Evaluation of anatomical landmarks for transanal total mesorectal excision based on MRI

Transanal total mesorectal excision (taTME) is a novel sphincter-preserving procedure for low rectal cancer. This "bottom to up" approach is unfamiliar to colorectal surgeons and the crucial anatomical landmarks also remain unclear.

Two hundred and five cases of pelvic magnetic resonance imaging (MRI) from 2015 to 2016 were reviewed. Curvature of posterior mesorectal fascia, distal mesorectal angle, length of posterior mesorectal fascia, main structures around the mesorectum were measured and analyzed. The landmarks identified on MRI were verified in taTME procedures of five rectal cancer patients.

The most of acute angles of posterior mesorectal fascia located at the joint of anococcygeal ligament-coccyx. Degree of distal mesorectal angle was independently correlated with gender and degree of angle of anococcygeal ligament-coccyx. Candidate landmarks evaluated by MRI with verification during taTME procedures included: anterior: seminal vesicle for male while cervix for female. And peritoneal reflection was a substitute landmark when cervix was hardly confirmed in operation; posterior: the joint of anococcygeal ligament-coccyx. The area between the joint of anococcygeal ligament-coccyx and S3S4 was a "transitional zone", the level of S3S4 could be the as the terminal landmark of transanal posterior dissection during taTME.

Preoperative MRI geometrical measurement of mesorectum might play an important role in evaluating the difficulty of taTME procedure before operation, as well as standardizing landmarks during taTME procedure.