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Current delineation of the gross tumor volume (GTV) in esophageal cancer relies on computed tomography (CT) and combination with 18 F-fluorodeoxyglucose (FDG) positron emission tomography (PET). There is increasing interest in integrating magnetic resonance imaging (MRI) in radiation treatment, which can potentially obviate CT-or FDG-PET/CTebased delineation. The aim of this study is to evaluate the feasibility of target delineation on T2-weighted (T2W) MRI and T2W including diffusion-weighted MRI (T2W þ DW-MRI) compared with current-practice FDG-PET/CT. Methods: Ten observers delineated primary esophageal tumor GTVs of 6 patients on FDG-PET/ CT, T2W-MRI, and T2W þ DW-MRI. GTVs, generalized conformity indices, in-slice delineation variation (root mean square), and standard deviations in the position of the most cranial and caudal delineated slice were calculated. Results: Delineations on MRI showed smaller GTVs compared with FDG-PET/CTebased delineations. The main variation was seen at the cranial and caudal border. No differences were Disclosures: The authors have no conflicts of interest to disclose.

Background Patients with a pathological complete response (pCR) after neoadjuvant chemoradiotherapy (nCRT) for oesophageal cancer may benefit from non‐surgical management. The aim of this study was to determine the diagnostic performance of visual response assessment of the primary tumour after nCRT on T2‐weighted (T2W) and diffusion‐weighted (DW) MRI. Methods Patients with locally advanced oesophageal cancer who underwent T2W‐ and DW‐MRI (1·5 T) before and after nCRT in two hospitals, between July 2013 and September 2017, were included in this prospective study. Three radiologists evaluated T2W images retrospectively using a five‐point score for the assessment of residual tumour in a blinded manner and immediately rescored after adding DW‐MRI. Histopathology of the resection specimen was used as the reference standard; ypT0 represented a pCR. Sensitivity, specificity, area under the receiver operating characteristic (ROC) curve (AUC) and interobserver agreement were calculated. Results Twelve of 51 patients (24 per cent) had a pCR. The sensitivity and specificity of T2W‐MRI for detection of residual tumour ranged from 90 to 100 and 8 to 25 per cent respectively. Respective values for T2W + DW‐MRI were 90–97 and 42–50 per cent. AUCs for the three readers were 0·65, 0·66 and 0·68 on T2W‐MRI, and 0·71, 0·70 and 0·70 on T2W + DW‐MRI ( P = 0·441, P = 0·611 and P = 0·828 for readers 1, 2 and 3 respectively). The κ value for interobserver agreement improved from 0·24–0·55 on T2W‐MRI to 0·55–0·71 with DW‐MRI. Conclusion Preoperative assessment of residual tumour on MRI after nCRT for oesophageal cancer is feasible with high sensitivity, reflecting a low chance of missing residual tumour. However, the specificity was low; this results in overstaging of complete responders as having residual tumour and, consequently, overtreatment.