Clinicopathologic significance of intrathyroidal epithelial thymoma/carcinoma showing thymus-like differentiation: a collaborative study with Member Institutes of The Japanese Society of Thyroid Surgery.

Intrathyroidal epithelial thymoma (ITET)/carcinoma showing thymus-like differentiation (CASTLE) is a rare malignant tumor having histopathologic features similar to those of squamous cell carcinoma of the thyroid and other organs and a more favorable prognosis. It is thought to originate from ectopic thymic tissue or embryonic thymic rest in or adjacent to the thyroid. We investigated clinicopathologic features of 25 cases of ITET/CASTLE. The sensitivity and specificity of the pathologic diagnosis of ITET/CASTLE by immunohistochemical staining with CD5, a marker of carcinoma of thymic origin, were 82% and 100%, respectively. The 5- and 10-year cause-specific survival rates were 90% and 82%, respectively. Nodal metastasis and tumor extension predict a worse prognosis. Of 22 patients who had curative surgery, 10 (45%) underwent adjuvant radiation therapy, and no locoregional recurrence was seen in any of them. This is the first study demonstrating the survival curve for patients with ITET/CASTLE. Our findings suggest that curative resection followed by radiation therapy may effectively prevent locoregional recurrence and CD5 immunostaining is useful for diagnosing ITET/CASTLE.