Prognostic value of lymph node yield and metastatic lymph node ratio in medullary thyroid carcinoma

Introduction: Lymphadenectomy and thyroidectomy is standard treatment for medullary thyroid carcinoma (MTC), but the prognostic importance of the number of lymph nodes removed (lymph node yield, LNY) and the proportion of metastatic lymph nodes resected (metastatic lymph node ratio, MLNR) is unknown. We hypothesized that MTC survival is influenced by LNY and MLNR. Methods: Patients (N = 534) who underwent thyroidectomy with lymphadenectomy for MTC between 1988 and 2004 were identified in the Surveillance, Epidemiology, and End Results (SEER) database. The Kaplan-Meier method was used for univariate comparisons of survival for LNY and MLNR with a maximum follow-up of 12 years. Cox regression models adjusted for age, sex, extent of disease, tumor size, nodal status, LNY, and MLNR. Results: By univariate analysis, increasing LNY was associated with improved survival in all patients (P < 0.002) and node-positive patients (P < 0.001). In a multivariate analysis using LNY and MLNR as categorical variables, significant factors influencing survival included: age (P < 0.001), tumor size (P < 0.001), LNY (P = 0.007), and MLNR (P < 0.02); in node-negative patients: age (P = 0.002); in node-positive patients: age (P < 0.001), tumor size (P < 0.001), and LNY (P = 0.001). Using LNY and MLNR as continuous variables, significant factors influencing survival included: age (P < 0.001), tumor size (P < 0.001), and MLNR (P = 0.01); in node-negative patients: age (P < 0.001); in node-positive patients: age (P < 0.001) and tumor size (P < 0.001). Conclusion: In patients undergoing thyroidectomy and lymphadenectomy for MTC, LNY and MLNR predict poorer survival, but their impact on survival was limited to node-positive patients and was otherwise dominated by the effects of age and extent of disease.