Level I constitutes lymph nodes above the anterior and posterior bellies of the digastric muscle cephalad to the hyoid bone and inferior to the inferior border of the mandible and includes the submental group of nodes. For Levels II–IV , the posterior border is the posterior edge of the sternomastoid muscle, and the anterior border is the laryngeal complex. Level II (upper jugular region) extends from the skull base and spinal accessory nerve superiorly to the hyoid bone inferiorly and is subdivided into IIA below the spinal accessory nerve and IIB above the spinal accessory nerve. Level III (mid-jugular region) extends from the hyoid superiorly to the cricoid cartilage inferiorly. Level IV (lower jugular region) extends from the cricoid cartilage down to the clavicle. Level V is a triangular region bounded anteriorly by the posterior edge of the sternocleidomastoid muscle (SCM) and posteriorly by the anterior edge of the trapezius muscle. It is subdivided into the region 5A above the level of the cricoid cartilage and 5B (supraclavicular region) below the level of the cricoid cartilage. Level VI (central compartment) is composed of four subcompartments: (1) prelaryngeal (Delphian nodal group), (2) pretracheal, (3) left paratracheal, and (4) right paratracheal. It extends from the hyoid bone superiorly to the clavicle. A Level VII or superior mediastinal group has also been described which appears to be encompassed in the lower part of the central neck dissection group described by ATA [2]. Typically standard neck dissection for papillary carcinoma of the thyroid excludes Levels I, IIB, and V due to the low prevalence of disease in these subareas. An exception to this is a patient with bulky lateral neck nodal disease where these areas may require dissection. In addition to the lymph node levels described above, many classification systems pertaining to the cervical lymph nodes have been published, including Union for International Cancer Control (UICC) classification, American Head and Neck Society (AHNS) classification, Japanese classification and the compartment classification (German classification) [3–6]. Cunnane et al. recently published a thyroid-specific, multidisciplinary, clinically validated nodal classification system that divides the neck into multiple stand-alone surgically distinct, side-specific compartments [7]. Distribution of nodal disease likely varies between primary presentation and recurrent disease. A recent study that mapped nodal distribution in PTC reported that at the first presentation, nodal disease is concentrated in the central neck, while in recurrent disease, this pattern is reversed with the nodes b eing more focused in the lateral neck [8] (Fig. 8.2). The same study reported that with recurrent nodal disease, ectopic nodal location outside of the typical central and lateral neck regions (retropharyngeal, sublingual region, subcutaneous location, axilla, and chest wall) occurred in almost 10% of patients, suggesting that axial imaging should be performed in patients with recurrent nodal disease [8].

Nodal subgroups can be further classified into the lateral (Levels II–V) or central (Levels VI ± VII) neck which correlates with the current prevailing philosophy of a “compartment-oriented dissection” in surgical management of nodal disease. Lateral neck dissection for thyroid carcinoma generally encompasses Levels II, III, and IV, and central neck dissection encompasses prelaryngeal, pretracheal, and at least one paratracheal r egion (Fig. 8.3). Note that, occasionally, especially in patients with aggressive subtypes of thyroid cancer, patients may present with nodal metastases which are outside of these known neck regions. However, the vast majority of patients with well-differentiated nodal metastasis can be described with this terminology.