Colorectal cancer (CRC) is a malignant tumour originating from the colorectal mucosal epithelium, with rising incidence and mortality rates. Currently, CRC ranks third in incidence and second in mortality among all cancers worldwide, making it the leading cancer in terms of global incidence and mortality. Approximately 90% of CRC develops from colorectal polyps, which are considered precancerous lesions of CRC, especially adenomatous polyps. If removed endoscopically during the polyp stage, 70%-90% of CRC can be prevented. However, current colonoscopy examinations have a high miss rate for polyps. Studies have shown that the miss rates for polyps and adenomas after colonoscopy can reach 22%-28% and 12%-26%, respectively.

The "2014 Chinese Guidelines for Early Screening and Endoscopic Diagnosis and Treatment of Colorectal Cancer" mentions that the observation method during colonoscopy starts from the rectum and progresses forward to the cecum, with observations made during withdrawal: from the cecum, ascending colon, transverse colon, descending colon, sigmoid colon to the rectum. Current quality control of colonoscopy mainly focuses on controlling withdrawal time exceeding 6 minutes, controlling cecal intubation rate, and ensuring adenoma detection rate, without specific requirements on the observation method. However, in actual clinical practice, it is found that single withdrawal observation is not enough, as this examination approach is prone to many missed polyps. The likely reason is that the colon is in a compressed state during withdrawal observation. Single-operator colonoscopy is currently the mainstream insertion method internationally, and the essence of the single-operator technique is "short-axis reductions", meaning that the colonoscope maintains a straight configuration throughout the entire examination. The average adult colon length is about 1.5m, but the distance reached by the colonoscope during the single-operator technique is often between 70-80cm, indicating compression of the colon. In addition, colonic folds become more dense when compressed, making it easier for lesions like polyps to hide within or near folds, leading to misses. Currently, in the clinical practice of colonoscopy, only withdrawal observation is performed, and only once, on a compressed colon. Many lesions can be easily missed.

The sigmoid colon, with the most turns in the entire large intestine, is also the part most prone to compression during colonoscopy insertion. Correspondingly, it is also more prone to misses during withdrawal observation. Although some scholars proposed repeating withdrawal to improve lesion detection rates, whether it is performed twice or three times, only compressed colons are observed. In actual clinical work, many polyps can only be found during advancement. The investigators propose performing a second advancement specifically for the easily compressed sigmoid colon. During the second advancement, the "short-axis reduction" technique should not be used. Instead, the folds should be deliberately advanced into, which helps fully extend the compressed sigmoid colon to shallow or eliminate the folds, allowing observation during advancement to achieve effects beyond multiple withdrawals, finding hidden lesions within or near folds to improve colonoscopy quality. Therefore, to explore whether observing during a second sigmoid colon advancement can further improve the adenoma detection rate (ADR) to improve colonoscopy quality and reduce interval cancers, the investigators conducted this study.