Clincal value of FDG PET/CT in response assessment of primary thyroid lymphoma

Objectives Primary thyroid lymphoma (PTL) is almost always associated with Hashimoto’s thyroiditis that accumulates FDG. This study was aimed to determine whether FDG PET/CT is valuable in assessment of therapy response in PTL.

Methods 36 patients with PTL (DLBCL 8, MALT lymphoma 28) were studied. Either or both of TgAb and TPOAb were positive in all. Clinical stage was IE in 19, IIE in 14, IIIE in 2, and IVE in 1. Radiotherapy (32-40Gy), chemotherapy (R-CHOP), and chemoradiotherapy was done in 24, 4, and 8 patients, respectively. PET/CT was performed within 4 weeks before therapy and 12-16 weeks after completion of therapy combined with neck US. Therapeutic efficacy was evaluated by varying methods. Either of the following was considered as a sign for absence of viable tumor: a) negative FDG uptake by visual assessment. b) more than 50% decrease in the SUV max compared with the pretreatment value. c) disappearance of the extremely hypo-echogenic lesion on US. Final outcome was determined by ether pathologic correlation (n=25) or comparison with other imaging modalities and clinical courses (n=11).

Results In the baseline study, all thyroid lesions were FDG avid. 2 patients showed new lesion outside of the thyroid after treatment and were defined as progressive disease. In the remaining 34 patients, visually negative FDG uptake, more than 50% decrease in the SUVmax, and disappearance of the extremely hypo-echogenic lesion on US was seen in 15, 22, and 19, respectively. Finally, viable tumor was confirmed only in 2 patients. PPV of visual analysis vs. semi-quantitative analysis vs. US for residual viable tumor were 10% vs.20% vs.13%, respectively. Every method showed NPV of 100%

Conclusions Our study suggests that both visual evaluation and semi-quantitative analysis of FDG have a limited value for response assessment of PTL probably due to co-existing Hashimoto’s thyroiditis mimicking viable tumor. Persistent thyroidal FDG uptake after therapy must be interpreted with caution.