Cutaneous metastases of follicular thyroid carcinoma: a report of four cases and a review of the literature.

Cutaneous metastasis of follicular carcinoma of the thyroid is very rare, and when it occurs, can exhibit a variety of histologic appearances. The 4 cases presented here were identified from the surgical pathology files of the James Homer Wright Laboratories of Pathology at the Massachusetts General Hospital (MGH). The cases consisted of 4 patients, 3 men and 1 woman, aged 52 to 75 years, with cutaneous metastasis of follicular thyroid carcinoma. The tumors include a conventional follicular carcinoma, a follicular carcinoma with anaplastic transformation following initial metastasis, the first reported cutaneous metastases of a follicular carcinoma with oncocytic features (Hürthle cell carcinoma), and a follicular carcinoma with a prominent insular carcinoma component. All 4 tumors were widely invasive within the thyroid gland. Sites of dermal metastases included a post-thyroidectomy scar, scalp, and sacral skin. Three metastases retained the morphologic and immunocytochemical features of the primary thyroid tumors. However, in one case there was high-grade transformation to anaplastic carcinoma following treatment of a sacral metastasis with accompanying loss of the characteristic immunophenotype of follicular thyroid carcinoma. Awareness of the varied morphologies of metastatic follicular thyroid carcinoma to the skin may prompt immunohistochemical analysis and the request for a complete clinical history, ultimately preventing misdiagnosis.