Case Report of a Clear-Cell Variant of Follicular Thyroid Carcinoma

Clear-cell variants of follicular carcinoma are rare subtypes of thyroid cancer. There is no unified view of the histopathological features of clear cell variants, but follicular carcinomas composed predominantly of clear cells are distinguished from clear cell variants. In clinical practice, it is important to determine whether clear cell variants arise primarily from the thyroid gland or are thyroid metastases of other clear cell carcinomas, such as renal cell carcinoma. We present a case in which a patient with initially suspected anaplastic thyroid carcinoma due to a rapidly progressive anterior neck mass was diagnosed with a clear cell variant of follicular carcinoma after a tissue biopsy. The patient was treated with lenvatinib, then his performance status improved, and he was discharged from the hospital. On day 188 after discharge, a contrast-enhanced computed tomography (CECT) scan of the neck showed further shrinkage of the tumor. However, a CECT scan of the chest revealed multiple lung metastases. On day 233 after discharge, the patient developed severe pneumonia resulting from tracheal rupture due to intratumoral necrosis. It was difficult to decide whether lenvatinib should have been discontinued or reduced when lung metastasis appeared. It is necessary to accumulate additional cases to make informed decisions about continuing lenvatinib therapy.