Value of 18F-FDG PET/CT in the Diagnosis of Primary Thyroid Squamous Cell Carcinoma: A Case Report

Abstract Primary squamous cell carcinoma (SCC) of the thyroid is an extremely rare aggressive malignancy, recently reclassified in the 2022 World Health Organization classification as a morphologic subtype of anaplastic thyroid carcinoma. We report a case highlighting the diagnostic value of fluorine-18 fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) in localizing the primary tumor and defining disease extent in a patient initially referred with metastatic SCC of unknown primary. A 78-year-old man presented with hoarseness of voice, exertional dyspnea, dysphagia mainly to solids, neck swelling/pain, and headaches as initial symptoms. Biopsy of the cervical lymph nodes revealed metastatic SCC, and thyroid fine-needle aspiration revealed SCC within the left thyroid lobe. 18F-FDG PET/CT demonstrated an intensely FDG-avid mass in the left thyroid lobe with widespread hypermetabolic cervical, supraclavicular, mediastinal, hilar, and axillary lymphadenopathy, without evidence of another primary site. On the basis of imaging and pathological findings, a diagnosis of primary thyroid SCC with nodal metastases was established. Unlike conventional anatomical imaging, which identified the thyroid mass but could not exclude another primary site, 18F-FDG PET/CT confirmed the thyroid as the site of origin, excluded secondary thyroid involvement, and provided comprehensive whole-body staging, underscoring its superiority over conventional imaging in resolving diagnostic uncertainty in this rare and aggressive disease.