Thyroid gland: a rare site of metastasis.

Purpose: Despite its rich vascularity, metastasis (MTS) to the thyroid tissue is unusual, ranging from 1 to 3%. This entity is not usually considered as differential diagnosis and is not included in the upfront approach in patients with thyroid nodules. Once diagnosed, treatment is controversial. The aim of this study was to evaluate diagnosis, treatment, and outcome at the end of follow-up in patients with a diagnosis of MTS to the thyroid.

Methods: A retrospective multicenter study was designed from 1985 to 2022; 29 patients with MTS to the thyroid gland were included in the analysis.

Results: Clinical presentation included the presence of a nodular goiter (65.5%), compression symptoms (17.2%), diffuse goiter (10.3%), and suspicious lymph nodes in the neck (7%). Primary tumor sites were: kidney (44.8%), breast (24.1%), lung (13.8%), neuroendocrine system (6.9%), colon (3.4%), cervix (3.4%), and ovary (3.4%). In 18/23 patients, suspicious ultrasound criteria for malignancy were described. Preoperative diagnosis was made in 23/27 patients by FNA and confirmed in 18 cases by immunohistochemistry. Seventeen patients underwent surgery. At the end of the follow-up, 19 patients had died of oncological disease, and six were alive (2/6 disease-free with isolated intrathyroidal MTS).

Conclusion: Renal carcinoma was the tumor that most frequently metastasized to the thyroid gland. Immunodiagnosis was a very useful tool for etiological confirmation. Patients with MTS to the thyroid gland as a unique site had a more favorable course compared to patients with multiple metastatic sites. Finally, outcomes and prognosis essentially depended on the biology of the primary tumor.