Longitudinal Changes in Body Composition and Fluid Distribution During Chemotherapy in Breast Cancer Patients: A Prospective Single-Center Longitudinal Observational Study Using Bioimpedance Spectroscopy.

Background: Anthracycline- and taxane-based chemotherapy regimens are widely used in the treatment of breast cancer; however, their effects on body composition and fluid distribution are not fully elucidated. Conventional assessment methods are often insufficient to distinguish true tissue changes from treatment-related fluid shifts. The primary objective of this study was to evaluate longitudinal changes in body composition and fluid distribution during chemotherapy in breast cancer patients using bioelectrical impedance spectroscopy. The secondary objective was to investigate the impact of anthracycline and docetaxel exposure on these changes and to identify patterns suggestive of masked sarcopenia. Methods: This prospective, single-center, longitudinal observational study was conducted between October 2024 and October 2025. Follow-up assessments at 3 and 6 months were completed by October 2025. A total of 51 female breast cancer patients undergoing systemic chemotherapy were evaluated using multifrequency bioelectrical impedance spectroscopy (BCM®). Measurements were performed at baseline, 3 months, and 6 months. Changes in total body water (TBW), extracellular water (ECW), intracellular water (ICW), extracellular-to-intracellular water ratio (E/I), lean tissue mass (LTM), adipose tissue mass (ATM), and volume status were analyzed longitudinally and according to treatment exposure. Results: The cohort consisted of 51 women (median age, 55 years), of whom 70.6% were postmenopausal, and the majority had stage II-III disease. While TBW remained stable, significant alterations in fluid distribution and body composition were observed. ECW increased, and ICW decreased, resulting in a significant rise in the E/I ratio. LTM declined significantly, particularly during the first 3 months, whereas ATM showed a gradual increase. Volume status increased progressively over time, indicating fluid accumulation. Anthracycline exposure was associated with greater reductions in LTM, while docetaxel treatment was linked to significant increases in extracellular fluid and volume, especially during the 3-6-month interval. At 6 months, a median increase of +1100 mL in volume was observed alongside a decrease in muscle mass (-1.4 kg), consistent with a pattern of masked sarcopenia. Conclusions: Chemotherapy in breast cancer patients is associated with concurrent muscle loss and fluid redistribution, which may obscure clinically relevant changes in body composition. Bioelectrical impedance spectroscopy enables differentiation between fluid and tissue compartments and provides a more accurate assessment than conventional methods. Early recognition of these changes may facilitate timely nutritional support and appropriate fluid management strategies.