SPINDLE CELL SUBTYPE OF MEDULLARY THYROID CARCINOMA WITH SOMATIC RET M918T MUTATION: A CASE REPORT AND LITERATURE REVIEW

The spindle cell subtype of medullary thyroid carcinoma (MTC) is an exceptionally rare morphologic pattern that can be mistaken cytologically and histologically for anaplastic thyroid carcinoma, sarcoma, melanoma, and other spindle cell lesions of the neck, creating a substantial diagnostic pitfall. We report a 72-year-old man who presented with an eight-month history of an enlarging, painless anterior neck mass accompanied by hoarseness, mild dysphagia, hot flushes, and unintentional weight loss. Examination revealed a firm 4 cm right thyroid nodule and a palpable left level VI lymph node; serum calcitonin exceeded 2000 pg/mL and carcinoembryonic antigen CEA) was 75 ng/mL. Fine-needle aspiration (FNA) demonstrated dispersed spindle-shaped cells within an amyloid-rich background, confirmed by Congo red birefringence and an immunohistochemical (IHC) profile positive for calcitonin, chromogranin A, synaptophysin, and CD56 and negative for thyroglobulin. Total thyroidectomy with comprehensive bilateral neck dissection confirmed bilateral spindle cell MTC with ten of thirty-two positive lymph nodes; molecular testing identified a somatic RET M918T mutation without RAS alteration, placing the patient in a high-risk prognostic category. Surveillance detected biochemical and structural relapse, with mediastinal and hepatic metastases at eighteen months, prompting initiation of a selective RET inhibitor. At two years, the patient remains on targeted therapy with stable disease and preserved quality of life. This case underscores the importance of a systematic cytologic and IHC approach to spindle cell thyroid lesions and illustrates the evolving role of RET-directed therapy in advanced disease.