Temporal change in skeletal muscle index as a predictor of recurrence for patients with locally advanced colorectal malignancy: A retrospective cohort study

Background: Colorectal cancer remains a leading cause of cancer-related morbidity and mortality with stage III disease carrying a substantial risk of recurrence despite curative resection. Accurate risk stratification is essential to optimize surveillance and guide adjuvant therapy. Traditional models rely heavily on pathologic features, but recent studies suggest that body composition metrics, particularly imaging-based assessments of skeletal muscle mass, may offer additional prognostic value. The skeletal muscle index, derived from routine CT imaging, has emerged as a promising surrogate marker of frailty. However, the relationship between temporal changes in SMI and cancer recurrence remains poorly defined. Methods: A retrospective cohort study was performed using single-institution data from over 500 patients aged 18 or greater who underwent resection for locally advanced stage III colorectal malignancy between years 2000 and 2020. Skeletal muscle index was measured at the third lumbar vertebral level using preoperative CT imaging from the time of initial diagnosis. For patients with recurrence, a follow-up measurement was obtained at their recurrence date, and using propensity score analysis, a patient without recurrence was selected to best match this follow-up duration. Temporal changes in skeletal muscle index were then calculated and compared with a conditional logistic regression. Receiver operating characteristics were then analyzed to identify clinically relevant thresholds of SMI decline. Results: A decrease in skeletal muscle index was independently associated with increased risk of disease recurrence with an odds ratio of 1.30 per 1-point decrease in SMI (95% CI: 1.09–1.54, p = 0.003). Receiver operating curve analysis suggests that an SMI decline of 2.5–6.0 cm²/m² within 1–5 years after resection may serve as a practical threshold for risk stratification. Conclusions: Postoperative skeletal muscle index decline is a significant, independent predictor of cancer recurrence and may serve as a clinically useful marker to personalize follow-up care and enhance risk stratification. These findings support the potential role of body composition monitoring to guide postoperative management and highlight a need for further prospective validation.