Lateral Neck Dissection: Indications and Technique

Anatomy of the Neck

Cervical and superior mediastinal lymph nodes are divided into seven levels delineated by anatomic boundaries ( Figure 39.1 ). Level I, which is rarely involved with thyroid malignancy, includes both submental (level IA) and submandibular (level IB) lymph node groups. Level IA, which comprises the submental triangle, lies between the anterior bellies of bilateral digastric muscles, superior to the hyoid. Level IB is situated posterior to the anterior belly of the digastric muscle, superior to the posterior belly of the digastric muscle, and anterior to the posterior border of the submandibular gland.

Levels II, III, and IV represent the upper, middle, and lower thirds of the jugular chain, respectively, and they comprise the region most commonly involved in thyroid cancer nodal metastasis. Level II is bound by the submandibular gland anteriorly, the posterior border of the sternocleidomastoid muscle posteriorly, the skull base superiorly, and the carotid bifurcation or hyoid bone inferiorly. The spinal accessory nerve divides this level into anterior (level IIA) and posterior (level IIB) compartments. Level IIB is not commonly involved in thyroid cancer. Level III extends from the lateral limit of the sternohyoid muscle to the posterior border of the sternocleidomastoid muscle and from the inferior limits of level II superiorly (i.e., carotid bifurcation or hyoid bone) to the superior limit of level IV inferiorly, as marked by the junction of the omohyoid muscle with the internal jugular vein, or alternatively the cricoid cartilage. Level IV extends from the latter surgical or clinical landmarks superiorly to the clavicle inferiorly. As with level III, this level extends from the lateral border of the sternohyoid muscle to the posterior border of the sternocleidomastoid muscle.

Level V lies posterior to the posterior border of the sternocleidomastoid muscle, anterior to the anterior border of the trapezius muscle, and superior to the clavicle. The cricoid cartilage divides this level into superior (level VA) and inferior (level VB) compartments.

Levels VI and VII constitute the central compartment bounded on both sides by the carotid sheath. Level VI extends from the hyoid bone superiorly to the suprasternal notch inferiorly. Level VII, often referred to as superior mediastinal lymph nodes, extends from the suprasternal notch superiorly to the brachiocephalic vein inferiorly.

In addition to understanding cervical lymphatic anatomy, it is important to know the cervical fascial planes. The superficial cervical fascia contains subcutaneous fat and sheaths the platysma muscle. The deep cervical fascia has three layers: superficial, middle, and deep. The superficial layer of the deep cervical fascia invests the posterior belly of the omohyoid muscle and the sternocleidomastoid and trapezius muscles. The middle layer of the deep cervical fascia envelops the strap muscles forming muscular subdivisions as well as the thyroid, larynx, trachea, and esophagus visceral subdivisions. The deep layer of the deep cervical fascia invests the deep paravertebral muscles of the neck. All three layers of the deep cervical fascia contribute to the carotid sheath.

Thyroid Gland Lymphatics

Thyroid gland lymphatics have been subdivided into three groupings. The lymphatics emerging from the inferior-medial aspect of the thyroid lobes follow the course of the inferior thyroid veins and drain to the primary echelon nodes in the pretracheal, paratracheal (also known as central compartment, level VI), and lower jugular and posterior neck regions (levels IV and VB). They drain secondarily into the nodes of the anterior-superior mediastinum (level VII) and less commonly into lower mediastinal nodes. The lymphatics emerging from the lateral aspect of the gland follow the middle thyroid vein and drain into paratracheal lymph nodes, as well as the jugular and posteroinferior lateral neck lymph nodes (levels II, III, IV, and VB). Lastly, the lymphatics from the superior aspect of the gland and the isthmus drain into the paratracheal lymph nodes, as well as the prelaryngeal and jugular lymph nodes, particularly to the midjugular nodes (level III).

Lymph node metastases from thyroid carcinoma tend to occur first in the prelaryngeal, para- and pretracheal nodes regardless of the location of the primary within the thyroid gland. In a prospective study using sentinel lymphoscintigraphy in papillary thyroid carcinoma (PTC), sentinel lymph nodes were located 83% to 88% of the time in level VI. This drainage pattern is so reliable that the prelaryngeal lymph node found anterior to the cricothyroid membrane is often referred to as the Delphian node, named after the Greek oracle Delphi, because an enlargement of this lymph node often predicts the presence of thyroid cancer. Although it is safe to consider levels VI (and VII) lymph nodes as the primary echelon of drainage for all thyroid cancers, skip metastases to the nodes of the lateral compartment of the neck can occur in as many as 22% of patients with thyroid cancer. Additionally, patients with superior thyroid tumors are at risk for skip metastases to lateral neck lymph nodes, most commonly level III.

This possibility of lateral-neck skip metastases further underlines the importance of rigorous preoperative screening for nodal metastases with preoperative high-definition ultrasound (US), including fine-needle aspiration (FNA) cytology for suspicious lymph nodes. Thyroglobulin washout measurements from lymph node biopsies can add value in cases where cytology is not conclusive. The 2015 American Thyroid Association Differentiated Thyroid Cancer Management Guidelines, hitherto referred to as 2015 ATA DTC [differentiated thyroid cancer] Guidelines, strongly recommended that all patients undergoing any thyroid surgery for malignant or suspicious nodules should have a neck US to evaluate central and especially lateral neck compartments (Recommendation). Furthermore, for DTC, a therapeutic lateral neck dissection should only be considered if biopsy-proven metastatic nodes are found in the lateral neck (Recommendation 37) or if the biopsy-proven positive node is ≥ 10 mm in the smallest dimension in a previously opeated neck (Recommendation 71). In patients with biopsy-proven lateral neck disease, a computed tomography (CT) scan of the head and neck region with contrast is generally also recommended. A CT with contrast can allow further lymph node mapping of the neck and provide the surgeon with an additional anatomic guide for the location of diseased node(s). Additionally, CT imaging can pick up retropharyngeal lymph nodes and superior mediastinal lymph nodes that cannot be seen on US.

Reported rates of metastases in the lateral neck compartments vary in the literature. Eskander et al. have however demonstrated with their systematic review and meta-analysis that levels III (71%) and IV (66%) had the highest frequency of metastasis, followed in order by levels II (53%) and V (25%). Involvement of lymph nodes in the submandibular triangle (level I) is uncommon, occurring mainly with regionally advanced and/or poorly differentiated disease and with involvement of nodes at other levels of the neck, particularly at level II.

Contralateral lymph node metastases occur in as many as 24% of patients, particularly in those patients with large tumors; those that involve the isthmus; with tumor recurrence ; or involving level VB lymph nodes. For small unifocal and unilateral primary tumors, the risk of contralateral metastases is low.

Risk Factors for Lymph Node Metastases

Several important factors in the patient’s history and tumor pathology are relevant to the rate of lateral neck metastases, including the patient’s age, gender, tumor size, presence of lymphovascular invasion, extrathyroidal extension, and histologic subtype. More specifically, men and younger patients (< 45 years old) are associated with significantly higher rates of locoregional metastases than women and older patients. Multifocal disease and large tumors (> 4 cm) in patients with well-differentiated thyroid carcinoma are also associated with increased risk for nodal involvement. Tumors that exhibit extrathyroidal extension or lymphovascular invasion can have more than two-fold or four-fold higher risk of lymph node metastases, respectively.

PTC is associated with the highest rate of lymph node metastases, ranging from 40% to 50%, and the mean incidence in children and adolescents is even higher, approximately 80%. A palpable lymph node metastasis may be the first clinical manifestation of a thyroid carcinoma in up to 30% of patients. The follicular variant of papillary carcinoma is associated with a lower risk of lymph node metastases than the conventional papillary variant and even less risk of lymph node metastasis if the tumor is fully encapsulated (see Chapter 23 , Noninvasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features). Follicular carcinoma has a propensity for hematogenous spread and a much lower risk of lymph node metastases (see Chapter 22 , Follicular Thyroid Cancer). Hürthle cell carcinoma has lower rates of nodal metastases than PTC, but higher than follicular thyroid carcinoma (FTC), occurring in approximately 15% of patients (see Chapter 25 , Hürthle Cell Tumors of the Thyroid). The tall-cell variant (TCV) of PTC is considered a more aggressive variant, with poor prognosis. Morris et al. undertook a matched-pair analysis, using the Surveillance, Epidemiology, and End Results (SEER) database, and they reported that rates of lymph node metastases of TCV did not significantly differ with conventional PTC.

Lymph Node Metasis and its Effect on Disease Outcome

The presence of lymph node metastasis in thyroid cancer is associated with an increased risk of locoregional recurrence and may be associated with decreased disease-specific survival, as suggested in several large studies. Mazzaferri and Jhiang studied 1355 patients with papillary and follicular cancer and followed them for 10 to 30 years. They reported that survival was significantly decreased in patients with lymph node metastases, and certain disease-associated deaths occurred 20 to 30 years after primary treatment. Podnos et al. completed a study of the SEER database in which they analyzed a cohort of 9904 patients with papillary thyroid carcinoma. Lymph node metastases, age > 45 years, distant metastasis, and large tumor size were significant predictors of overall survival on multivariate analysis. Overall survival at 14 years was 79% for patients with lymph node metastases and 82% for patients without ( p < 0.05). In a subsequent study of the SEER database, Zaydfudim et al. concluded that cervical lymph node metastases were independently associated with decreased overall survival, specifically in patients with follicular cancer or in patients with papillary cancer older than 45 years. Other studies have shown that the rate of regional recurrence is higher in patients with lymph node metastases, especially in patients with metastases in multiple lymph nodes or with extracapsular spread.

Management of the N0 Lateral Neck in Differentiated Thyroid Cancer

In the past, elective lateral neck dissection of levels II to V was considered for thyroid cancer. However, this practice is no longer recommended among contemporary high-volume thyroid surgeons for well-differentiated thyroid cancer. Lateral neck compartment dissection should only be performed for well-differentiated thyroid cancer in the presence of clear-cut lateral neck metastases. If there is an ultrasonographically suspicious lymph node in level III, IV, or VB of the lateral neck that remains indeterminate after attempt(s) at FNA biopsy, consideration may be given to excisional biopsy with frozen section evaluation of the lymph node at the time of thyroid surgery, with concurrent compartment-oriented lateral neck dissection if frozen section is positive for metastasis. Some surgeons will offer empiric lateral neck surgery without FNA confirmation based on clear-cut suspicious nodal finding on nodal US and/or CT mapping studies.