The value of [18F]FDG PET/CT in avoiding overtreatment of 131l avidity pulmonary metastasis of differentiated thyroid cancer.

INTRODUCTION

We usually use 131I-whole body scan (131I-WBS) and serum thyroglobulin (Tg) values to determine whether differentiated thyroid cancer (DTC) patients need to receive 131I treatment, but not all ¹³¹I-avid (functioning) patients have a good response to ¹³¹I therapy. Our study aims to assess the data of [¹⁸F]fluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography ([¹⁸F] FDG PET/CT) to research the status of 131I-avid pulmonary metastases (PMs) and the prognosis of the patients.

MATERIAL AND METHODS

The 131I-avid PMs of DTC patients who underwent [18F]FDG PET/CT scans were included. The maximum standardized uptake value (SUVmax), metabolic tumour volume (MTV), and total lesion glycolysis (TLG) were used to estimate [¹⁸F]FDG uptake. The mean follow-up period was 34.14 ± 18.64 months. 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 [¹⁸F]FDG uptake, which was defined as abnormal foci (SUVmax > 1.0) in the lungs. SUVmax, MTV, TLG, and tumour size were the factors that influenced the outcome of 131I treatment based on Tg levels (p = 0.000, 0.016, 0.000, 0.000) in per-lesion analysis. The only independent factor was the size of the lesion. There was a significant difference in response to ¹³¹I therapy between PMs with F-I+ and F+/I+ according to both Tg levels and Response Evaluation Criteria in Solid Tumours (RECIST) (version 1.1) (p = 0.044, 0.001), in the per-lesion analysis. When the changes in size or metabolism of some lesions are inconsistent the prognosis of these patients is poor (p = 0.003).

CONCLUSIONS

We concluded that higher [18F]FDG uptake and larger tumour size predict poor therapeutic effects and a high risk of disease progression in ¹³¹I-avid PMs of DTC. For evaluating the efficiency of ¹³¹I treatment, per-lesion analyses and assessing the data of [¹⁸F] FDG PET/CT would be more reliable than per-patient evaluation only. And early focal treatment modalities may improve their life span.