A Case of Carcinoma Showing Thymus-Like Differentiation (CASTLE) in the Thyroid

Carcinoma Showing Thymus-Like Differentiation (CASTLE) is a very rare malignant neoplasm of the thyroid, and this resembles lymphoepithelioma or squamous cell carcinoma of the thymus. It originates from ectopic thymic tissue or remnants of the branchial pouches. We recently experienced a case of CASTLE in the thyroid gland of a 61-year-old woman. She presented with an asymptomatic mass in the right thyroid gland and she was diagnosed with 'poorly differentiated carcinoma' of the thyroid by fine needle aspiration cytology (FNAC). Total thyroidectomy was performed for both diagnostic and therapeutic purposes. Histologic examination of the resected tumor showed that the tumor was lobulated with expanding fibrous bands, and it was infiltrated by lymphocytes and plasma cells. The tumor cells had oval, large vesicular nuclei and prominent nucleoli, and the immunohistochemical staining was positive for CD5 and bcl-2, so the patient was diagnosed with thyroid CASTLE. We report here on a case of CASTLE in the thyroid gland treated by surgery and external neck radiation therapy. (J Korean Endocr Soc 23:272~276, 2008)