ObjectiveTo determine if preoperative ⁶⁸Ga-fibroblast activation protein inhibitor (FAPI)-04 PET/MR could contribute to predicting pathological complete response (pCR) in patients with gastrointestinal cancer undergoing neoadjuvant immunotherapy.

MethodsIn this retrospective study, 35 patients (23 males, 12 females, age (59.1±7.9) years) with gastrointestinal cancer who underwent ⁶⁸Ga-FAPI-04 PET/MR after receiving neoadjuvant immunotherapy between February 2021 and January 2024 were enrolled. Clinical data, PET imaging parameters including SUV, peak of SUV normalized by lean body mass (SUL _peak), FAPI-positive tumor volume (FTV), and total FAPI-positive lesion burden (TLF), and pathological data were collected and analyzed. Patients were divided into pCR group and non-pCR group, and the independent-sample t test or Mann-Whitney U test was performed to compare those parameters between the 2 groups. ROC curve analysis (Delong test) was performed to evaluate the diagnostic efficiency of each parameter to predict pCR.

ResultsThe overall pCR rate of the neoadjuvant therapy was 40.0%(14/35). In the visual evaluation, ⁶⁸Ga-FAPI-04 PET was limited in predicting pCR, showing false positivity in 12 patients and false negative in 1 patent. While SUV _max( t=2.50, P=0.018), SUL _peak( t=3.11, P=0.004), FTV( U=3.00, P=0.030) and TLF( U=2.96, P=0.042) in non-pCR group were all higher than those in pCR group. The predictive efficiency of FTV <1.925cm ³ for pCR was better than the efficiency of PET visual evaluation ( Z=3.61, P<0.001), with the prediction accuracy of 82.86%(29/35).

Conclusions ⁶⁸Ga-FAPI-04 PET/MR may provide an effective clinical tool for guiding further treatment of patients with gastrointestinal cancer undergoing neoadjuvant immunotherapy. The quantitative features derived from ⁶⁸Ga-FAPI-04 PET appear promising in predicting pCR, which are expected to provide a reference for avoiding surgery.