This study found that robot-assisted surgery significantly reduced postoperative complications compared with laparoscopic surgery. In particular, urinary dysfunction, which is one of the most frequent postoperative complications, tended to less frequent in the Robot group than in the Lap group. The novelty of this study lies in its focus on anastomotic leakage and urinary dysfunction, which are clinically important morbidities. Furthermore, this result has a statistical impact when the IPTW method is used to remove various patient background biases.

Although a few studies have demonstrated the safety of robot-assisted surgery for rectal cancer, no strong evidence supports its superiority. Many randomized trials and meta-analyses have reported that robot-assisted surgery is superior to laparoscopy and associated with a significantly lower rate of conversion to laparotomy. Conversely, many reports have identified no significant differences in intraoperative blood loss, postoperative complications, or postoperative hospital stay, whereas some studies have reported inferior outcomes, such as a prolonged operation time [6, 10,11,12,13,14,15]. In the 2017 ROLARR trial, Jayne et al. conducted a large multicenter randomized controlled trial (RCT) comparing the short-term results of robot-assisted versus laparoscopic surgery for rectal cancer, determining the superiority of robot-assisted surgery over laparoscopic surgery in terms of perioperative outcomes [16], but only in a limited subgroup of obese or male patients. Recently, Feng et al. conducted a large multicenter RCT of more than 1000 patients in 2022. That study reported that the patients in the robotic group had fewer postoperative complications than those in the laparoscopic group (p = 0.003) [17]. However, few reports have examined the complications in detail, including large-scale clinical trials.

Techniques and instruments for rectal cancer surgery have undergone major changes in recent years, and endoscopic and robot-assisted surgeries have become widely available. At our institution, since the number of qualified surgeons for endoscopic and robotic surgery has increased over the past five years and surgical procedures have become standardized, comparing data older than five years might have revealed a large bias. Therefore, we deemed a comparison within the most recent five-year period appropriate for analysis.

In this study, using the IPTW method, robot-assisted surgery resulted in significantly fewer postoperative complications of Clavien–Dindo grade ≥ II and ≥ III than those associated with laparoscopic surgery. Furthermore, we examined the major postoperative complications in detail. Although we observed a trend toward lower rates of AL robot-assisted surgery than in laparoscopic surgery, the difference was not significant. Regarding urinary dysfunction, although there was no statistically significant difference between the groups, the rate tended to be even lower than AL in the Robot group. Urinary dysfunction and infection were the most common postoperative complications in our study, being found in 18 cases, suggesting that robot-assisted surgery was significantly associated with a lower rate of these complications than laparoscopic surgery.

Preoperative treatment, especially neoadjuvant chemoradiation therapy (NACRT), is a risk factor for pelvic dysfunction, such as urinary dysfunction, after rectal cancer surgery [18, 19]. Therefore, a sub-analysis excluding the 12 patients with NACRT in this study was conducted, and postoperative complications of urinary dysfunction (OR, 0.54; 95% CI 0.33–0.90; p = 0.02) were found to be significantly less frequent in the Robot group than in the Lap group (Supp. Tables 1, 2, 3, 4). These results indicate that the robotic approach does indeed contribute to the prevention of urinary dysfunction.

In rectal cancer resection, the urological function must be preserved even while complete tumor resection is pursued. Postoperative urinary dysfunction is often caused by intraoperative injury to the pelvic visceral nerves or pelvic plexus. Robot-assisted surgery has a more stable high-resolution field of view and multi-joint capability than laparoscopic surgery and enables accurate visualization of the anatomy and a safe approach, which can help preserve the pelvic nerves [20]. Most reports of urogenital dysfunction after rectal cancer surgery used the International Prostate Symptom Score (IPSS) and/or the International Index of Erectile Function Scores (IIEF-5), and many have reported that both the IPSS and IIEF-5 are better in robot-assisted surgery than in laparoscopic surgery [21,22,23,24,25]. Tang et al. reported a significantly faster recovery rate for urinary dysfunction with robot-assisted surgery than with laparoscopic surgery [26].

In the present study, although robot-assisted surgery was associated with a lower incidence of AL than laparoscopic surgery, the incidence was also low in the Lap group (6.5%), suggesting that patient background, improvements and innovations in anastomotic devices, and evaluation of anastomotic blood flow might have contributed more to the improvement than the surgical approach itself. For anastomotic devices, there is some concern that the number of staples used for anastomosis increases the number of small defects between staple lines, which may cause AL. Furthermore, Kim et al. reported in their prospective study that the use of two or more staples in anastomosis was associated with AL, and Fukada et al. reported that the number of staples used for anastomosis was significantly higher in male patients than in females, in procedures performed close to the anal verge than in other procedures, and in patients with a longer operative time than in those with a shorter time [27, 28].

Recently, indocyanine green fluorescence angiography (ICG-FA) has been widely used in colorectal surgery to evaluate the blood flow at anastomotic sites. ICG-FA is a near-infrared fluorescent dye that can be detected by imaging systems. It can be used to detect areas of vascular failure and, if necessary, accurately perform anastomoses in areas with a good blood flow. Some reports have demonstrated the usefulness of ICG-FA in preventing AL during colorectal surgery [29].

Robot-assisted rectal cancer surgery has only been performed for a relatively short period of time thus far, and few studies have reported the long-term outcomes [30]. Various randomized trials and meta-analyses have reported significantly lower circumferential resection margin (CRM) positive rates in robot-assisted surgery than laparoscopic surgery [10, 11], but many others have reported no significant differences in the long-term outcomes [12, 14]. Therefore, the results remain controversial. Establishing the superiority of robot-assisted rectal cancer surgery over laparoscopic surgery in terms of the long-term outcomes will require large-scale randomized controlled trials, such as the ongoing ROLARR or COLRAR trials. Postoperative complications, especially intra-abdominal infections, such as AL, are often reported to be poor long-term prognostic factors. In a meta-analysis, Wang et al. reported that the occurrence of postoperative AL significantly increased the local recurrence rate and was a poor prognostic factor for both the overall and cancer-specific survival [31]. The results suggest that reducing postoperative complications, as in this study, may contribute to prolonging the long-term prognosis.

In addition, in the present study, the number of lymph nodes collected was significantly lower in the Robot group than in the Lap group. One reason for this may be that there was a significant difference in tumor progression between the two groups, which led to a difference in the level of dissection. Second, in Japan, the mesorectal ligament around the tumor is peeled off from the resected specimen, and the lymph nodes are collected. In recent years, to evaluate CRM, the mesentery near the tumor is fixed in formalin without being removed, and in the end, pathologists often count the number of lymph nodes. Having multiple parties handle the resected specimens during our study may have affected the results.

Although this was a single-center study, the surgeons were all certified by the Japan Society for Endoscopic Surgery, a medical advisor always served as the primary surgeon or the first assistant, and the entire team was made up of surgeons who had attended designated training certification sessions. Furthermore, the patients’ background characteristics, oncological factors, and surgical factors, including the surgical equipment used, might have resulted in some biases; however, we believe that such biases were minimized between the two groups due to the implementation of the IPTW method.

One major limitation of this study was that it was not a prospective, randomized study. With the rapid spread of robotic devices, especially in high-volume center hospitals, most rectal tumor surgeries are now performed using robot-assisted approaches; therefore, it is difficult to conduct multicenter randomized controlled trials.