Features of mixed medullary thyroid tumors: An NCDB analysis of clinicopathologic characteristics and survival

Background: Mixed medullary-papillary thyroid carcinoma (MMPTC) and mixed medullary-follicular thyroid carcinoma (MMFTC) are rare variants with little known regarding behavior and prognosis. Methods: Using the National Cancer Database (NCDB), demographics, clinicopathologic features, treatment, and overall survival (OS) from patients with MMPTC and MMFTC were compared to more prevalent subtypes. Results: There were 296,101 patients: 421 MMPTC (0.14%), 133 MMFTC (0.04%), 263,140 PTC (88.87%), 24,208 FTC (8.18%) and 8,199 MTC (2.77%). Compared to PTC, MMPTC and MMFTC patients were older (p < 0.001) with a higher Charlson-Deyo comorbidity index (p < 0.001). Mixed tumors exhibited lower rates of nodal disease but more distant metastases compared to PTC (p < 0.001). MMPTC demonstrated lower estimated 10year OS than PTC and FTC (76.04%vs 89.04% and 81.95%,p < 0.001), yet higher than MTC (70.29%,p < 0.001). MMFTC had a worse OS compared to all groups (63.32%,p < 0.001). Conclusion: Patients with MMFTC had significantly worse OS compared to DTC, portending a worse prognosis.