Preoperative Assessment of Peritoneal Carcinomatosis with [68Ga]Ga-FAPI-46 PET/CT: Agreement with Surgicopathologic PCI and Comparison with MRI and [18F]FDG PET/CT.

Accurate preoperative mapping of peritoneal carcinomatosis (PC) is critical for selecting patients for cytoreductive surgery and guiding treatment planning. Conventional imaging, including [18F]FDG PET/CT (FDG PET), may underestimate PC extent. This study evaluated [68Ga]Ga-fibroblast activation protein inhibitor (FAPI)-46 PET/CT (FAPI PET) for preoperative PC assessment, examining agreement with the surgicopathologic reference standard and comparing its performance with MRI and FDG PET. Methods: In this prospective phase 2 study (FAPeCa, NCT06061874), patients with colorectal or ovarian cancer with known or suspected PC underwent FAPI PET, FDG PET, and MRI with diffusion-weighted imaging within 4 wk before diagnostic laparoscopy or cytoreductive surgery. PC extent was quantified using a 10-segment peritoneal cancer index (PCI) adaptation. Agreement with the reference standard was assessed using intraclass correlation coefficients (ICCs), and segment-level diagnostic performance was evaluated after dichotomization (PCI, 0 vs. ≥1). Subgroup analyses were performed based on recent chemotherapy status. Results: The analysis included 61 patients. Agreement with the reference standard was highest for FAPI PET (ICC, 0.81), followed by MRI (ICC, 0.76) and FDG PET (ICC, 0.54). Segment-level sensitivity, specificity, and accuracy were 77.6%, 71.2%, and 74.8% for FAPI PET; 67.0%, 84.4%, and 74.7% for MRI; and 47.5%, 91.8%, and 67.2% for FDG PET, respectively, with significantly higher sensitivity for FAPI PET (P < 0.001 for both comparisons) but lower specificity than for MRI and FDG PET. In the recent chemotherapy subgroup, agreement remained good for FAPI PET (ICC, 0.75), compared with lower agreement for MRI (ICC, 0.65) and FDG PET (ICC, 0.46). Conclusion: FAPI PET demonstrated the strongest overall agreement with surgicopathologic PCI and higher detectability than MRI with diffusion-weighted imaging and FDG PET, with preserved performance after chemotherapy, supporting its role for preoperative PC mapping.