Diagnostic performance of FAPI PET/CT for detecting peritoneal metastasis in gastric cancer: an exploratory prospective study and systematic review with meta-analysis

Abstract Background Peritoneal metastasis (PM) in gastric cancer is associated with poor survival, but conventional imaging has limited sensitivity for its early detection. Although staging laparoscopy with peritoneal cytology, supplemented by histopathological examination of suspicious lesions when present, is considered the reference standard, it is invasive. Fibroblast activation protein (FAP), which is highly expressed in tumor stroma, can be targeted using FAP inhibitor (FAPI) positron emission tomography/computed tomography (PET/CT). The high target-to-background contrast of FAPI PET/CT may improve PM detection; however, its diagnostic value in gastric cancer remains uncertain. We prospectively evaluated FAPI PET/CT for detecting PM in gastric cancer and conducted a systematic review and meta-analysis to comprehensively assess its diagnostic performance. Methods We evaluated [¹⁸F]AlF-NOTA-FAPI-04 PET/CT for detecting peritoneal metastasis in gastric cancer in an exploratory prospective cohort using a laparoscopy-based composite reference standard. For the meta-analysis, PubMed, Embase, the Cochrane Library, and Web of Science were searched through July 10, 2026. Eligible studies evaluated FAPI-04 PET/CT and provided per-patient diagnostic 2 × 2 data. Risk of bias and applicability concerns were assessed using QUADAS-2. Pooled sensitivity and specificity were jointly estimated using a bivariate random-effects model. The primary analysis included datasets using a laparoscopy-based composite reference standard. Sensitivity analyses included datasets using multidisciplinary team-based assessment and all datasets eligible for bivariate pooling. Potential sources of heterogeneity were explored using separate univariable meta-regression models. Results In the exploratory prospective cohort, FAPI PET/CT yielded a sensitivity of 0.40 (95% CI, 0.12–0.77) and specificity of 1.00 (95% CI, 0.72–1.00). The accuracy was 0.80 (95% CI, 0.55–0.93), with a positive predictive value of 1.00 (95% CI, 0.34–1.00) and negative predictive value of 0.77 (95% CI, 0.50–0.92). In the primary meta-analysis of datasets using a laparoscopy-based composite reference standard, the pooled sensitivity was 0.66 (95% CI, 0.53–0.77), and the pooled specificity was 0.90 (95% CI, 0.76–0.96). In studies using multidisciplinary team-based assessment as the reference standard, the pooled sensitivity and specificity were 0.92 (95% CI, 0.85–0.96) and 0.98 (95% CI, 0.94–0.99), respectively. When all 11 datasets eligible for bivariate pooling were analyzed, irrespective of the reference standard used, the pooled sensitivity and specificity were 0.86 (95% CI, 0.75–0.92) and 0.96 (95% CI, 0.91–0.98), respectively. Conclusions FAPI PET/CT demonstrated high specificity in the prospective cohort (1.00; 95% CI, 0.72–1.00) and primary meta-analysis (0.90; 95% CI, 0.76–0.96). However, sensitivity was lower in both the cohort (0.40; 95% CI, 0.12–0.77) and meta-analysis (0.66; 95% CI, 0.53–0.77). The wide confidence intervals surrounding the prospective estimates warrant cautious interpretation. Large-scale, well-designed prospective studies are needed to validate the diagnostic performance and clinical utility of FAPI PET/CT for detecting PM in gastric cancer.