Carcinoma showing thymus-like differentiation of the thyroid (CASTLE): a comparative study: evidence of thymic differentiation and solid cell nest origin.

Carcinoma showing thymus-like differentiation (CASTLE) is a rare intrathyroidal neoplasm, a member of a tumor family probably arising from ectopic thymus or branchial pouch remnants. Thyroid solid cell nests (SCNs) may also be derived from branchial pouch remnants. SCNs express p63, carcinoembryonic antigen (CEA), and high molecular weight keratin (HMWK). To determine whether CASTLE and SCNs derive from similar embryologic origins/lines of differentiation, and to better differentiate CASTLE from other thyroid neoplasms, we compared p63, CD5, HMWK, and CEA staining of CASTLE and SCNs with other thyroid and thymic lesions. Seven CASTLE, 11 SCNs, 10 thymic carcinoma, 11 invasive thymoma, 12 thymoma, 28 papillary thyroid carcinoma, 4 thyroid squamous cell carcinoma, 2 childhood sclerosing carcinoma, 4 follicular adenoma, 6 follicular carcinoma, 4 poorly differentiated carcinoma, and 20 lymphocytic thyroiditis cases were analyzed. In normal thyroid, only SCNs stained for p63, HMWK, and CEA. The only CD5-positive cells in normal thyroid were T cells. Thymomas and normal thymus stained similarly to SCNs. All CASTLE and thymic carcinomas exhibited diffuse p63 and HMWK staining and all CASTLE cases and the majority of thymic carcinomas were positive for CEA and CD5. In contrast, none of the other thyroid neoplasms examined exhibited consistent staining for all 4 markers studied. These findings provide further evidence that CASTLE is distinct from other thyroid neoplasms, is probably of thymic origin, and may arise from branchial pouch remnants, the thyroid SCNs. Moreover CD5, HMWK, CEA and p63 can be used to help distinguish CASTLE from other thyroid neoplasms.