Differential [¹⁸F]FAPI-42 and [¹⁸F]FDG uptake patterns distinguish desmoid tumours from GISTs on PET/CT imaging

Intra-abdominal desmoid tumours (IADT) can closely mimic gastrointestinal stromal tumours (GISTs), particularly in recurrence and metastases. However, differentiating these two diseases using radiological methods remains challenging. This study aimed to investigate the value of [¹⁸F]FAPI-42 PET/CT in differentiating IADT and GISTs compared to [¹⁸F]FDG PET/CT.

This study retrospectively included a total of 24 patients (12 patients with IADT and 12 patients with recurrent/metastatic GISTs) who underwent two separate PET/CT scans using [¹⁸F]FAPI-42 and [¹⁸F]FDG, respectively. The differences in tumour SUV_max on [¹⁸F]FAPI-42 PET/CT (SUV_max-FAPI) and [¹⁸F]FDG PET/CT (SUV_max-FDG) and the ratio of tumour SUV_max on FAPI PET/CT to SUV_max on FDG PET/CT [SUV_max ratio (FAPI/FDG)] were compared between IADT and GISTs. Receiver operating characteristic (ROC) curve analyses were performed to evaluate the diagnostic performance of these parameters in differentiating IADTs from GISTs. Immunohistochemistry was used to verify FAP expression in all IADT lesions and GISTs lesions.

The SUV_max of IADTs on [¹⁸F]FAPI-42 PET/CT was significantly higher than that of GISTs [8.8 (7.2, 10.6) vs. 3.9 (2.8, 7.6), P = 0.045], whereas the SUV_max of IADTs on [¹⁸F]FDG PET/CT was significantly lower than that of GISTs [2.5 (1.9, 3.5) vs. 6.1 (3.6, 8.3), P = 0.007]. The SUV_max ratio (FAPI/FDG) of IADTs was significantly higher than that of GISTs [3.3 (2.2, 4.1) vs. 0.8 (0.3, 1.4), P < 0.001]. SUV_max-FAPI, SUV_max-FDG, and the SUV_max ratio (FAPI/FDG) achieved AUCs of 0.743, 0.819, and 0.896, with corresponding accuracies of 75.0%, 83.3%, and 87.5%, respectively, for distinguishing IADT from GISTs. Immunohistochemistry revealed significantly higher FAP expression in IADTs compared to GISTs (P = 0.002), with most IADTs showing strong (3+) staining (8/12, 66.7%), while GISTs primarily demonstrated weak (1+) or no staining (7/12, 58.3%).

The distinct uptake characteristics of [¹⁸F]FAPI-42 and [¹⁸F]FDG observed in IADTs and GISTs provide preliminary evidence that these two tracers may offer complementary diagnostic information when evaluating desmoid tumours that mimic recurrent or metastatic GISTs.