Definition of the rectum

The total mesorectal excision technique has been the greatest single prognostic factor improving long-term outcome in patients with cancer of the rectum, by a decrease in locoregional recurrence and an increase in overall survival. Moreover, the prognosis of patients with rectal cancer has further improved with the addition of perioperative therapies, specifically neoadjuvant therapy based on chemo and especially radiotherapy. However, the rectum is fixed both distally and proximally, and this targeted radiotherapy is challenged, for example, in mobile tumors from the distal sigmoid. It is mandatory to optimize and standardize the definition of the rectum and the transition between sigmoid and rectum to better plan preoperative staging and treatment.

There exist different historic and pragmatic definitions of the rectum, mostly based on textbooks of anatomy and classical descriptions. Some examples are:

• The 3rd valve of Houston as seen in endoscopy
• The coalescence of colonic teniae, and the loss of haustrations and appendices epiploicae
• The transition from sigmoidal vessels to superior rectal vessel
• The transition from sigmoid mesocolon to mesorectum
• The location of the anterior peritoneal reflection
• The sacral promontory
• The distance from the anal verge, which can range from less than 13cm, to 15cm or less

A recent study has concluded that the sigmoid take-off based on imaging, which is defined as the junction of the mesorectum and mesocolon, might be considered as an appropriate and standardized definition. The final suggestion was that sigmoid tumors are considered those that arise above the sigmoid take-off, rectosigmoid tumors are those purely at the take-off, and high rectal tumors are those below the take-off but above the peritoneal reflection. This is of great importance especially because of two significant variations in the treatment of distal colorectal cancer: neoadjuvant therapy, and partial mesorectal excision. However, it is too soon to determine if tumors of the higher rectum, which might similar but not equivalent to tumors above the anterior peritoneal reflection, are also good candidates for neoadjuvant radiotherapy.

The rectum is surrounded by an embryologically defined envelope of tissue with its inherent lymphovascular supply. In rectal cancer surgery, the anatomical definition of the mesorectum is of utmost importance, as shown by Professor Heald in the 1980s and verified in several other studies. A mesorectal excision as an en-bloc dissection for cancer, with boundaries founded in embryology, has been the most important variable that has allowed for a significant improvement in recurrence and survival outcomes in these patients.

In this video the definition of the rectum and mesorectum, together with practical examples based on MRI, is reviewed.