Advanced Unresectable Differentiated Thyroid Cancer With Anaplastic Transformation: A Case Report and Review of the Literature.

Papillary thyroid carcinoma (PTC) is the most common subtype of thyroid cancer (TC). Although surgery and radioactive iodine therapy (RAI) generally yield favorable outcomes, advanced cases with extensive local invasion and metastases pose significant challenges. We report the case of a 65-year-old male with advanced, inoperable PTC characterized by extensive local invasion and distant metastases. A whole-body radioiodine scan revealed no iodine-avid disease. However, following recombinant human thyrotropin (rhTSH) preparation, the patient developed severe compressive symptoms, necessitating glucocorticoid treatment. Lenvatinib monotherapy was initiated, resulting in initial symptom relief. Unfortunately, the disease rapidly progressed due to anaplastic transformation of the PTC, ultimately leading to the patient's death. This report highlights the challenges of managing advanced differentiated TC (DTC) and the risk of aggressive transformation into anaplastic TC (ATC), even with targeted therapy. It underscores the critical need for close monitoring and a multidisciplinary approach. Further research is needed to explore the role of targeted therapies, such as lenvatinib, in altering tumor biology and their long-term effects on disease progression.