Therapeutic Outcome of Second Primary Malignancies in Patients with Well-Differentiated Thyroid Cancer

Background. The aims of this study were to analyze the clinical characteristics of SPM in patients with well-differentiated thyroid cancer and to determine the long-term prognosis in patients with double malignancies. Materials and Methods. We retrospectively analyzed 2,864 patients with well-differentiated thyroid cancer and a mean age of 44.0 ± 14.4 years. Of these, 200 (7.0%) were diagnosed with SPM, 115 of which were diagnosed with metachronous SPM. Results. Of 2,864 patients, 163 (5.7%) patients died of thyroid cancer and 301 (10.5%) died of any cause by the end of the follow-up period. Multivariate analysis identified age, SPM, external radiotherapy, TNM stage, and postoperative serum Tg level to be factors independently associated with decreased survival. Of 200 patients with SPM, 74 (37.0%) died. In comparison to the anachronous and synchronous groups, the metachronous SPM group had a higher mean age; more advanced tumor, node, and metastasis stage; lower remission rate; higher postoperative radioactive iodide (¹³¹I) accumulated dose; a higher proportion of patients who underwent external radiotherapy; and higher thyroid cancer and total mortality rates. Conclusions. Patients with well-differentiated thyroid carcinoma and metachronous SPM had worse prognoses compared to patients without SPM.