Unusual Soft Tissue and Muscle Metastases in Papillary Thyroid Carcinoma: Insights from 131I Scintigraphy and 18F-FDG PET/CT

Distant metastases from well-differentiated thyroid carcinoma typically involve the lungs and bones. We report a rare case of metastatic papillary thyroid carcinoma presenting as soft tissue lesion, detected by Iodine-131 whole-body scintigraphy in a 44-year-old asymptomatic male. The patient had previously undergone a total thyroidectomy and bilateral modified radical neck dissection, receiving radioiodine for pT3bN1bMx multifocal classical papillary thyroid carcinoma. Further evaluation with ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography revealed additional sites of soft tissue and muscle metastasis, along with extensive nodal, lung, and skeletal involvement. These findings indicated advanced disease with a poor prognosis, necessitating a change in the treatment plan.