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

Metastasis to the thyroid gland is a rare occurrence that may pose a diagnostic challenge. In this study, we aimed to report the clinicopathological features, immunoprofile, molecular alterations and outcome of 30 patients treated at our centre from 2003 to 2019. The most common site of the primary tumour 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 tumours. Six patients (20%) had thyroid as the sole metastatic site. Three (10%) patients harboured tumour-to-tumour metastasis; 71% had a unilateral unifocal thyroid mass, which might be mistaken for a primary thyroid tumour. Among the 13 cases that were initially diagnosed at an outside hospital, four (31%) were misinterpreted as a thyroid primary. An immunohistochemical panel of thyroid follicular cell markers was most useful to differentiate primary thyroid tumours from metastasis. Molecularly, the metastasis showed alterations characteristic of the primary tumour, which may be helpful in establishing the 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 successfully treated with surgery. 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 tumour, posing a diagnostic challenge. Although the overall prognosis is poor, a subset of patients with oligometastasis can be managed surgically.

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