Primary Thyroid Lymphoma: External Beam Radiation Therapy Induced Thyroiditis Mimics Residual Disease on Serial 18F-FDG PET/CT Imaging

A 67-year-old female patient with no prior history of benign thyroid disease was diagnosed with primary thyroid lymphoma and was staged with 18F-fluoro-2-deoxy-D-glucose (18F-FDG) positron emission tomography/computed tomography (PET/CT). She was treated with chemotherapy and external beam radiation therapy, and a follow-up PET/CT showed significant reduction in the size of the thyroid lymphoma with persistent intense 18F-FDG uptake, which was interpreted as partial response to therapy. However, two subsequent PET/CT studies showed no change in the persistent intense 18F-FDG uptake in the thyroid and a biopsy confirmed the presence of thyroiditis with no evidence of residual lymphoma. Follow-up PET/CTs performed over the subsequent three years showed stable intensely 18F-FDG avid thyroiditis with no evidence of lymphoma recurrence. We present the imaging characteristics of a long term radiation treatment induced thyroiditis mimicking 18F-FDG avid residual disease on PET/CT.