Lower Preoperative Skeletal Muscle Index in Patients with Pathological T4 Colorectal Cancer: An Exploratory Retrospective Cohort Study

Background/Objectives: Low skeletal muscle index (SMI) has been linked to adverse outcomes in colorectal cancer, but its association with local pathological tumor extent is less clear. This study examined whether preoperative CT-derived SMI was associated with pathological T4 disease in patients undergoing colorectal cancer resection. Methods: This retrospective single-center observational study included 147 consecutive adults who underwent colorectal resection for histologically confirmed adenocarcinoma between January 2022 and November 2025 and had suitable preoperative abdominal or abdomino-pelvic CT imaging within 90 days before surgery. Skeletal muscle area was measured on a single axial CT image at the L3 level, and SMI was calculated as muscle area/height2. Patients were classified as having pathological T1–3 or T4 disease. Logistic regression assessed the association between SMI, expressed per 5 cm2/m2 increase, and pathological T4 stage. Results: Patients with pathological T4 tumors had lower SMI than those with T1–3 disease (37.22 vs. 42.85 cm2/m2, p = 0.016). Higher SMI was associated with lower odds of T4 disease in univariable analysis (OR 0.79, 95% CI 0.66–0.94; p = 0.008) and after adjustment for age and sex (OR 0.77, 95% CI 0.63–0.94; p = 0.009). Conclusions: Lower preoperative SMI was associated with pathological T4 colorectal cancer in this cohort. Because of the retrospective observational design, causality cannot be inferred. The association should be interpreted as hypothesis-generating and may reflect reverse causality, shared inflammatory–nutritional pathways, or residual confounding.