Comparison of [Ga]Ga-FAPI and [F]FDG Uptake in Patients with Gastric Signet-ring-cell Carcinoma: a Multicenter Retrospective Study

Objective: In this study, we investigated the role of [Ga]Ga-FAPI PET imaging in the detection of primary and metastatic gastric signet-ring-cell carcinoma (GSRCC) and compared with [F]FDG PET.

Methods: This retrospective multicenter analysis included 34 patients with histologically confirmed GSRCCs from four medical centers. The maximum standard uptake value (SUV), tumor-to-background ratio (TBR), and diagnostic accuracy were compared between the two modalities. [F]FDG and [Ga]Ga-FAPI uptakes were compared by using the Wilcoxon signed-rank test. McNemar's test was used to compare the diagnostic accuracy between the two techniques.

Results: Data were analyzed from 27 paired PET/CT and 7 paired PET/MRI scans for 34 GSRCC patients (16 men and 18 women) who had a median age of 51 years (range: 25-85 years). [Ga]Ga-FAPI PET showed higher SUV and TBR values than did [F]FDG PET in the primary tumors (SUV: 5.2 vs. 2.2, p = 0.001; TBR: 7.6 vs. 1.3, p < 0.001), involved lymph nodes (SUV: 6.8 vs. 2.5, p < 0.001; TBR: 5.8 vs. 1.3, p < 0.001), and bone and visceral metastases (SUV: 6.5 vs. 2.4, p < 0.001; TBR: 6.3 vs. 1.3, p < 0.001). In diagnostic performance, [Ga]Ga-FAPI PET exhibited higher sensitivity than [F]FDG PET for detecting primary tumors (73% [16/22] vs. 18% [4/22], p < 0.001), local recurrences (100% [7/7] vs. 29% [2/7], p = 0.071), lymph node metastases (77% [59/77] vs. 23% [18/77], p < 0.001), and distant metastases (93% [207/222] vs. 39% [86/222], p < 0.001).

Conclusion: The results from this multicenter retrospective analysis justify the clinical use of [Ga]Ga-FAPI tracers for GSRCC diagnosis and staging.

Key Points: • [Ga]Ga-FAPI PET/CT is a promising imaging modality for the detection of primary and metastatic disease and has implications for TNM staging in GSRCC. • In this multicenter study of 34 patients with GSRCC, [Ga]Ga-FAPI PET exhibited greater radiotracer uptake, tumor-to-background ratios, and diagnostic accuracy than [F]FDG PET for detecting primary/recurrent tumors and metastatic lesions.