19.1 Introduction

Cervical lymph node metastases are known to significantly correlate with both the persistence and the recurrence of thyroid cancer, and they may impact survival. ¹ Central neck dissection (CND) plays an important role in the management of thyroid malignancy. The primary treatment for locally advanced differentiated thyroid cancer (DTC) should consist of a total thyroidectomy, with both a therapeutic neck dissection and thyroid remnant ablation as indicated. ² Yet, because occult lymph node metastases are common in DTC, some have advocated elective CND (in the absence of any clinically or radiologically detectable nodal metastases) as part of routine surgery for DTC.

Nevertheless, the pertinent medical literature indicates that there is no standardization of this operation among surgeons. Boundaries and compartments are not well defined, and the operative reports often fail to mention whether these procedures were being performed in the presence or absence of gross lymph node metastases (clinically or radiographically apparent). These shortcomings prompted the convergence of a subgroup of thyroid cancer specialists under the auspices of the American Thyroid Association (ATA) to formulate a consensus statement on the anatomy and terminology pertinent to CND. ³ The group concluded that CND should consist of levels VI and VII of the neck and must contain the prelaryngeal, pretracheal, and at least one paratracheal nodal basin. The surgery should be designated as elective or therapeutic.

Clinically or radiographically apparent cervical lymph node metastases in the central neck should be treated with a therapeutic intent accomplished by a compartmental dissection. Although elective CND is advocated for medullary thyroid cancer (MTC) and other aggressive histologies, controversy still exists for its application to DTC. ⁴ This chapter discusses the technique for performing a CND and the considerations for when it should be performed.

19.2 Incidence and Prevalence

DTC, specifically papillary thyroid cancer (PTC), has a tendency for cervical lymphatic spread. Cervical metastases are found in 20 to 50% of patients, with the use of standard pathological techniques, and have been reported by some authors to occur in up to 90% of those examined for micrometastases. ^{5 , 6} Thyroid tumor cells spread through the lymphatic system in a sequential fashion, starting in the perithyroidal, pretracheal, paratracheal, and prelaryngeal lymph nodes of the central neck compartment, and then progress to the lymph nodes of the lateral cervical compartments and the superior mediastinum. ⁷

Although occult micrometastases in the lymph nodes of the central neck have been reported to occur in 31 to 62% of patients with PTC, the available evidence suggests that most remain dormant, rarely become clinically apparent, and are of little clinical significance. ^{8 , 9} The incidence of lymph node metastases in follicular thyroid cancer is 20 to 25% in the American population. In patients with MTC, clinically detectable cervical lymph node metastases are found in at least 50% of patients.

19.3 History, Physical Examination, and Preoperative Planning

Patients will usually present with a mass in the thyroid that has been detected by palpation or discovered incidentally on radiographic imaging for evaluation of other disease processes (e.g., carotid ultrasound, imaging of the cervical spine). A diagnosis is usually established by ultrasound-guided fine-needle aspiration (FNA) biopsy. Most patients with a thyroid malignancy, with or without central neck lymphadenopathy, are usually asymptomatic. Patients with larger or more aggressive tumors can present with hoarseness, dysphagia, and dyspnea.

A patient with suspected thyroid cancer should undergo a detailed examination of the thyroid gland and the cervical lymph node compartments. Cervical ultrasound is often the initial imaging modality employed in the assessment because it is readily accessible, inexpensive, and noninvasive. It is also an important tool for postoperative surveillance of patients with PTC. ^{10 , 11} High-resolution ultrasonography can detect cervical nodal metastasis in up to 20% of patients with PTC. These ultrasound findings may alter the planned surgical procedure in up to 39% of thyroid cancer patients. ² Pathological lymph nodes usually have concerning sonographic features that include a round shape, absent hilum, calcification, intranodal necrosis, reticulation, matting, and peripheral vascularity. ¹² A detailed cervical ultrasound to include nodal levels II through VI should be performed, ideally by a dedicated clinician, such as the thyroid endocrinologist, the operating surgeon, or a radiologist with a vested interest, to detect nonpalpable lymph node metastases in patients undergoing surgical evaluation for any thyroid cancer. ¹¹ However, ultrasound can miss as many as 50% of the involved lymph nodes in the central neck because the overlying thyroid gland may hinder adequate visualization. ¹³

In patients with suspected mediastinal disease or with bulky (clinically palpable, > 3 cm in size) cervical lymphadenopathy, cross-sectional imaging with computed tomography (CT), magnetic resonance imaging (MRI), and/or positron emission tomography (PET) should be considered. Though the sensitivities of these modalities for the screening and detection of cervical lymph node metastases can be relatively low (30–40%), ¹⁴ they can aid in the planning of a neck dissection and often identify pathological level VII lymph nodes within the superior mediastinum and parapharyngeal, retropharyngeal nodes that are not detected on cervical ultrasound or physical examination. ¹¹

A CT scan with iodinated contrast may be extremely helpful in evaluating the extent of cervical lymphadenopathy when there is gross nodal disease present, and can help define the extent of surgery necessary to plan clearance of all gross disease in the neck. ¹¹ CT with contrast may delay postoperative thyroid scanning and radioactive iodine (RAI) administration for 4 to 8 weeks, but in the scenario of bulky lymphadenopathy or concern for locally invasive disease, it appears justified because complete surgical resection of gross disease is of paramount importance for disease control. ¹⁵

19.4 Indications for Central Neck Dissection

There are several indications for therapeutic or elective CND. Patients with thyroid cancer and evidence of central neck lymphadenopathy confirmed by physical examination, radiologic imaging, or intraoperative inspection should undergo therapeutic CND. CND is also indicated in patients with recurrent laryngeal nerve (RLN) invasion or medullary thyroid cancer. Elective CND may be considered in select patients with T3 and T4 differentiated thyroid cancer cases or when there is suspicion of a more aggressive variant of thyroid cancer (including medullary thyroid cancer).

19.5 Contraindications

Just as there are times that CND may be indicated, there are also times where the potential morbidity of the procedure may outweigh the benefit. Surgeons should be very cautious when considering elective CND of the paratracheal nodal basin on the side of the only functioning RLN (i.e., when there is a preexisting vocal fold paralysis contralateral to the paratracheal nodal basin where the positive lymph node is located). Patients with stable recurrent or persistent disease that is away from vital structures, such as the trachea, may be considered for observation rather than surgery. Finally, CND may not be appropriate in patients with known systemic metastases that are progressive and outpacing the recurrent or persistent central nodal metastasis.

19.6 Elective versus Therapeutic Central Neck Dissection

A therapeutic CND should be performed for patients with DTC and pathological lymph node involvement noted on preoperative clinical exam or imaging, or during intraoperative inspection. ² The goal of removing these lymph nodes is to aid in local control, prevent recurrence, and perhaps improve survival.

The role of elective CND remains a contentious issue regarding its benefits and risks. An essential component of any discussion about the need for lymphadenectomy is whether patients derive any additional benefit from having a CND with total thyroidectomy and whether this can be done without significantly increasing the morbidity of the operation. Because microscopic nodal disease is rarely of clinical importance, many authors argue that elective CND of microscopic lymph node metastases that are not clinically identifiable at the time of surgery may not improve long-term outcome and could subject patients to more risk than benefit. ² Also, if removal of subclinical metastases alone were an indication for surgery without an appreciation for the clinical significance of this disease (which certainly is debatable), then the same should theoretically apply to the lateral neck. Opponents also fear that, if elective CND were universally adopted by all surgeons performing thyroidectomy, the risk of parathyroid and nerve injury may increase in the absence of significant oncological benefit to the patients. ^{4 , 5 , 6}

Nonetheless, no consensus yet exists regarding the addition of CND for clinically node-negative patients with DTC. To date, no study has demonstrated significantly reduced recurrence or mortality rates with elective CND. A recent meta-analysis found no difference in the recurrence rates between those treated with or without elective CND at the time of thyroidectomy. ⁴ A different study suggested that when elective CND is performed by experienced surgeons it may be associated with a lower risk of disease recurrence, and that the number of patients that would need to be treated to prevent a single recurrence is 31. ¹⁶ In light of these discordant findings, the rationale for routine elective CND to prevent either recurrence or death is questionable in DTC. Conversely, there are strategic uses for elective CND that may have justification. These include patients with higher-risk tumors, such as T3 or T4 lesions, or those exhibiting extrathyroidal extension, or more aggressive histologies, such as diffuse sclerosing, insular, or poorly differentiated tumors. Elective central neck dissection is also recommended for medullary thyroid cancer. The pros and cons of therapeutic and elective CND are outlined in Table 19.1.