Metastasis to the thyroid gland: a single institution 16-year experience.

Aims Metastasis to the thyroid gland is a rare occurrence that may pose a diagnostic challenge. In this study, we aimed to report the clinicopathologic features, immunoprofile, molecular alterations, and outcome of 30 patients treated at our center from 2003 to 2019. Results The most common site of the primary tumor was the kidney, followed by the lung, lower gastrointestinal tract, and breast. In seven (23%) patients, the thyroid metastases were resected prior to the diagnosis of the primary tumors. Six patients (20%) had thyroid as the sole metastatic site. Three (10%) patients harbored tumor-to-tumor metastasis. 71% had unilateral unifocal thyroid mass, which might be mistaken for primary thyroid tumors. Among the 13 cases that were initially diagnosed at an outside hospital, 4 (31%) were misinterpreted as a thyroid primary. An immunohistochemical panel of thyroid follicular cell markers was most useful to differentiate primary thyroid tumors from metastasis. Molecularly, the metastasis showed alterations characteristic of the primary tumor, which may be helpful in establishing diagnosis and primary site. Although the prognosis was poor with a 5-year disease specific survival of 58%, a long-term cure was possible in cases with oligometastasis treated successfully with surgery. Conclusions Metastasis to the thyroid gland is an uncommon phenomenon with an incidence of 0.36% in all thyroid malignancies. It may present as a solitary thyroid mass before the discovery of the primary tumor, posing a diagnostic challenge. Although the overall prognosis is poor, a subset of patients with oligometastasis can be managed surgically.