Intrathyroid thymic carcinoma/carcinoma showing thymus-like elements (CASTLE): An extremely rare presentation of thyroid malignancy

Intrathyroid thymic carcinoma, also known as carcinoma showing thymus-like elements/CASTLE, is an uncommon malignancy that arises in the thyroid gland. This tumour shares both morphological and immunohistochemical characteristics with thymic carcinomas. Preoperative diagnosis can be challenging as the clinical and histological features may mimic other high-grade thyroid malignancies. Differentiating CASTLE from other thyroid lesions is important as it has a better prognosis.
A 46-year-old man presented with progressive dysphagia, difficulty in breathing and cough for eight months. The ultrasound scan (USS) revealed a hypoechoic lesion in the right lobe of the thyroid. The cytology specimen obtained through non-US-guided fine needle aspiration (FNA) was reported as Thy5/Bethesda VI, consistent with papillary thyroid carcinoma and a total thyroidectomy was performed. Macroscopic examination revealed a white, non-encapsulated tumour in the right lobe of the thyroid, measuring 45x40x25 mm, with gross extrathyroidal extension. Microscopically, this lesion predominantly showed a squamous morphology with admixed lymphocytes. The tumour cells tested positive for CD5, CD117, CK5/6, CK19 and synaptophysin, and were negative for TTF1 and calcitonin, confirming its classification as CASTLE.
This case brings attention to the difficulties encountered in diagnosing uncommon lesions in both cytology and histology and emphasizes the crucial contribution of immunohistochemistry in navigating through such complex scenarios.