Anaplastic transformation of follicular thyroid carcinoma in a metastatic skeletal lesion presenting with paraneoplastic leukocytosis

Background: Anaplastic transformation of differentiated thyroid carcinoma (DTC) is a rare event with a poor clinical outcome. It usually occurs in the primary site or in regional lymph nodes, but rarely in distant metastatic lesions. Summary: A 55-year-old woman with persistent pain in the left hip joint visited our hospital. She had a history of DTC that had been surgically removed 12 years earlier. Clinical images showed a tumorous mass in the left pelvis, indicative of bone metastasis. The patient underwent surgery to remove the tumor and remained stable until local recurrence was found 5 weeks after the surgery. The patient subsequently underwent radiation therapy; however, she died of respiratory failure due to lung metastases 2 months after the surgery for the recurrent lesion. The surgical specimens were diagnosed as anaplastic thyroid carcinoma, indicating that anaplastic transformation of thyroid follicular carcinoma occurred in the metastatic skeletal lesion. In addition, the patient had an unus...