Response Prediction Using18F-FAPI-04 PET/CT in Patients with Esophageal Squamous Cell Carcinoma Treated with Concurrent Chemoradiotherapy

This prospective study examined whether imaging results obtained using the tracer ¹⁸F-AlF-NOTA-fibroblast activation protein inhibitor (FAPI)-04 (denoted as ¹⁸F-FAPI-04) in PET/CT can predict the short-term outcome in patients with locally advanced esophageal squamous cell carcinoma (LA-ESCC) treated with concurrent chemoradiotherapy (CCRT). Methods: The 18 enrolled LA-ESCC patients underwent ¹⁸F-FAPI-04 PET/CT scanning before CCRT. The SUV_max, SUV_mean, SUV_peak, metabolic tumor volume, and total lesion fibroblast activation protein expression of the primary tumor were recorded. Additionally, the SUV_max of the primary tumor and SUV_mean of normal tissue (muscle and blood) were measured, and their ratios were denoted as target-to-background ratios (TBR_muscle and TBR_blood). Patients were classified as responders or nonresponders according to RECIST (version 1.1), and variables were compared between the 2 groups. Results: The TBR_blood, TBR_muscle, and SUV_mean were significantly higher in nonresponders than in responders (all P < 0.05). Receiver-operating-characteristic curve analysis identified TBR_blood (area under the curve [AUC], 0.883; P = 0.008), TBR_muscle (AUC, 0.896; P = 0.006) and SUV_mean (AUC, 0.870; P = 0.010) as significant predictors of the response to CCRT, with cutoffs of 10.68, 10.95, and 6.88, respectively. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were also determined for TBR_blood (100.0%, 72.7%, 66.7%, 88.9%, and 77.8%, respectively), TBR_muscle (100.0%, 72.7%, 66.7%, 88.9%, and 77.8%, respectively), and SUV_mean (85.7%, 81.8%, 75.0%, 90.0%, and 83.3%, respectively). On univariate logistic regression analysis, TBR_blood (P = 0.026), TBR_muscle (P = 0.036), SUV_mean (P = 0.045), and tumor site (P = 0.032) were significantly correlated with the short-term outcome. On multivariable logistic regression analysis, TBR_blood (P = 0.046) was an independent prognostic factor for short-term outcome. Conclusion: A higher baseline TBR_blood on ¹⁸F-FAPI-04 PET/CT scans was associated with a poor response to CCRT in LA-ESCC patients, and thus, TBR_blood may be useful for screening LA-ESCC patients before CCRT treatment.