The Value of 18F-FDG PET/CT in avoiding overtreatment of 131l Avidity Pulmonary Metastasis of Differentiated Thyroid Cancer

Abstract Background: We usually use 131 I whole body scan and serum thyroglobulin (Tg) values to determine whether differentiated thyroid cancer (DTC) patients need to receive 131 I treatment, but not all 131 I-avid (functioning) patients have good responses, which is more likely to cause the 131 I-avid patient to receive overtreatment. Our study aims to assess the date of 18 F-FDG PET/CT to avoid 131 I overtreatment and research the status of 131 I-avid pulmonary metastases (PMs) and the prognosis of the patients. Methods: The 131 I-avid PMs of DTC patients who underwent 18 F-FDG PET/CT scans were included. The SUVmax (maximum standardized uptake value), MTV (metabolic tumour volume) and TLG (total lesion glycolysis) were used to estimate 18 F-FDG uptake. The mean follow-up period was 34.14 ± 18.64months. Progression-free survival (PFS) was estimated by the Kaplan-Meier method.The study was based on per-patient and per-lesion analyses. Results: Among the 42 included patients, 34 (34/42, 81%) showed 18 F-FDG uptake, which was defined as abnormal foci (SUVmax >1.0) in the lungs. SUVmax, MTV and TLG and tumour size were the factors that influenced the outcome of 131 I treatment based on Tg levels (p=0.000,0.016,0.000,0.000). The only independent factor was the size of the lesion. There was a significant difference in response to 131 I therapy between PMs with F-I+ and F+/I+ according to both Tg levels and RECIST (version 1.1) (p= 0.044,0.001), according to the per-lesion analysis. When the changes in size or metabolism of some lesions are inconsistent with therapeutic efficacy of patients, it indicates that these patients have a poor prognosis (P=0.003). Conclusions: We concluded that higher 18 F-FDG uptake and larger tumour size predict poor therapeutic effects and a high risk of disease progression in 131 I-avid PMs of DTC. For evaluating the efficiency of 131 I treatment, per-lesion analyses and assessing the data of 18 F-FDG PET/CT would be more reliable than per-patient evaluation only. The increasing 18 F-FDG uptake or diameter of PMs may lead to a poor prognosis for the patient, and early focal treatment modalities may improve their life span.