Rectal Anatomy for Staging
Definition
The rectum is the terminal segment of the large bowel, extending from the anorectal ring (top of puborectalis) inferiorly to the sigmoid takeoff (STO) superiorly. Defining the rectum precisely is critical because the rectum, anal canal, and sigmoid have distinct vascular supplies, lymphatic drainage, and treatment approaches.
Anatomic vs Surgical Anal Canal
| Feature | Anatomic Anal Canal | Surgical Anal Canal |
|---|---|---|
| Superior border | Dentate line | Anorectal ring (top of puborectalis) |
| Inferior border | Anatomic anal verge (anal-perianal skin junction) | Inferior intersphincteric groove |
| Lining | Squamous epithelium | Mixed (upper: columnar rectal mucosa; lower: squamous) |
| Embryology | Ectoderm | Functional unit, not embryologic |
| Vascular supply | Middle and inferior rectal vessels | — |
| Lymphatic drainage | Superficial inguinal nodes | — |
| Length | Lower 1/3 to 1/2 of surgical anal canal | ~2.5-5.5 cm (variable) |
Board Pearl
The surgical anal canal contains the anatomic anal canal (lower portion) AND a portion of the anatomic rectum (above the dentate line). This explains how rectal adenocarcinomas can occur within the surgical anal canal.
Defining the Rectum
The definition varies significantly between societies:
| Organization | Proximal Border | Distal Border |
|---|---|---|
| AJCC (2017) | Coalescence of taeniae / sacral promontory | Anorectal ring |
| NCCN (2020) | Sacral promontory to pubic symphysis | Anorectal ring |
| ESMO (2017) | <15 cm from anal verge | Anal verge |
| JSCCR | Lower edge of S2 | Anorectal ring |
| NCI (2000) | <12 cm from anal verge | Anal verge |
Problems with Measurement-Based Definitions
- Significant individual variation in rectal length (gender- and habitus-dependent)
- STO ranges from 9.4-19 cm from anal verge in a 100-patient study
- If standard <15 cm cutoff is used, 84% of STO-defined sigmoid tumors would be misclassified as upper rectal
- Landmark-based sacral promontory found at 21-23 cm from verge (vs 12-15 cm measurement cutoff)
Recommended Definition
Distal border: Anorectal ring (top of puborectalis) Proximal border: Sigmoid Takeoff (where rectum flexes anteriorly from sacrum)
Subdivisions of the Rectum
Using the Anterior Peritoneal Reflection as the landmark:
| Segment | Location | Peritoneal Status |
|---|---|---|
| Upper rectum | Above APR | Intraperitoneal |
| Mid rectum | At APR | Transitional |
| Lower rectum | Below APR | Extraperitoneal |
This is distinct from the measurement-based approach (0-5 cm = low, 5-10 cm = mid, 10-15 cm = upper), which does not account for individual variability.
Vascular Supply and Lymphatic Drainage
| Region | Arterial Supply | Lymphatic Drainage |
|---|---|---|
| Sigmoid colon | Sigmoid arteries (from IMA) | Along sigmoid arteries → sigmoid mesocolon |
| Entire rectum | Superior rectal arteries (from IMA) | Along superior rectal → inferior mesenteric nodes |
| Lower rectum (additional) | Middle rectal vessels | Internal iliac and obturator nodes |
| Anatomic anal canal | Middle and inferior rectal vessels | Superficial inguinal nodes |
Key MRI Landmarks
- Anorectal ring: On sagittal T2WI, line from inferior pubic symphysis to anorectal angle
- Puborectalis: On coronal T2WI, arises from inferior pubic symphysis, extends on either side of rectum, fuses posteriorly
- Dentate line: Occasionally seen as junction of T2-hyperintense columnar epithelium with T2-hypointense squamous epithelium
- Anal verge at MRI: Inferior-most intersphincteric groove (most reproducible landmark)