Internal and External Iliac Nodes Are Locoregional in Anal Canal Carcinoma
Internal iliac and external iliac lymph nodes are classified as locoregional (N1) nodes in anal canal carcinoma, not distant metastases. Specifically, internal iliac nodes are categorized as N1a disease, while external iliac nodes are classified as N1b disease according to the AJCC 9th edition staging system 1.
Staging Classification Details
The current AJCC 9th edition defines the nodal staging as follows 1:
- N1a: Metastasis in inguinal, mesorectal, superior rectal, internal iliac, or obturator nodes
- N1b: Metastasis in external iliac nodes
- N1c: Metastasis in external iliac nodes combined with any N1a nodes
This classification explicitly places both internal and external iliac nodes within the regional nodal basin, making them locoregional rather than distant disease.
Anatomic Drainage Patterns
The lymphatic drainage patterns support this classification 1, 2:
- Proximal anal canal tumors (at or above the dentate line) drain to perirectal, paravertebral, and internal iliac nodes
- Distal anal canal tumors drain primarily to superficial inguinal nodes but can also involve the iliac system
- The external iliac nodes receive drainage from the inguinal and femoral nodes, particularly from infra-dentate and perianal skin lesions 2
Clinical Implications for Treatment
Both internal and external iliac nodes are routinely included in elective radiation treatment volumes 3:
- Standard radiotherapy target volumes include the primary tumor, anal canal, rectum, and mesorectal, presacral, internal and external iliac, obturator, and inguinal lymph nodes
- Recent evidence suggests that in early-stage disease (cT1-2N0), external iliac and upper pelvic nodes may potentially be omitted from elective coverage given extremely low recurrence rates (0.9% for upper pelvic, 0% for external iliac) 4, 5
- However, for advanced disease (T3-4 or node-positive), these regions should be included as failure rates increase 6
Important Distinction from Rectal Cancer
This differs fundamentally from rectal carcinoma staging, where iliac node involvement would typically be considered non-regional (M1) disease. The anal canal has a distinct lymphatic drainage pattern that incorporates the iliac nodal basins as part of the regional spread.