Collision tumor of poorly differentiated thyroid carcinoma and intrathyroid thymic carcinoma arising in association with a branchial cleft-like cyst: a case report.

Background: Primary squamous carcinoma of the thyroid is an exceptionally rare malignancy, and its coexistence with poorly differentiated thyroid carcinoma (PDTC) is exceedingly uncommon. The pathogenesis of thyroid squamous carcinomas remains controversial, particularly when associated with congenital epithelial remnants such as branchial cleft-like cysts or thymic/ultimobranchial remnants. We report an unusual collision tumor composed of PDTC and intrathyroid thymic carcinoma (ITTC) arising in association with a longstanding cystic thyroid lesion. Case presentation: A 59-year-old man initially presented with a left thyroid lesion that was interpreted as a benign branchial cleft cyst on ultrasound-guided fine-needle aspiration. After being lost to follow-up for more than four years, he re-presented with progressive enlargement of the lesion. Repeat cytology and core biopsy demonstrated a poorly differentiated carcinoma with squamous differentiation. Cross-sectional imaging revealed local invasion, retrosternal extension, and multiple bilateral pulmonary nodules suspicious for metastatic disease. Near total thyroidectomy was performed for symptomatic airway compression. Histopathological examination demonstrated two morphologically and immunophenotypically distinct malignant components: PDTC with solid, trabecular, and insular architecture, necrosis, increased mitotic activity, angioinvasion, lymphatic invasion, perineural invasion, and extrathyroidal extension; and a squamous carcinoma arising in association with a benign squamous-lined cyst. Immunohistochemically, the PDTC component expressed thyroglobulin, TTF-1, and PAX8, whereas the squamous component was positive for CK5/6, p63, and CD117 and negative for thyroid follicular markers, supporting ITTC. Outcome: The final diagnosis was a collision tumor composed of PDTC and ITTC arising in association with a branchial cleft-like cyst or other embryologic cystic remnant. Although pulmonary biopsy and systemic treatment were planned, the patient developed severe bronchopneumonia and died before further staging could be completed. Conclusion: This case illustrates an exceptionally rare collision tumor of the thyroid and highlights the diagnostic limitations of cytologic assessment in predominantly cystic lesions. It emphasizes the importance of repeat evaluation of enlarging cystic thyroid masses and comprehensive histopathological and immunohistochemical assessment for accurate diagnosis and appropriate clinical management.