OBJECTIVETo investigate the surgical endpoint of separation of mesorectum during total mesorectal excision (TME), suggesting the concept of "terminal line", in order to perform above separation better for middle-low rectal cancer.

METHODSGross anatomy of mesorectum endpoint from 81 surgical specimens of low anterior resection (LAR, 5 to 6 cm of distance from low margin of cancer to anal edge) and 71 surgical specimens of abdominal perineal resection(APR, <5 cm of distance from low margin of cancer to anal edge) was observed. Clinicopathological, magnetic resonance imaging(MRI) morphological and operative video data of 108 low rectal cancer patients undergoing TME at Department of Colorectal Surgery of Affiliated Union Hospital of Fujian Medical University between March 2016 and March 2017 were retrospectively analyzed. Rates of the "terminal line" exposure of TME between different surgical procedures(robot or laparoscope) and different anatomical instruments (ultrasonic knife or electric hook) were compared for evaluating the site of separation endpoint.

RESULTSThe gross anatomical findings of specimens from LAR showed that the rectal wall below the levator hiatus level had no mesorectum attachment, and gross anatomical finding of specimens from APR showed that the levator hiatus was the most terminal attachment margin of the mesorectum whose thickness was only 2 millimeters in levator hiatus level. MRI morphological findings of 108 low rectal cancer patients showed that high signal intensity of mesorectal tissue on T2 MRI gradually thinned to the level of levator hiatus. High quality laparoscopic and robotic operation revealed a white linear structure formed by pelvic fascia, which covered and surrounded levator hiatus, so the "terminal line" of TME was defined. The operation video of 108 revealed that the overall exposure rate of the "terminal line" was 45.4%, the exposure rate of "terminal line" in robotic surgery was similar to that in laparoscopic surgery [(60.0%(18/30) vs. 39.7%(31/78), P=0.058], while such rate in ultrasonic knife was superior to electric hook [55.4%(41/74) vs. 23.5%(8/34), P=0.002]. Laparoscopy combined with ultrasonic knife can also obtained a high exposure rate of 52.3%(23/44).

CONCLUSIONSThe white linear structure referring to pelvic fascia which covers and surrounds levator hiatus is the "terminal line" of TME. The use of an ultrasonic knife is easier to expose this structure and to guarantee the quality of TME.