Background: In papillary thyroid cancer, the role of lymph node dissection remains controversial, and staging systems consider metastatic lymph nodes as a binary entity. The purpose of this study was to determine a threshold lymph node ratio (LNR) that impacted disease-specific mortality (DSM).
Methods: We utilized the surveillance, epidemiology, and end results (SEER) database to analyze adult patients who underwent thyroidectomy with lymph node dissection. A LNR (metastatic lymph nodes to total lymph nodes) was calculated after eliminating patients with less than three nodes collected. Kaplan-Meier estimates for DSM were plotted for LNRs and compared by the log rank test. The Cox proportional hazards model was used to evaluate LNR with other known clinical and pathologic determinants of prognosis.
Results: A total of 10,955 cases contained data on lymph nodes. Median follow-up time was 25 months (range 0-59 months), and the mean LNR was 0.28 ± 0.37. After comparing Kaplan-Meier survival estimates and overall DSM rates, we found that a LNR ≥0.42 best divided those with lymph node metastasis based on DSM (p < 0.01). Those with a LNR ≥0.42 experienced a DSM rate of 1.72 % while those with a LNR <0.42 had a DSM rate of 0.65 % (p < 0.01). In addition, patients with a LNR ≥0.42 experienced a 77 % higher DSM rate compared to those with metastatic lymph nodes as a whole. When considered with other known determinants of prognosis, we found that LNR was strongly associated with DSM (hazard ratio 4.33, 95 % confidence interval 1.68-11.18, p < 0.01).
Conclusions: LNR is a strong determinant of DSM, and a threshold LNR of 0.42 can be used to risk-stratify patients with metastatic lymph nodes.