First Echelon Lymph Nodes in Oral Cavity and Oropharyngeal Cancers
The first echelon lymph nodes (sentinel lymph nodes) in oral cavity and oropharyngeal cancers are the initial lymph nodes encountered in the regional draining basin via lymphatic spread from the primary tumor, which are not necessarily the nodes closest to the tumor and may be multiple in number. 1
Definition and Concept
The sentinel node concept defines first echelon nodes as those lymph nodes that receive direct lymphatic drainage from the primary tumor site and represent the nodes most likely to harbor occult metastases. 1 These nodes can be identified through lymphoscintigraphy, which visualizes lymphatic channels and distinguishes nodes on a direct drainage pathway. 1
Anatomical Distribution by Primary Site
Oral Cavity Cancers
For oral cavity primary tumors, the first echelon nodes are predominantly located in levels I, II, and III of the neck. 2
- Level IB (submandibular triangle) is a common first echelon site, particularly for buccal mucosa and retromolar trigone primaries 3
- Level IIA (upper jugular chain) serves as the primary first echelon node for tongue carcinomas 3
- The submental region (Level IA) receives drainage from anterior floor of mouth, anterior tongue (especially midline lesions), lower lip, and anterior mandibular gingiva 4
- Perifacial lymph nodes in the submandibular triangle show 35% metastasis rate in oral cavity carcinoma with clinically node-positive necks 5
Oropharyngeal Cancers
For oropharyngeal primary tumors, the first echelon nodes are predominantly in levels II, III, and IV. 2
- Level II (upper jugular chain) represents the highest risk zone for oropharyngeal drainage 2
- Perifacial lymph node metastasis occurs in only 8% of oropharyngeal cancers, significantly lower than oral cavity primaries 5
Important Clinical Considerations
Multiple First Echelon Nodes
There may be multiple sentinel lymph nodes draining a single primary tumor, and all must be identified and evaluated. 1, 6 The European Association of Nuclear Medicine emphasizes that sentinel nodes need not be those closest to the tumor anatomically. 1
Detection Methodology
Dual detection technique using both radiotracer (99mTc-labeled colloid) with preoperative lymphoscintigraphy and intraoperative blue dye injection plus gamma probe guidance provides optimal identification. 6 This combined approach identified 57 of 59 positive nodes versus 44 of 59 for blue dye alone. 6
Second and Third Echelon Nodes
Beyond first echelon nodes, second echelon nodes can be identified by injecting blue dye into the sentinel node itself. 7 For tongue primaries, second echelon nodes show equivalent distribution across levels II, III, and IV on imaging, while gamma probe detection identifies level IIA most commonly, followed by III and IV. 3 Third echelon nodes have been detected only with gamma probe for tongue carcinoma at level IV. 3
Clinical Pitfalls
Gross lymphatic involvement can distort normal drainage patterns, leading to aberrant pathways and false sentinel node identification. 1, 6 This is why advanced imaging (CT, MRI, ultrasound-guided FNA, or PET/CT) is mandatory to confirm clinical N0 status before proceeding with sentinel node biopsy. 1, 6
The close proximity of primary tumors to first echelon nodes, particularly in hypopharynx and supraglottic larynx, creates "shine-through" effects that can obscure true sentinel node location. 1, 4 This technical limitation makes sentinel node biopsy investigational for these subsites. 1