Clear Cell Follicular Adenoma of the Thyroid – A Challenge in Intra-operative Diagnostics

Pure clear cell lesions of the thyroid gland are rare and prone to pose severe diagnostic challenges. We report the case of a 61-year-old man with a clear cell adenoma of the thyroid. The patient presented with a hypoechoic thyroid nodule. Fine needle aspiration cytology rendered the primary diagnosis of a follicular neoplasia, and right thyroid lobectomy was performed. By intra-operative frozen section, the diagnosis given was clear cell lesion of unknown malignant potential. Based on the light microscopic findings and the immunohistochemical profile, the lesion was diagnosed as clear cell follicular adenoma of the thyroid. A follicular thyroid lesion presenting with clear cell changes in fine needle aspiration cytology or intra-operative frozen section consultation constitutes a diagnostic challenge to every surgical pathologist. As immunohistochemistry of cytologic specimens is hampered by several methodological problems, any thyroid lesion with clear cell features warrants further histologic assessment to render the correct diagnosis.