Lateral Pelvic Lymph Nodes (LPLNs)
Definition
LPLNs are lymph nodes in the lateral pelvic compartments that receive lymphatic drainage from the extraperitoneal (lower) rectum. They are distinct from mesorectal lymph nodes and have different prognostic implications.
Which Tumors Are at Risk
- Only tumors below the APR (extraperitoneal rectum) have lymphatic drainage to LPLNs
- LPLN metastasis is rare in T1/T2 tumors: incidence 5.4-8.2%
- T3/T4 tumors below APR: incidence 16.5-37.2%
- 92.3% of LPLN metastases develop in the internal iliac and obturator node compartments
LPLN Compartments
Internal Iliac Lymph Node Compartment
All lymph nodes abutting the internal iliac artery and its branches from its origin to the point where terminal branches exit the infrapiriformis foramen.
Terminal branches exiting the infrapiriformis foramen:
- Internal pudendal artery
- Inferior gluteal artery
Board Pearl
Internal iliac nodes are sentinel lymph nodes for rectal cancer. Enlarged internal iliac nodes are more likely to be metastatic than comparable obturator nodes.
Obturator Lymph Node Compartment
Defined by three levels:
| Level | Definition |
|---|---|
| Pelvic brim | Lateral to a line along the lateral aspect of internal iliac vessels |
| Midpelvis | Lateral to a line from internal iliac branches to the obliterated umbilical artery; posterior to external iliac vessels |
| Low pelvis | All nodes below the infrapiriformis foramen |
Key landmarks:
- At midpelvis: obliterated umbilical artery is medial (do not confuse with laterally located vas deferens)
- Obturator nodes in the low pelvis (below infrapiriformis foramen) were first described by the LNSC
External Iliac Nodes (NOT LPLNs)
- Flat oblong nodes adjacent to external iliac vessels at the level where they exit the pelvis
- Do NOT receive visceral lymphatics from pelvic organs
- M category (distant metastasis) — NOT regional
- Significant survival difference vs obturator nodes
- NOT candidates for LPLN dissection
Board Pearl
Distinguishing obturator nodes from external iliac nodes is critical: obturator nodes are regional (N category with favorable survival after resection), while external iliac nodes are M category (distant metastasis, not candidates for LPLN dissection).
Radiologist Agreement
- Only 46-62% of radiologists correctly localized internal iliac and obturator nodes
- Improved to 72-77% after 2-hour training session
- Significant learning curve for LPLN compartment identification
Size Criteria
Pretreatment MRI (T3/T4 tumors below APR)
| LPLN Size (Short Axis) | Lateral LR Rate at 4-5 Years |
|---|---|
| >10 mm | 33-36.7% |
| 5-<10 mm | 10.1-20% |
| <5 mm | 6.4% |
- LNSC: LPLNs >=7 mm → 19.5% lateral LR despite nCRT (nCRT alone insufficient)
- Linear relationship between size, metastasis likelihood, and lateral LR
Post-nCRT MRI
| LPLN Compartment | Size >4 mm (Short Axis) | 5-Year Lateral LR Rate |
|---|---|---|
| Internal iliac | >4 mm | 52.3% |
| Obturator | >4 mm | 9.5% |
Board Pearl
Residual internal iliac nodes >4 mm after nCRT carry a 52.3% 5-year lateral LR rate — far worse than obturator nodes at the same size (9.5%). Even small residual internal iliac nodes are significant.
Interobserver Variability
Significant measurement variability exists: a 5.7-mm node (expert measurement) showed ranges of 2-7 mm across readers; a 9.3-mm node showed ranges of 8-11 mm.
AJCC Classification
- Internal iliac nodes: N category (regional) for rectal cancer
- Survival comparable to N2 mesorectal lymph nodes
- Obturator nodes: More controversial but resection shows excellent survival → supports classification as regional