Diagnostic Performance of FAPI PET in Patients with Breast Cancer: A Systematic Review.

We evaluated the diagnostic performance of fibroblast activation protein inhibitor (FAPI) PET for lesion detection at breast cancer diagnosis and response assessment after neoadjuvant chemoimmunotherapy (NAC) and compared these findings with those from [18F]FDG PET or histopathology after the completion of NAC. Methods: A systematic search across PubMed, EMBASE, Web of Science, and Cochrane Trial databases up to January 12, 2026, was conducted. Study quality was assessed using the Quality Assessment of Diagnostic Accuracy Studies version 2 tool. Extracted data included SUVmax, tumor-to-background ratio, and lesion detection in studies comparing FAPI PET with [18F]FDG PET. For studies comparing FAPI PET with histopathology, diagnostic performance parameters were obtained. Data on FAPI uptake, stratified by histology type, molecular subtype, and tumor grade, were extracted. Results: The literature search yielded 3662 studies, 13 of which were included in this review. Of these, 10 compared FAPI PET with [18F]FDG PET and 3 compared FAPI PET findings with those of histopathology after the completion of NAC. The reported mean SUVmax range was 6.5-17.1 versus 3.9-8.3 for primary tumors, 5.1-17.1 versus 1.2-10.3 for axillary lymph node metastases, and 4.9-21.7 versus 1.7-8.1 for distant metastases on FAPI PET versus [18F]FDG PET, respectively. FAPI PET detected more lesions compared with [18F]FDG PET in all evaluated anatomic sites. In post-NAC response assessment of the breast tumor, FAPI PET reported a sensitivity and specificity of 73%-100% and 71%-100%, respectively. Lower FAPI uptake was observed in low-grade, luminal A tumors and in those with lobular histology. Conclusion: FAPI PET demonstrated increased lesion detection compared with [18F]FDG PET at the time of breast cancer diagnosis and demonstrated considerable diagnostic performance for assessing response evaluation after NAC.