Medullary Thyroid Carcinoma That Progressed From Low‐ to High‐Grade During Treatment: A Case Report

Medullary thyroid carcinoma (MTC) with high proliferative potential or tumor necrosis is classified as high‐grade MTC. Progression from low‐ to high‐grade MTC has rarely been documented. We report a case in which a sporadic MTC transitioned from low‐ to high‐grade after the initial therapy. A 62‐year‐old man with low‐grade MTC underwent total thyroidectomy and bilateral neck dissection. Recurrence was identified postoperatively, and the surgical specimen of the lymph node revealed metastatic MTC with high‐grade features. Treatment was initiated with vandetanib followed by selpercatinib, with no significant efficacy. Debulking surgery was performed for five recurrent lesions. Surgical specimens of the tumor after chemotherapy showed increased aggression, characterized by high Ki‐67 index and decreased calcitonin expression. The patient eventually died due to brain metastasis. This case highlights the potential for MTC to progress from low‐ to high‐grade and underscores the importance of ongoing evaluations for recurrent disease.