[18F]FAPI-04 PET/CT for detection of recurrent breast cancer after surgery: a post hoc analysis comparing [18F]FDG PET/CT.

PURPOSE: To compare the diagnostic performance of [18F]FAPI-04 PET/CT with that of [18F]FDG PET/CT in patients with suspected recurrence of breast cancer following surgical treatment. METHODS: This is a post‑hoc analysis of data from a prospective single-center clinical trial. Patients with clinical suspicion of recurrence of breast cancer after surgery were consecutively enrolled and underwent both [18F]FDG and [18F]FAPI-04 PET/CT. Paired comparisons of sensitivity and accuracy were performed using McNemar's test. Differences in semi-quantitative parameters, including maximum standardized uptake value (SUVmax) and tumor-to-background ratio (TBR), were assessed using the Wilcoxon signed-rank test. RESULTS: A total of 44 patients with 880 lesions were included, of whom 40 patients (782 lesions) were confirmed to have recurrent disease. Compared with [18F]FDG PET/CT, [18F]FAPI-04 PET/CT showed higher sensitivity and accuracy across most metastatic sites. The differences were most pronounced for lymph node, pleural, and bone metastases, with significantly higher sensitivity (98.7% vs. 62.7%, 94.0% vs. 64.2%, and 99.6% vs. 63.4%, respectively; all P < 0.001) and accuracy (93.6% vs. 54.3%, 91.5% vs. 64.3%, and 98.4% vs. 62.7%; all P < 0.001). Quantitatively, [18F]FAPI-04 demonstrated significantly higher tracer uptake than [18F]FDG across most lesion categories, with consistently elevated SUVmax and TBR values (all P < 0.001 for TBR; SUVmax differences significant except for liver lesions). These findings indicate improved lesion conspicuity and contrast with [18F]FAPI-04. Importantly, [18F]FAPI-04 PET/CT resulted in a change in clinical management in 12 of 40 patients (30%) with confirmed recurrence. CONCLUSION: In this cohort, [18F]FAPI-04 PET/CT demonstrated higher lesion detectability than [18F]FDG PET/CT, particularly for lymph node, pleural, and bone metastases. These findings suggest that [18F]FAPI-04 PET/CT may serve as a valuable complementary imaging tool to [18F]FDG PET/CT in the post‑treatment surveillance setting, with the potential to refine patient stratification and inform therapeutic decisions.