Superiority of [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT to [ 18 F]FDG PET/CT in the evaluation of thymic epithelial tumours

Purpose The purpose of this study is to compare the ability of [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT and [ 18 F]FDG PET/CT to stratify the malignancy and invasiveness of thymic epithelial tumours (TETs). Methods From April 2021 to November 2022, participants with suspected TETs confirmed by histopathology or follow-up imaging were prospectively analysed. All participants underwent [ 18 F]FDG and [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT within 1 week. Clinical characteristics, CT features, and metabolic parameters (maximum standardized uptake value [SUV max ] and tumour-to-mediastinum ratio [TMR]) of subjects with different pathological types and stages were compared. The diagnostic capacities of [ 18 F]FDG and [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT were compared using receiver operating characteristic (ROC) curves and McNemar’s test. Results Fifty-seven participants were included. [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT was superior to [ 18 F]FDG PET/CT in differentiating thymomas from thymic carcinomas (TCs) (AUC: 0.99 vs. 0.90, P = 0.02). Logistic regression revealed that SUV max-FAPI ( P = 0.04) was a significant predictive factor for TCs. SUV max-FAPI and TMR -FAPI showed an excellent ability to differentiate low-risk thymomas (types A, AB, and B1), high-risk thymomas (types B2 and B3), and TCs (both P < 0.001). In thymomas, only SUV max-FAPI ( P < 0.001), TMR -FAPI ( P < 0.001), and nonsmooth edges ( P = 0.02) were significantly higher in the advanced-stage (Masaoka-Koga [MK] stage III/IV) group than in the early-stage group (MK stage I/II). Compared with [ 18 F]FDG PET/CT, [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT showed significantly higher specificity (67% [46 of 69] vs. 93% [64 of 69], P < 0.001) in the detection of lymph node metastases and higher sensitivity (49% [19 of 39] vs. 97% [38 of 39], P < 0.001) in evaluating distant metastases. Both SUV max-FAPI and TMR -FAPI were correlated with FAP expression (both r = 0.843, P < 0.001). Conclusion [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT was superior to [ 18 F]FDG PET/CT in evaluating the World Health Organization (WHO) classification, MK staging, and metastatic status of TETs. Trial registration ChiCTR2000038080, registration date 2020-09-09, https://www.chictr.org.cn/com/25/showproj.aspx?proj=61192