RESUMO
Carcinoma showing thymus-like differentiation (CASTLE) is a very rare malignant neoplasm of the thyroid, which 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 65-year-old woman. The patient presented with a non-tender mass in the right thyroid gland and dyspnea and coughing upon bending. The patient was diagnosed with ‘cystic change of adenomatous goiter’ of the thyroid by fine needle aspiration cytology. Right thyroidectomy was performed because of nodular hyperplasia on frozen biopsy. Histologic examination of the resected tumor showed that the tumor was lobulated and expansive growth pattern, with fibrous septa dividing the tumor and infiltrated by lymphocytes and plasma cells. Tumor cells possessed oval, large vesicular nuclei and prominent nucleoli, and the immunohistochemical staining was positive for CD5. The patient was diagnosed with thyroid CASTLE. We performed complete thyroidectomy. There has been no local regional recurrence.