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Our purpose was to prospectively investigate optimal evaluation of qualitative and quantitative 18 F-FDG PET/CT in response evaluations 12-14 wk after neoadjuvant chemoradiotherapy (nCRT) in esophageal cancer patients. Methods: This was a side study of the prospective diagnostic pre-SANO trial. 18 F-FDG PET/CT scans at baseline and at 12-14 wk after nCRT were qualitatively assessed for the presence of tumor. Maximum SUVs normalized for lean body mass (SUL max ) were measured in all scans. The primary endpoint was the proportion of false-negative patients with tumor regression grade (TRG) 3-4 (>10% vital residual tumor) in qualitative and quantitative analyses. Receiver-operating-characteristic curve analysis for TRG1 versus TRG3-4 using SUL max , SUL max tumor-to-esophagus ratio, and Δ%SUL max was performed to define optimal cutoffs. Secondary endpoints were sensitivity, specificity, negative predictive value, and positive predictive value for TRG1 versus TRG2-4. Results: In total, 129 of 219 patients were analyzed. Qualitative 18 F-FDG PET/CT was unable to detect TRG3-4 in 15% of patients. Sensitivity, specificity, negative predictive value, and positive predictive value in qualitative analysis for detecting TRG1 versus TRG2-4 was 80%, 37%, 42%, and 77%, respectively. In 18 of 190 patients (10%) with follow-up scans after nCRT, 18 F-FDG PET/CT identified new interval metastases. Quantitative parameters did not detect TRG3-4 tumor in 27%-61% of patients. The optimal cutoff for detecting TRG1 versus TRG2-4 was a post-nCRT SUL max of 2.93 (area under receiver-operating-characteristic curve, 0.70). Conclusion: Qualitative and quantitative analyses of 18 F-FDG PET/CT are unable to accurately detect TRG3-4 and to discriminate substantial residual disease from benign inflammation-induced 18 F-FDG uptake after nCRT. However, 18 F-FDG PET/CT is useful for the detection of interval metastases and might become useful in an active surveillance strategy with serial 18 F-FDG PET/CT scanning., (© 2019 by the Society of Nuclear Medicine and Molecular Imaging.)