What is the anatomy of the colon in a patient with mid to upper rectal cancer?

Anatomy of the Colon in Mid to Upper Rectal Cancer

Rectal Boundaries and Definition

The rectum is defined as the area below the line connecting the sacral promontory to the upper edge of the pubic symphysis on mid-sagittal MRI, with its inferior boundary at the anorectal ring (palpable upper border of the anal sphincter and puborectalis muscles, approximately 3-5 cm from the anal verge). 1

• Mid to upper rectal cancers typically lie above the peritoneal reflection anteriorly, which has critical implications for surgical planning and recurrence patterns 1
• The rectosigmoid boundary is marked by fusion of the three taeniae coli of the sigmoid colon to form the circumferential longitudinal muscle of the rectal wall 2

Wall Layers and Histologic Structure

The rectal wall consists of distinct layers that are critical for pathologic staging 2:

• Mucosa and submucosa: The innermost layers where tumors originate
• Muscularis propria: Clearly delineated at most sites, making pT1-pT3 classification straightforward 2
  • Internal sphincter represents a continuation of the muscularis propria in the low rectum 2
• Mesorectum: The fatty tissue envelope containing lymph nodes and vessels
• Mesorectal fascia (MRF): The outermost boundary, a multi-layered membrane surrounding the mesorectum 3

Peritoneal Coverage

For mid to upper rectal cancers, understanding peritoneal coverage is essential as it predicts both local and peritoneal recurrence risk. 1

• The anterior aspect of the rectum has peritoneal covering down to the level of the anterior peritoneal reflection 2, 1
• Posteriorly, the nonperitonealized margin is represented by a triangular-shaped bare area extending superiorly in continuity with the sigmoid mesentery 2
• Tumors above the peritoneal reflection anteriorly have risk of peritoneal metastases if they breach the serosa (pT4a) 2
• Tumors below the reflection have risk of local recurrence through extraperitoneal spread 1

Mesorectal Envelope and Surgical Planes

The optimal surgical plane for rectal cancer follows the mesorectal fascia, producing an intact bulky mesorectum with a smooth surface—this is the "complete" or "optimal" plane. 2, 1

Surgical plane quality directly impacts outcomes 2:

• Mesorectal fascia plane (complete): Intact bulky mesorectum with smooth surface, only minor irregularities, no defects >5mm depth 2
• Intramesorectal plane (near complete): Moderate bulk with irregularities >5mm but not extending to muscularis propria 2
• Muscularis propria plane (incomplete): Little mesorectal bulk with defects down to the muscle layer—associated with higher local recurrence 2

Tumor involvement of the MRF is defined as tumor, metastatic lymph nodes, or extramural vascular invasion within ≤1mm of the fascia 1

Muscular Anatomy

The levator ani muscle complex forms the pelvic floor and includes the puborectalis muscle 1:

• The puborectalis creates the anorectal angle and forms the superior boundary of the anal canal 1
• The conjoined longitudinal muscle consists of smooth muscle from the rectal longitudinal layer and striated muscle from the levator ani 3
• The rectourethralis muscle connects directly to the conjoined longitudinal muscle at the top of the external anal sphincter 3

Lymphatic Drainage

Regional lymph nodes for mid to upper rectal cancer include mesorectal lymph nodes, distal sigmoid mesentery lymph nodes, para-rectal vessel lymph nodes, and internal iliac lymph nodes. 1

• Lymph node size is not a reliable predictor of nodal involvement; nodes as small as 4.5mm may harbor metastases 2
• Lower rectal cancers have higher risk of lateral lymph node metastasis including obturator and internal iliac nodes 1

Adjacent Pelvic Structures

When rectal cancer invades beyond the visceral peritoneum or mesorectal fascia (pT4b), it may involve 1:

• Urogenital organs: Bladder, prostate, seminal vesicles, vagina, uterus
• Gastrointestinal structures: Small bowel, sigmoid colon
• Musculoskeletal structures: Sacrum, pelvic sidewall muscles
• Neurovascular structures: Autonomic nerves pass between the mesorectal fascia and parietal fascia 3

Critical Clinical Pitfalls

Always document the distance between rectal tumors and the puborectal muscle, and specify the involved quadrant, particularly the anterior quadrant. 1

• Do not assume all rectal tumors are extraperitoneal; anterior tumors above the peritoneal reflection behave differently regarding peritoneal recurrence risk 1
• Distinguish peritoneal involvement through direct continuity (pT4a) from discontinuous peritoneal deposits (pM1c distant metastases) 2
• Sharp pelvic dissection must follow the anatomic fascial plane to avoid autonomic nerve damage 3
• Tumor perforation requires penetration of the serosal surface; contained peritumoral abscesses without serosal breach are pT3, not pT4a 2