Rectal Anatomy and Its Clinical Significance

Anatomical Boundaries and Definition

The National Comprehensive Cancer Network (NCCN) defines the rectum as the area below the line connecting the sacral promontory to the upper edge of the pubic symphysis on mid-sagittal MRI 1, 2
The anorectal ring marks the inferior boundary, formed by the palpable upper border of the anal sphincter and puborectalis muscles, approximately 3-5 cm from the anal verge 3, 4, 5

The anterior aspect of the rectum has peritoneal covering down to the level of the anterior peritoneal reflection 6, 7, 8, 9
The relationship of rectal tumors to this reflection is clinically critical, as it predicts risk of both local and peritoneal recurrence 6, 7, 8, 9

Tumor involvement of the mesorectal fascia (MRF) is defined as tumor, metastatic lymph nodes, or extramural vascular invasion (EMVI) within ≤1 mm of the fascia 1, 2
The optimal surgical plane for rectal cancer resection follows the MRF, producing an intact bulky mesorectum with a smooth surface 6, 7, 8, 9

The levator ani muscle complex forms the pelvic floor and includes the puborectalis muscle 1, 2, 5
The puborectalis muscle creates the anorectal angle and forms the superior boundary of the anal canal; palpable as a distinct muscular ring during digital examination 3, 5

When rectal cancer invades beyond the visceral peritoneum or mesorectal fascia, it may involve urogenital organs, gastrointestinal structures, musculoskeletal structures, and neurovascular structures 1, 2, 6

Regional lymph nodes include mesorectal lymph nodes, distal sigmoid mesentery lymph nodes, para-rectal vessel lymph nodes, and internal iliac lymph nodes 1, 2
Lower rectal cancers have a higher risk of lateral lymph node metastasis, including obturator and internal iliac nodes 1, 2

Always annotate the distance between rectal tumors and the puborectal muscle, and specify the involved quadrant, particularly the anterior 1/4 quadrant 1, 2
Do not assume all rectal tumors are extraperitoneal; anterior tumors above the peritoneal reflection behave differently regarding peritoneal recurrence risk 6, 7, 8, 9

The rectum extends approximately 12-15 cm from the anal verge to the rectosigmoid junction, though in a distended lumen during endoscopy, 16-20 cm of endoscope insertion may be required to reach the rectosigmoid junction, as defined by the American Gastroenterological Association 10
The rectum is formally defined as tumors with distal extension to 15 cm from the anal margin as measured by rigid sigmoidoscopy, with more proximal tumors classified as colonic, according to the European Society for Medical Oncology 11

The rectum is divided into three distinct segments based on distance from the anal verge: Lower rectum (up to 5 cm), Middle rectum (greater than 5 cm to 10 cm), and High (upper) rectum (greater than 10 cm up to 15 cm), as classified by the European Society for Medical Oncology 11

The peritoneal coverage of the rectum differs significantly by anatomical level, with the Upper third covered by peritoneum anteriorly and laterally, the Middle third covered only on the anterior wall, and the Lower third having no peritoneal covering, as described by the American Gastroenterological Association 10

Location must be documented in relation to specific anatomical landmarks, and the distance should be measured using rigid sigmoidoscopy rather than flexible endoscopy, as recommended by the European Crohn’s and Colitis Organisation 11, 12, 13