Multiple Bone Metastasis Occurred 35 Years after Initial Lobectomy in a Patient with Follicular Thyroid Carcinoma: a Case Report

Differentiated thyroid cancer (DTC), which primarily consists of papillary thyroid cancer (80%) and follicular thyroid cancer (10-20%) worldwide, is the most common form of thyroid cancer originating from follicular cells.Although DTC generally has a favorable prognosis, patient survival rates significantly decrease with distant metastases, which occur particularly in FTC through hematogenous spread.A 67-year-old female patient presented with back and thigh pain in October 2020, leading to the discovery of an L2 vertebral osteolytic lesion, with a history of thyroid lobectomy 35 years prior.Multiple bone metastases were identified through FDG PET/CT, and biopsy confirmed metastatic FTC, leading to completion thyroidectomy which revealed an 8 mm encapsulated follicular variant of PTC.The patient underwent three sessions of radioactive iodine therapy (RAIT) with a cumulative dose of 500 mCi and external beam radiation for bone metastasis.Despite treatment, subsequent imaging showed no significant improvement in metastatic lesions, and the patient developed persistent bicytopenia.Given the limited treatment response and ongoing complications, further high-dose RAIT was discontinued, and the patient continued follow-up with regular tumor marker assays and imaging studies.This case illustrates the importance of considering thyroid cancer when investigating bone metastasis with an unknown primary site.Furthermore, it highlights the necessity of accurate diagnosis, proper treatment, and vigilant follow-up in FTC management, particularly regarding the possibility of delayed distant metastasis in DTC patients.