Anterior Peritoneal Reflection (APR)
Definition
The APR is the point where the visceral peritoneum reflects from the bladder/seminal vesicles (or uterus) onto the anterior rectal wall. It is NOT interchangeable with the visceral peritoneum itself.
Imaging Appearance
- On axial T2WI: V-shaped or “seagull” configuration at the point where peritoneum reflects onto the anterior rectum
- Identifiable in 81.6-88.5% of rectal MRI studies
- Seminal vesicles and uterocervical angle serve as reliable landmarks
- Excellent interreader agreement for APR localization
Significance
The APR divides the rectum into two clinically distinct regions:
| Feature | Above APR (Upper Rectum) | Below APR (Lower Rectum) |
|---|---|---|
| Peritoneal status | Intraperitoneal | Extraperitoneal |
| MRF relationship | Posterolateral only | Circumferential |
| Lymphatic drainage | Superior rectal → inferior mesenteric nodes | Also to internal iliac + obturator nodes |
| LPLN metastasis | Not at risk | At risk (T3/T4) |
| MRF/organ involvement | Lower incidence (wider pelvis) | Higher incidence (tapering mesorectal fat) |
| Pulmonary metastasis | Lower incidence | Higher (systemic drainage via middle rectal veins) |
| Peritoneal spread | Possible (T4a) | Not applicable |
| Typical treatment (stage II/III) | Upfront surgery increasingly preferred | nCRT or TNT generally required |
Board Pearl
The APR separates two distinct lymphatic drainage zones. Only the extraperitoneal rectum (below APR) drains to lateral pelvic lymph nodes. This is why LPLN size criteria apply only to tumors below the APR.
Peritoneal Involvement and T Staging
- Tumor involving the peritoneum = T4a (upstaging)
- Abutment of peritoneum does NOT equal involvement
- Must identify the tumor base (point of muscularis propria transgression) to accurately assess involvement
- Pitfall: On sagittal images, rolled tumor edges can create false impression of peritoneal contact
Progressive Peritoneal Investment Above the APR
As the rectum ascends from the APR to the STO:
- Peritoneum gradually extends from anterior to anterolateral coverage
- MRF correspondingly reduces from circumferential to posterolateral only
- At the STO, peritoneum nearly encircles the rectum
- Above the STO (sigmoid), peritoneum completely encircles except at mesocolon root
- The peritoneum and MRF merge laterally with no clear demarcation on MRI or CT
Board Pearl
Tumors above the APR may involve peritoneum (T4a) AND/OR MRF, as these structures merge laterally. Radiologists must report both when relevant.
Trial Evidence
- OCUM trial: 89% of upper rectal tumors (above APR) went directly to surgery with no LR at 3 years
- Multiple trials show stage II/III tumors above APR can proceed to surgery without nCRT
- This supports the APR as a clinically meaningful division point