A Case of Intrathyroid Thymic Carcinoma Coexisting with Papillary Thyroid Carcinoma

Intrathyroid thymic carcinoma (ITTC) is a rare malignant tumor of the thyroid that histologically resembles thymic carcinoma.A 74-year-old man presented with a two-month history of neck discomfort.Imaging revealed a 2.9 cm mass in the left thyroid lobe, and cytology suggested a follicular cell-derived malignancy.The patient underwent total thyroidectomy with central neck dissection.Pathologic examination confirmed ITTC with extrathyroidal extension, strap muscle invasion, lymphovascular and perineural invasion, and a positive surgical margin, while no lymph node metastasis was noted (pT3bN0).Immunohistochemistry showed positivity for CD5, GLUT-1, and c-kit, confirming thymic differentiation.An incidental 0.2 cm papillary thyroid microcarcinoma was also identified.Postoperatively, concurrent chemoradiation therapy (60 Gy/30 fractions with etoposide and cisplatin) achieved complete remission.This case emphasizes the diagnostic importance of immunohistochemistry and suggests that complete surgical resection followed by adjuvant therapy can achieve favorable outcomes in ITTC.