Segmental bioimpedance and arm-volume asymmetry after breast cancer surgery: a longitudinal analysis of within-patient associations

Background Segmental bioimpedance and perometry assess different aspects of arm asymmetry after breast cancer surgery. The within-patient relationship between these measurements during extended postoperative follow-up remains incompletely characterized. We examined the association between segmental extracellular water/total body water (ECW/TBW) asymmetry and concurrent arm-volume asymmetry across postoperative periods. Methods This retrospective, exploratory cohort included 246 patients with 1,642 same-day perometry and segmental bioimpedance visits. Volume asymmetry was calculated as the affected-minus-unaffected arm volume divided by unaffected-arm volume, expressed as a percentage. ECW/TBW asymmetry was the affected-minus-unaffected arm ratio difference. Each patient’s mean ECW/TBW asymmetry was separated from visit-specific deviations. Generalized estimating equations with working independence and patient-clustered robust standard errors adjusted for postoperative period, body mass index, nodal positivity, axillary surgery, and regional nodal irradiation. Sensitivity analyses examined exchangeable correlation and native perometry measurements. Secondary volume-course analyses included 2,045 visits from the same patients. Results A 0.01 higher ECW/TBW asymmetry relative to a patient’s own mean was associated with 3.78 percentage points higher volume asymmetry (95% confidence interval [CI], 1.79–5.77). Beyond 3 years, the corresponding estimate was 3.86 (95% CI, 2.17–5.54). Associations differed across postoperative periods (p = 0.00087), with the largest estimate during 0–3 months, based on 35 patients and 46 visits. No clear evidence of heterogeneity was found among the three later periods (p = 0.368). Evidence of period heterogeneity in native-only paired measurements depended on working correlation. Adjusted volume asymmetry beyond 3 years was 9.48 percentage points higher than during 0–3 months (95% CI, 5.36–13.61), although the native-only contrast included zero under exchangeable correlation. Conclusions Segmental ECW/TBW asymmetry was positively associated with concurrent arm-volume asymmetry within patients, including beyond 3 years after breast cancer surgery. Period-specific estimates were sensitive to sparse early observations and analytic specification. These findings extend the characterization of serial fluid-fraction and volume measurements during routine follow-up; their implications for surveillance decisions require prospective validation.