Although CSPO is also divided into transabdominal and transanal surgery, it is different from ISR  or ultra-low Dixon. The first is to retain the internal anal sphincter and dentate line as much as possible. Then, the design of tumor resection line and conformal resection were carried out on the premise of the safety of tumor distal margin. In addition, the mechanical suture and manual suture of the excised conformal bowel are also different. These are not only the key reasons for the increased difficulty of CSPO, but also the key points need to be overcome in the process of learning and training. Therefore, the definition of CSPO learning curve is crucial for beginners to formulate training plan, select appropriate cases and set phased objectives. In addition, it also concerns the further promotion of CSPO.
Compared with previous studies on learning curve , this study explores CSPO learning curve through moving average, CUSUM, which are two more accurate statistical methods. It not only proved the existence of CSPO learning curve, but also defined the cut-off point of learning curve in 45 cases. According to this, the patients were divided into learning period and learning completion period. Subsequent statistical analysis proved that there was no significant difference between the two groups in curative resection (distance of distal margin, number of harvested lymph nodes), postoperative complications and prognosis. This reflects the safety of learning period, and the surgical effect and prognosis will not be affected by surgical proficiency. Therefore, beginners can actively try this operation. In addition, it also proved that the learning completion period not only achieved a significant reduction in the operation time and intraoperative bleeding, but also achieved a faster recovery of postoperative gut function and a shorter postoperative hospital stay. This is not solely related to the improvement of surgical proficiency, but also related to the enhancement of the cooperation ability of the overall operation team, which also reflects the concept of fast-track surgery .
This study also further investigated the influencing factors of CSPO operation time, in order to use our experience in CSPO to help beginners choose the appropriate cases and get through the learning curve faster.
Many previous studies have proved that obesity is an important risk factor for prolonged operation time . Especially for ultra-low rectal cancer, obesity and narrow pelvis will increase the difficulty of free and anastomosis, thus increasing the operation time . However, this study found that BMI was not an independent risk factor affecting the operation time of CSPO. On the one hand, this may be related to the bias between patient selection and enrollment in this study. That is, when doctors perform CSPO, the obesity degree is also a potential enrollment tendency. It can be seen that the body types of patients in this study are relatively moderate, and the BMI value is 23.0 ± 3.3kg/m ². On the other hand, it is related to the fact that CSPO does not require cutting and anastomosis in the abdominal cavity. That is, the anastomosis process of coloanal canal is carried out through the anus, which breaks through the limitation of the patient's narrow pelvic space . This reduces the impact of obesity on the operation time, which is also the advantage of CSPO.
At present, neoadjuvant therapy combined with TME and postoperative consolidation chemotherapy has become the standard treatment mode for advanced rectal cancer recommended by NCCN and ESMO guidelines. For ultra-low rectal cancer, nCRT can not only reduce the tumor size and the difficulty of operation, but also lower the tumor stage and achieve the criteria of anus preservation. However, many studies have shown that radiotherapy can lead to rectal wall fibrosis [26-27]. This fibrosis will disturb the normal anatomical layer of perianal tissue, reduce the tactile feedback, increase intraoperative bleeding, prolong the operation time, reduce the postoperative anal function [28-29]. However, multivariate analysis in this study showed that nCRT was not an independent risk factor affecting the operation time. On the one hand, this is because the sphincter space is dissected through the anus under direct vision, which can facilitate hemostasis and clarity of anatomical layer, which is also the advantage of CSPO. On the other hand, many studies have shown that nCRT will cause microscopic morphological changes of internal anal sphincter [30-31], but have little effect on its macroscopical morphology. Therefore, as long as we pay attention to the anatomical layer, the difficulty of operation will not increase too much.
In terms of the distance of tumor from anal verge, this study showed that the distance of tumor from anal verge≥4cm was an independent risk factor affecting the operation time of CSPO. This is different from the previous cognition that the closer the tumor is to the anal margin, that is, the lower the tumor location, the greater the difficulty of operation. There are several reasons. Firstly, the higher the tumor location, the more difficult it is to expose the limited space through anus under direct vision, which is easy to bleed and confuse the anatomical layer, increasing the difficulty. In addition, the patients included in this study were all with ultra-low rectal cancer whose lower edge of the tumor less than 2cm from the dentate line. The distance of tumor from anal verge≥4cm means that the patient's anal canal is longer and the tumor is deeper under direct vision, which is also related to the patient's obesity. These will increase the difficulty of surgery. This is a significant difference between CSPO and other rectal cancer surgery, and also shows the advantage that CSPO can achieve anal preservation in a lower position.
In terms of tumor staging, all patients included in this study met the criteria of type II and III of rullier classification . It is required that the external anal sphincter of the patients is not invaded, so all the enrolled patients are T1-3 stage patients. Consistent with previous studies, multivariate analysis showed that T3 stage was an independent risk factor for CSPO operation time. On the one hand, the late tumor stage is often accompanied by nCRT for a long time, which has a certain impact on the operation. On the other hand, many previous studies have shown that the T stage of the tumor, that is, the depth of tumor invasion, is significantly and independently related to the local recurrence of the tumor . In addition, late T stage is related to the distal invasion, lateral invasion and lymph node metastasis. Therefore, for patients with late T stage, surgeons often need to clean lymph nodes and perform local operations more carefully, which will prolong operation time.
There are still some limitations in this study. First, this is a single center non random retrospective study, so there are some biases and deficiencies in the design itself. Second, this study mainly focuses on one surgeon's experience, and the operative quantity is not particularly large. Considering the heterogeneity between surgeons, other surgeons may have different results in CSPO. In addition, the surgeon in this study has accumulated experience in more than 100 cases of laparoscopic surgery, so the CSPO learning curve may be longer for novice surgeons. Third, this study takes the operation time as the main index to measure the operator's experience accumulation, but in fact, there are many other factors that may affect the operation time. In addition to the operation time, other indicators, such as the amount of bleeding, can also be used to reflect the operator's experience. Fourth, the prolongation of liquid diet time and hospital stay in the learning period may be partly due to the selection bias of the researchers. Because more conservative strategies are often adopted in the early stage of a new operation to ensure its safety. Fifth, the cut-off value of learning curve and the results of univariate and multivariate analysis need to be further proved in clinical practice. For these reasons, a prospective, multi-center and multi-index randomized controlled study is needed to further confirm the conclusions of this study and explore the methods to shorten the learning curve.