18F-FDG PET/CT for initial assessment and response monitoring in a case of high grade primary lymphoma of the thyroid gland: A case report and review of literature
A 64-year-old man with a history of hypothyroidism and proven Hashimoto's thyroiditis presented with painless progressive asymmetric enlargement of the thyroid gland. Laboratory evaluation revealed an elevated erythrocyte sedimentation rate (ESR), elevated thyroid peroxidase antibody titer of 850 U/mL (normal <30 U/mL), free triiodothyronine (FT3) 2.34, free thyroxine (FT4) 0.97, and thyroid stimulating hormone (TSH) level of 22.11. A contrast enhanced CT (CECT) exam of the neck revealed a large heterogeneous mass lesion predominantly involving the left lobe of thyroid and partly involving the isthmus and anterior part of right lobe. A needle biopsy from the thyroid mass revealed features of non-Hodgkin's lymphoma (DLBCL) with the tumor cells staining positive for CD20, negative for cytokeratin (CK) and having a high proliferative index (MIB-1) of 35-40%. 18F-FDG PET/CT was performed for staging. It revealed a large heterogeneous mass lesion predominantly involving the left lobe of thyroid and partly involving the isthmus and anterior part of right lobe with intense radiotracer uptake in the thyroid mass (SUVmax-18.7) with no other abnormal identifiable areas [Figure –]. The patient underwent four cycles of combination chemotherapy (rituximab-cyclophosphamide, doxorubicin, vincristine, and prednisone) and was reevaluated with 18F-FDG PET/CT. Post therapy 18F-FDG PET/CT [Figure –] done 6 weeks after completion of chemotherapy revealed significant decrease in the size of the enlarged thyroid gland associated with no significant FDG avidity (SUVmax-2.0) of the residual thyroid mass (arrow), suggestive of complete metabolic response. The patient is in complete remission at 18 month follow-up.