Objective:To investigate the influence of computer tomography (CT) examination bony parameters and surgical methods on difficult pelvis and perioperative efficacy of mid-low rectal cancer.Methods:The retrospective case-control study was conducted. The clinicopathological data of 191 patients who underwent robotic or laparoscopic total mesorectal excision (TME) for mid-low rectal cancer in the First Affiliated Hospital of Nanchang University from January 2021 to December 2022 were collected. There were 123 males and 68 females, aged (60±11)years. According to the surgical difficulty score, 191 patients were divided into the difficult pelvis group of 41 patients and the non-difficult pelvic group of 150 patients. Thirteen pelvic parameters were obtained on CT images of the 191 patients. Observation indicators: (1) comparison of intraoperative and postoperative conditions of patients in the difficult pelvis group and the non-difficult pelvis group; (2) analysis of factors affecting difficult pelvis in patients with mid-low rectal cancer; (3) stratified analysis of different surgical methods for patients with difficult pelvis. Measurement data with normal distribu-tion were represented as Mean± SD, and comparison between groups was conducted using the t test. Measurement data with skewed distribution were represented as M(range), and comparison between groups was conducted using the Mann-Whitney U test. Count data were expressed as absolute numbers, and comparison between groups was conducted using the chi-square test or Fisher exact probability. Comparison of ordinal data was conducted using the Mann-Whitney U test. Univariate and multivariate analyses were conducted using the Logistic regression model. Results:(1) Com-parison of intraoperative and postoperative conditions of patients in the difficult pelvis group and the non-difficult pelvis group. There were significant differences in operation time, volume of intra-operative blood loss, sum of the product of the longest diameters (SPD) of tumor lesions, postopera-tive complications, duration of postoperative hospital stay, and hospitalization expenses between the two groups ( t=-4.853, -5.816, Z=2.838, χ2=81.498, t=-5.897, -2.770, P<0.05). (2) Analysis of factors affecting difficult pelvis in patients with mid-low rectal cancer. Results of multivariate analysis showed that gender, age, pelvic inlet, distance between the sacral promontory and the coccyx, depth of the pelvis, line AB, angle A, and angle B were independent factors affecting difficult pelvis in patients with mid-low rectal cancer ( odds ratio=0.080, 1.067, 3.065, 0.004, 10 165.062, 0.019, 0.476, 0.662, 95% confidence interval as 0.016-0.409, 1.008-1.130, 1.388-6.767, 0.001-0.024, 477.574-216 361.071, 0.003-0.131, 0.358-0.632, 0.551-0.794, P<0.05). (3) Stratified analysis of different surgical methods for patients with difficult pelvis. ① Comparison of general preoperative data of patients with difficult pelvis receiving different surgical methods. Of the 41 mid-low rectal cancer patients with difficult pelvis, 15 underwent robotic TME, and 26 underwent laparoscopic TME. There was a significant difference in the body mass index between patients receiving the two surgical methods ( t=-2.055, P<0.05). ② Comparison of intraoperative and postoperative conditions of patients with difficult pelvis receiving different surgical methods. There was no significant difference in operation time, volume of intraoperative blood loss, SPD of tumor lesions, number of lymph nodes dissected, number of lymph node metastases, distance between tumor and anal verge, combined organ resection, postoperative complications, duration of postoperative hospital stay, hospitalization costs, T staging, N staging, M staging, vascular tumor thrombus, and nerve invasion ( P>0.05). Conclusions:Gender, age, pelvic inlet, distance between the sacral promontory and the coccyx, pelvic depth, AB line, angle A, and angle B are independent factors affecting difficult pelvis in patients with mid-low rectal cancer. There is no difference in perioperative outcomes between mid-low rectal cancer patients of difficult pelvis who received robotic surgery and laparoscopic surgery.