limited pairwise associations and should not be considered interchangeable.