Thymic carcinoma involving the thyroid gland: report of two cases.

Two new cases of thymic carcinoma involving the thyroid gland and manifesting as thyroid tumors are presented. One tumor occurred within the thyroid gland and the other totally involved the thyroid gland. Microscopically they showed lobular architecture separated by fibrous bands and consisted of solid islands of squamoid to spindled epithelial cells with whorl formation resembling Hassall's corpuscle and variable amounts of lymphocytes. The squamoid to spindled epithelial cells were positive for cytokeratin but negative for thyroglobulin and calcitonin and the majority of lymphocytes showed a T-cell phenotype. One of the two cases had been diagnosed initially as squamous cell carcinoma of the thyroid. This type of thyroid carcinoma is relatively rare, but it should be differentiated from other types of thyroid carcinomas, especially squamous cell carcinoma, medullary carcinoma, or anaplastic carcinoma of the thyroid.