Exceptional Thyroid Recurrence of Solitary Bone Plasmacytoma Detected by 18F-FDG PET/CT

A 70-year-old man with prior occupational radiation exposure and a history of solitary bone plasmacytoma of the left hemipelvis treated with radiotherapy underwent ¹⁸ F-FDG PET/CT for follow-up. Imaging revealed focal hypermetabolism in the right thyroid lobe, with no additional abnormalities detected. Ultrasound revealed an EU-TIRADS 5 nodule, and the cytologic analysis was indeterminate (Bethesda III). Core biopsy confirmed thyroid plasmacytoma. Although solitary bone plasmacytoma usually progresses to multiple myeloma, distant extramedullary relapse is uncommon. Thyroid involvement is exceedingly rare. This case highlights the value of ¹⁸ F-FDG PET/CT and the need for histologic confirmation of suspicious thyroid lesions.