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

Abstract

Thyroid carcinoma showing thymus-like differentiation (CASTLE) is a rare histological type of malignant tumor of the thyroid gland and similar to thymic epithelial tumors. Thyroid CASTLE accounts for 0.075% of all malignant thyroid tumors, and preoperative diagnosis is difficult. We report the case of a 64-year-old woman with thyroid CASTLE. An elastic hard nodule was palpated on the right anterior neck. Blood tests showed no abnormalities, and thyroid function and parathyroid function were normal. Computed tomography showed an iso-absorbing nodular shadow measuring 16 mm located caudal to the right lobe of the thyroid gland. Neck echography showed a nodule with indistinct borders and slightly irregular margins on the caudal aspect of the right lobe of the thyroid gland. It was impossible to make a diagnosis, but since malignancy could not be ruled out, we decided to perform diagnostic and therapeutic surgery consisting of resection of the right lobe of the thyroid gland and ipsilateral lymph node dissection in the central region of the neck. A poorly differentiated 16-mm carcinoma was found at the inferior pole of the thyroid gland. Immunostaining was positive for CD5, and a diagnosis of thyroid CASTLE was made. If a tumor of the thyroid gland lacks the features of more common thyroid tumors such as papillary carcinoma or follicular tumor and is localized caudally to the inferior pole of the thyroid gland, thyroid CASTLE should be included in the differential diagnosis.