Anatomical Patterns of Nodal Spread in Unilateral Papillary and Medullary Thyroid Cancer.

Background: Skip metastases, node metastases in the lateral neck sparing the ipsilateral central neck, challenge the current concept of central-to-lateral lymphatic spread. This study sought to delineate patterns of central and lateral neck involvement in unilateral papillary thyroid cancer (PTC) and medullary thyroid cancer (MTC). Methods: This was a retrospective correlative analysis of nodal patterns in surgical specimens from patients with unilateral PTC or MTC who had undergone thyroidectomy with at least ipsilateral central neck dissection between November 1994 and January 2024 at a tertiary referral center. Results: Included were 833 patients with unilateral PTC and 640 patients with unilateral MTC. Simultaneous presence or absence of node metastases was noted in ipsilateral central and lateral neck compartments in 76.6-78.1% of patients with PTC (both node positive in 27.0-54.7% and both node negative in 23.4-49.6%) and 77.3-80.0% of patients with MTC (both node positive in 26.6-33.2% and both node negative in 44.1-53.4%). Only one ipsilateral neck compartment was node positive in 21.9-23.4% of patients with PTC and 20.0-22.7% of patients with MTC. The ipsilateral central, but not the ipsilateral lateral compartment, was node positive in 8.8-16.9% with PTC and 8.6-8.8% of patients with MTC, whereas the ipsilateral lateral, but not the ipsilateral central compartment, was node positive in 6.5-13.1% with PTC and 11.3-14.1% with MTC. Ipsilateral lateral neck involvement sparing the ipsilateral central neck was 1.5-2 times more frequent in patients with node positive MTC than patients with node positive PTC (24.2-25.2% vs. 12.9-17.1%). Greater numbers of node metastases in the ipsilateral central neck compartment were associated with more frequent involvement of the ipsilateral lateral, contralateral central, and contralateral lateral neck compartments. Thyroid tumor diameter intensified nodal spread without changing nodal spread patterns. Conclusions: These histopathological findings, which need to be interpreted in light of the respective tumor biology, offer an unprecedented glimpse at the metastatic patterns of unilateral PTC and MTC. Customizing neck dissection to the patterns of nodal spread, considering operative status (initial vs. reoperative surgery) and experience with neck dissection, may require more frequent concomitant dissections of ipsilateral central and ipsilateral lateral neck compartments.