Intrathyroidal epithelial thymoma (ITET)/carcinoma showing thymus-like differentiation (CASTLE) exhibits CD5 immunoreactivity: new evidence for thymic differentiation

Aims: Cases of intrathyroidal epithelial thymoma (ITET)/carcinoma showing thymus-like differentiation (CASTLE) were examined for CD5 immunoreactivity, a feature of true thymic carcinoma, but not other thymic epithelial neoplasms or carcinomas of other sites, ITET/CASTLE, a rare, low-grade malignant neoplasm arising within the thyroid gland which resembles lymphoepithelioma-like and squamous cell carcinoma of the thymus, is postulated to arise from remnants of branchial pouch capable of thymic differentiation, but thymic differentiation in this neoplasm remains unproven. Methods and results: The largest published series of cases of ITET/CASTLE was examined for CD5 immunoreactivity using an anti-CD5 antibody reactive in fixed, paraffin-embedded tissue with microwave antigen retrieval, Neoplastic cells in all five cases of ITET/CASTLE studied were immunoreactive for CD5, including foci of tumour metastatic to lymph node and lung, In contrast, none of five cases of thyroid carcinoma with squamous differentiation was immunoreactive for CD5, A minority of cases of typical thyroid carcinomas showed some weak immunoreactivity for CD5, Other carcinomas of the head and neck were nonimmunoreactive for CD5. Conclusions: CD5 immunoreactivity in ITET/CASTLE is new evidence in support of thymic differentiation in this neoplasm.