The impact of qualitative [18F]FDG PET/CT in predicting clinical outcomes of post-surgical differentiated thyroid cancer patients with elevated thyroglobulin and negative radioiodine whole-body scan

Background: [¹⁸F]FDG PET/CT has been widely used as a diagnostic tool in detection and localization of recurrent non-avid radioiodine lesions in post-operative differentiated thyroid cancer (DTC) patients with elevated serum thyroglobulin but negative radioiodine whole-body scan (TENIS) syndrome. The aim of our study was to evaluate the role of [¹⁸F]FDG PET/CT in prediction on outcomes of these DTC patients.

Methods: Post-operative DTC patients with TENIS syndrome were collected in the department of nuclear medicine, Hospital 108 from 2019 to 2023. Patients underwent [¹⁸F]FDG PET/CT with standard protocol following EANM guideline for tumor imaging version 2.0. The qualitative [¹⁸F]FDG PET/CT imaging characteristics were classified into three categories: (i) negative [¹⁸F]FDG PET/CT, (ii) minimal [¹⁸F]FDG PET/CT volume of lesions, (iii) extensive [¹⁸F]FDG PET/CT volume of lesions. Progression-free survival (PFS) and overall survival (OS) were the end point of the study. The prognosis of qualitative [¹⁸F]FDG PET/CT in predicting PFS and OS was illustrated by Kaplan-Meier survival analysis. The independent factors predicting PFS and OS were determined by univariate and multivariate analysis using logistic regression.

Results: There were 164 consecutive patients, 51.2% female and 48.8% female. The most common histopathological type was papillary accounting for 91.5%. The median time of follow-up was 33.3 months, (range 6.57 - 82.5). There was 70 (36.6%) progressions and 12 (7.35%) deaths. Negative [¹⁸F]FDG PET/CT uptake patients had median PFS with median 57.1 months which was higher than that of minimal category (46.2 months), and extensive category (37.6 months) (p < 0,001). 1-year OS and 5-year OS in extensive PET/CT category was 97.8% and 86.2% respectively which were significantly lower than that of negative and minimal categories (p = 0.053). In multivariate analysis, age at the time of diagnosis, pulmonary, bone metastases and extensive [¹⁸F]FDG PET/CT volume of lesions were the independent factor predicting PFS. Bone metastasis was only the factor could predict OS in multivariate analysis.

Conclusions: The minimal and negative [¹⁸F]FDG PET/CT categories had better prognosis than extensive category in PFS and OS. Extensive [¹⁸F]FDG PET/CT category was an independent factor for predicting PFS. Bone metastasis was only the independent factor that could predict both PFS and OS.