Synchronous diagnosis of metastatic cancer to the thyroid is associated with poor prognosis.

The incidence and histopathological characteristics of metastatic cancers to the thyroid (MCT) are different in various geographic areas. The aim of this study was to elucidate the clinical features of MCT, including histocytological diagnosis and therapeutic outcomes. A retrospective analysis of patients with thyroid cancer treated and followed up at the Chang Gung Medical Center in Linkou was performed. Among 3957 patients with thyroid cancer, a total of 56 patients with MCT were evaluated. Of them, 47 patients (83.9 %) were diagnosed with malignancy or suspected malignancy via fine needle aspiration cytology of the thyroid. Synchronous primary cancers were diagnosed in 44 of the patients with MCT. Of the MCT, metastasis of lung cancer to the thyroid was the leading category. Other primary sites of MCT were the head and neck, gastrointestinal tract, kidneys, breast, cervix, and unknown primary site. The mean 5-, 10-, 20-, and 60-month survival rates were 46.4, 32.1, 21.4, and 7.9 % for the patients. Patients with metachronous thyroid carcinoma had significantly better survival than patients with synchronous cancer. In conclusions, the incidence of MCT in patients with thyroid cancer is low; however, the prognosis of patients with MCT is poor, especially in patients diagnosed with synchronous primary tumors. In this study, MCT commonly originated in organs located near the thyroid, such as the lungs, head, and neck. Close monitoring of these malignancies may improve the prognosis of patients with MCT in the future.