The Association of Number of Retrieved Lymph Nodes with Oncologic Outcomes in Patients with Papillary Thyroid Cancer with Lateral Cervical Nodal Metastasis

Background: The number of retrieved lymph nodes (rLN) is an important surgical metric associated with survival in cancer. However, its association with oncological outcomes in patients with papillary thyroid cancer (PTC) with lateral lymph node (LN) metastasis (N1b disease) has not been investigated. Methods: This retrospective cohort study included patients with PTC and N1b disease (N = 1003) who underwent lateral neck dissection and categorized them by total number of rLNs. We investigated the association of the categories with recurrence or cancer-specific mortality (CSM) using multivariable Cox proportional hazard models. The optimal number of rLNs associated with low recurrence rate or CSM were identified using restricted cubic spline analysis. Survival analyses across subgroups were performed according to adequate (the estimated total number of rLN at which CSM appeared to be lowest) and inadequate groups. Results: At initial surgery, the median total number of rLNs and positive number of LNs was 36.0 (25.0-49.0) and 9.0 (5.0-14.0), respectively. During a median follow-up of 90.0 (73.0-129.0) months, recurrence and CSM were detected in 165 and 30 patients, respectively. A higher total number of rLNs was inversely associated with the adjusted hazard ratio (aHR) for recurrence (0.50, 0.51, 0.33, and 0.28; p for trend = 0.001) and CSM (0.33, 0.13, 0.02, and 0.04; p for trend 0.002). The estimated total number of rLNs associated with the lowest recurrence rate and CSM were 32 and 33, respectively. Compared with the inadequate rLNs group, the adequate rLNs group (≥33 total rLNs) showed better prognosis in the entire group (aHR 0.51 [0.35-0.73], p < 0.001; recurrence) (aHR 0.14 [0.04-0.47], p = 0.001; CSM) and various subgroups. Conclusions: rLNs are associated with recurrence and CSM in patients with PTC and N1b disease, with outcomes appearing to improve at approximately 32-33. This finding suggests that rLNs may serve as a potential quality indicator for neck dissection in PTC. Our finding is mostly applicable to more aggressive disease, and further prospective confirmatory research is warranted.