**Table 3.**

*Classification of neck dissections.*

#### **2.2 Therapeutic neck dissection**

Therapeutic neck dissection is performed when patients have preoperatively confirmed nodal metastasis (cN+). It can be performed during initial thyroidectomy or as a staged secondary procedure after initial thyroidectomy.

#### *2.2.1 When nodal metastasis was detected in the central compartment preoperatively*

For DTC with preoperatively confirmed central compartment nodal metastasis, therapeutic central neck dissection should be performed bilaterally. This should extend superiorly from the hyoid bone to the innominate artery inferiorly. It is laterally bound by bilateral carotid arteries, anteriorly bound by the superficial layer of the deep cervical fascia, and posteriorly bound by the deep layer of the deep cervical fascia [12]. A compartment-oriented lymphadenectomy with the removal of all fibroadipose tissues within the compartment while identifying and preserving the ipsilateral recurrent laryngeal nerve (RLN), superior, and inferior parathyroid glands should be performed. In the past, non-anatomical nodal dissection ("berry picking") was performed but studies have shown a significantly greater recurrences rate (100% vs. 9%) with non-anatomical dissection versus compartment-oriented dissection, and similar surgical morbidities [60].

The most common morbidities of central neck dissection are transient hypocalcaemia (4–60%) and transient RLN injury (0–5%) but permanent hypocalcaemia and permanent RLN injury can occur in up to 15 and 12% of patients, respectively [61]. Although isolated studies demonstrated no additional complication risk from extra central neck dissection, most series had reported higher rates of complication when central neck dissection was performed concurrently with thyroidectomy and the outcomes were associated with surgeon's experience [62–65].

#### *2.2.2 When nodal metastasis was detected in the lateral compartment preoperatively*

For DTC with preoperatively confirmed lateral compartment nodal disease, the chance of concomitant central compartment disease is high. Therefore, patients should undergo both therapeutic lateral and concomitant central neck dissection.

The extent of therapeutic lateral neck dissection is not well agreed upon. While the consensus statement from the ATA recommends the removal of levels IIA, III, IV, and VB in a comprehensive therapeutic neck dissection, other authors routinely recommend a full dissection of levels II−V [11]. In DTC, the rate of nodal metastasis at the various lateral neck levels differs widely. The incidence of nodal metastasis at level III (62–67%) and level IV (50–67%) were significantly higher than that at level II (42–56%) and level V (29–40%) [21, 66–68]. A further distinction of sublevels IIA/B and VA/B were reported. Farrag et al. stated that level IIB rarely had nodal metastasis (8.5%) while all level V metastases were within level VB [68]. Others reported that level IIB nodal metastasis was exclusively accompanied by level IIA nodal metastasis [68, 69]. To date, no randomized controlled trials were published to determine the most appropriate operative extent. But available evidence supports a selective approach where level IIB and VA are only dissected if there is preoperative or intraoperative suspicion of level II or level V involvement. There is an added advantage that avoiding routine level IIB and VA dissection can minimize the risk of spinal accessory nerve injury.

Only a few studies have dealt with the surgical morbidities of lateral neck dissection. Two key complications after lateral neck dissection are lymphatic leakage secondary to thoracic duct damage (0.5–8%), and spinal accessory nerve injury (25−50%) resulting in shoulder dysfunction [27]. Other less common nerve injuries can be related to greater auricular nerve (48%), cervical plexus, sympathetic trunk (5%), and phrenic, hypoglossal, and vagal nerves [28, 63].

#### **2.3 Prophylactic central neck dissection: why and who?**

The rationale for prophylactic neck dissection is based on the fact that occult nodal metastasis is common. Despite comprehensive preoperative imaging, there is clinically node-negative (cN0) patients who are found to have unexpected nodal metastasis (pN+) on pathology. The incidence of this occult nodal disease was reported in up to 54% of patients who underwent elective bilateral central neck dissection during total thyroidectomy for DTC [14]. About 50% of which were a bilateral occult nodal disease. Prophylactic neck dissection allows accurate disease staging, improves postoperative risk stratification, and improves serum thyroglobulin levels facilitating postoperative surveillance. Prophylactic neck dissection at the initial thyroid surgery may additionally help to avoid reoperation in the future.

The evidence supporting routine prophylactic central neck dissection is controversial. While some earlier studies reported that prophylactic central neck dissection could reduce the risk of nodal recurrence and cancer-specific survival [29], others did not show such benefit. Aggregating these heterogenous nonrandomized studies, a meta-analysis showed that prophylactic central neck dissection had a lower risk of locoregional recurrence (risk ratio 0.66) than those without neck dissection [30]. However, prophylactic central neck dissection was associated with higher rates of overall morbidity, especially transient hypoparathyroidism. To date, three randomized trials have been published on prophylactic central neck dissection and all failed to show improvement in oncological outcomes or recurrence-free survival [31, 32, 57]. While it was shown that the prevalence of operative morbidities was similar in the group with prophylactic central neck dissection, all these randomized trials were underpowered to demonstrate the difference in survival outcomes and morbidity rates. To this end, ATA examined the feasibility of a multi-institutional prospective randomized controlled trial and concluded that the sample size required would be prohibitively large (>5800) given the low rate of disease recurrence and operative morbidities [33].

Both the ATA guidelines and the American Association of Endocrine Surgeons guidelines recommend a selective approach based on assessment of a patient's risk factors [59, 70]. For patients with T1/2 tumor, the risks of central nodal metastasis are relatively small, and thus prophylactic central neck is not recommended. Patients with T3/4 tumor, or extrathyroidal extension, or BRAF mutation may be considered at higher risk, and option of prophylactic central neck dissection should be considered. The British Thyroid Association on the other hand recommended personalized decision-making for prophylactic central neck dissection on the basis of one or more high-risk factors (adverse histological subtype, age ≥ 45 years, multifocal, tumors >4 cm, extra-thyroidal extension). This highlights the variability and uncertainty in this aspect of management of DTC.
