Carcinoma Showing Thymus-Like Differentiation (CASTLE): A Rare Tumor of the Thyroid

Abstract

Carcinoma showing thymus-like differentiation (CASTLE) is a rare neoplasm of the thyroid or the adjacent tissues in the neck. It was first described by Miyauchi et al. in 1985 as an intrathyroidal epithelial thymoma. In 1991 Chan and Rosai classified these tumors into four types including CASTLE. World Health Organization (WHO) declared it as an independent clinicopathologic entity in 2004. The tumor arises from ectopic thymus tissue or remnants of branchial pouch. Both sexes are affected similarly with a slight female dominance. It is usually encountered in the fourth and fifth decades of life. It does not have specific symptoms or radiologic findings which makes preoperative diagnosis difficult. It has a higher tendency to be located in the lower poles of thyroid lobes. Immunohistochemistry helps differentiate it from other malignant neoplasms, CD5 being an important marker. The tumor is negative for thyroid specific markers as thyroglobulin, TTF-1 or calcitonin. Surgery is considered the mainstream therapy. Radiotherapy may be reserved for gross disease or recurrence. The role of chemotherapy is unclear. The prognosis of CASTLE is favourable.