Diffuse FDG uptake in the thyroid in rapidly enlarging goiter does not guarantee benign disease

423 Objectives It has been reported that diffuse uptake of F-18 fluorodeoxyglucose (FDG) in the thyroid gland is suggestive of autoimmune thyroiditis with high likelihood. However, primary thyroid lymphoma (PTL), which arise from Hashimoto’s thyroiditis(HT), could show diffuse thyroid FDG uptake. The aim of the study was to investigate association between FDG uptake and conditions of disease in rapidly enlarging goiter. Methods Between July 2007 and December 2011, total of 68 patients with HT, who had visited our institutes for investigation of rapidly enlarging goiter, were enrolled. All patients underwent neck ultrasound (US), US-guided core needle biopsy, and PET/CT using FDG. Thyroid function at FDG PET/CT was euthyroid in 45, subclinical hypothyroid in 15, hypothyroid in 5 and hyperthyroid in 1. Specimen of core needle biopsy was subjected to immunohistochemical staining (CD20,CD3,CD79a,Ki-67, etc.) along with H-E staining. IgH re-arrangement was analyzed by PCR. All patients were fasted overnight prior to injection of FDG. On PET/CT images, FDG uptake in the thyroid was categorized as nodular or diffuse by visual interpretation. Additionally, FDG uptake was semi-quantitatively determined in form of SUVmax. Results Pathological diagnosis was PTL in 33 (MALT lymphoma23, DLBCL10) and HT in 35. Of 33 PTL, clinical stage was IE in 19 , IIE in 12, IIIE in 1 and IV E in 1. In PTL, FDG uptake was nodular in 10 and diffuse in 23, In HT, FDG uptake was diffuse in all. There was no significant difference in SUVmax between PTL and HT (7.9± 2.8 vs. 7.1 ±2.6) and that between nodular uptake and diffuse uptake in PTL (6.7± 2.5 vs. 8.3 ±2.8). Conclusions In our study, about 50% of HT associated with rapidly enlarging goiter had PTL. Additionally, about two-thirds of patients with PTL showed diffuse thyroid FDG uptake. Thus, diffuse FDG uptake in enlarging goiter does not guarantee benign conditions of disease and should be interpreted with caution