A preoperative MRI pelvic multiparameter nomogram for predicting sphincter preservation and operative difficulty in laparoscopic total mesorectal excision for ultra-low rectal cancer

Abstract Background Sphincter preservation (SP) during laparoscopic total mesorectal excision (LaTME) for low rectal cancer is challenged by a deep, narrow pelvis. Robust, MRI-based predictors for both SP success and operative difficulty are lacking. Methods We conducted a retrospective study of 275 consecutive patients undergoing LaTME (development cohort, Jan 2022-Feb 2024). An independent, temporal validation cohort (n = 118; Mar-Dec 2024) from the same institution was used. Eight predefined pelvic MRI parameters were measured. The primary outcome was SP failure (conversion to abdominoperineal resection). Surgical difficulty was defined as operative time ≥ 245 min. Independent predictors from multivariable analysis were used to construct a predictive nomogram. Model performance was evaluated by discrimination (AUC), calibration, and clinical utility (decision curve analysis). Results Shorter tumor distance from the anal verge (OR, 0.26), greater tumor diameter (OR, 1.45), and narrower intertuberous distance (OR, 0.75) were independent predictors of SP failure. The nomogram showed good discrimination (AUC: 0.85 development, 0.84 validation) and calibration. Among patients with successful SP (n = 278), neoadjuvant radiotherapy (OR, 2.11), shorter tumor distance (OR, 0.65), and larger mesorectal anteroposterior diameter (OR, 1.73) were independently associated with surgical difficulty. Conclusion We developed and validated an MRI-based nomogram that quantifies the likelihood of SP and identifies factors associated with surgical difficulty in LaTME for low rectal cancer. This tool facilitates preoperative risk stratification and personalized surgical planning.