Impact of Circular Stapler Diameter on Anastomotic Leakage in Left-Sided Colorectal Cancer: A Retrospective Single-Center Case–Control Analysis

<i>Background and Objectives</i>: Anastomotic leakage (AL) is a major complication following sphincter-preserving surgeries for left-sided colorectal cancer. In this study, we aimed to evaluate the association between circular stapler diameter and the risk of AL. As a secondary objective, we investigated whether preoperative serum protein levels were associated with leakage development. <i>Materials and Methods</i>: We conducted a retrospective case-control study including 99 patients who underwent elective colorectal surgery with stapled anastomosis for left-sided colorectal cancer between January 2020 and May 2024. A total of 99 patients were included (60.6% male), with a mean age of 66.1 ± 10.7 years. The patients were categorized into small (≤29 mm) and large (≥30 mm) stapler groups. Demographic, clinical, and laboratory variables were collected. Anastomotic leakage was defined as an International Study Group of Rectal Cancer (ISREC) Grade B or C leak requiring intervention. Univariate analyses and multivariate logistic regression analyses were performed, and results were reported as odds ratios (ORs) with 95% confidence intervals (CIs). A STROBE-compliant flow diagram was prepared. <i>Results</i>: Anastomotic leakage occurred in 10 patients (10.1%), and leakage rates were not significantly different between stapler-size groups (≤29 mm: 10.9% vs. ≥30 mm: 7.5%, <i>p</i> = 0.365). In multivariate analysis, stapler size was not independently associated with leakage (OR 1.68, 95% CI 0.40-6.97, <i>p</i> = 0.480). Lower preoperative serum protein levels were identified as the only independent predictor of leakage (OR 0.28, 95% CI 0.10-0.74, <i>p</i> = 0.011). Postoperative hospital stay was significantly longer for patients with leakage (median 17 vs. 7 days, <i>p</i> < 0.001). <i>Conclusions</i>: We found no significant associations between circular stapler diameter and anastomotic leakage in left-sided colorectal cancer surgery. Conversely, low serum protein levels were independently associated with increased leakage risk, highlighting the importance of preoperative nutritional assessment. Given the retrospective design, small number of leakage cases, and unmeasured confounders, these findings should be interpreted with caution. Further multicenter, prospective studies should be conducted to clarify the influence of stapler size and patient-related factors on anastomotic outcomes.