A case report of the very rare intrathyroid thymic carcinoma

A 51-year-old male had an incidental finding of a mass in the right lobe of the thyroid gland during a chest CT scan. Subsequently, a thyroid gland ultrasonography with fine-needle aspiration was performed; cytological material was suspicious for malignancy. A total thyroidectomy was performed. Macroscopically, a solid, dense, white nodule (4,0 cm in greatest dimension) was identified on the surface of the right lobe of the thyroid gland. Microscopically, the tumor was composed of irregular nests and solid structures. It was unencapsulated, but well demarcated from surrounding thyroid tissue. Cells had scant cytoplasm, round to oval vesicular nuclei with prominent cherry-red nucleoli. Mitotic activity was low. Tumor had fibrotic stroma with dense lymphocytic infiltrate with germinal center formation. Focal comedo-type necrosis was present. The tumor did not have the papillary-like nuclear features, follicular architecture or nuclear pleomorphism characteristic of the more commonly encountered thyroid neoplasms e.g., papillary thyroid carcinoma and its variants, follicular thyroid carcinoma, anaplastic thyroid carcinoma. Since the tumor was on the surface of thyroid gland, a parathyroid gland or thymic remnant neoplasm was also considered. Based on the similarities in morphology, an intrathyroid thymic carcinoma was suspected. Immunohistochemically, the tumor was CD117, p63, CD5, BCL2 positive and TTF-1, thyroglobulin, calretinin negative. Ki67 proliferation index was 30%. Thus, immunohistochemical findings confirmed the morphological suspicion of intrathyroid thymic carcinoma and a very rare histological diagnosis was made.