Preoperative assessment of peritoneal carcinomatosis with 68 GaGa-FAPI-46 PET/CT.

5553 Background: Accurate preoperative mapping of peritoneal carcinomatosis (PC) is essential for selecting patients for cytoreductive surgery (CRS) and guiding treatment planning. Conventional imaging including 18 FFDG PET/CT may underestimate PC tumour burden. This study evaluated 68 GaGa-FAPI-46 PET/CT for preoperative PC assessment using a surgico-pathological reference standard and compared its performance with MRI and 18 FFDG PET/CT. Methods: In this prospective phase II study (FAPeCa, NCT06061874), patients with colorectal or ovarian cancer with known or suspected PC underwent 68 GaGa-FAPI-46 PET/CT, 18 FFDG PET/CT (FDG PET), and MRI within 4 weeks before diagnostic laparoscopy and/or CRS. PC extent was quantified using a 10-segment adaptation of the Peritoneal Cancer Index (PCI). Reference standard was intraoperative PCI with histopathological confirmation, with pathology adjudicating discordant findings. Agreement between imaging-derived total PCI and the reference-standard PCI was assessed using intraclass correlation coefficient (ICC, two-way mixed, absolute agreement). Segment-level diagnostic performance was evaluated after dichotomisation (PCI 0 vs ≥1). Subgroup analyses were performed according to recent chemotherapy (<3 months). Results: Sixty-one patients were enrolled. Median (IQR) total PCI values were 11 (18.5) for the reference standard, 12 (15.5) for FAPI PET, 6 (15.5) for MRI, and 4 (6.5) for FDG PET; FAPI-PCI was higher than MRI-PCI and FDG-PCI (Wilcoxon signed-rank, p<0.001). Agreement with the reference standard was highest for FAPI PET (ICC 0.81; BCa 95% CI 0.67–0.89), followed by MRI (ICC 0.76; BCa 95% CI 0.63–0.85) and FDG PET (ICC 0.54; BCa 95% CI 0.29–0.74). In the recent chemotherapy subgroup, FAPI PET maintained good agreement with the reference standard (ICC 0.75, BCa 95% CI 0.56–0.86), whereas agreement declined to moderate for MRI (ICC 0.65, BCa 95% CI 0.41–0.82) and was lower for FDG PET (ICC 0.46, BCa 95% CI 0.17–0.75). 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 . Overall segment-level sensitivities and specificities differed across the three modalities (Cochran’s Q, p<0.001). Conclusions: 68 GaGa-FAPI-46 PET/CT demonstrated the strongest agreement with surgico-pathological PCI and maintained good agreement after recent chemotherapy. It showed higher detectability than MRI and 18 FFDG PET/CT, albeit at the expense of lower specificity. These findings support 68 GaGa-FAPI-46 PET/CT as a promising modality for preoperative PC mapping. Clinical trial information: NCT06061874 .