72 Chapter 5 less valuable. Secondary aims are to evaluate interobserver agreement and the effect of radiologists’ experience levels on ymrT-staging accuracy. Methods Patient selection This study was performed as a side-study of a larger institutional review board-approved retrospective multicentre project, of which further details are provided in previous publications [14,15]. In short, 90 patients (from 10 centres) were selected using the following inclusion criteria: non-mucinous rectal adenocarcinoma (as mucinous tumours are known to exhibit completely different response patterns than solid tumours), neoadjuvant long course CRT (50-50.4 Gy with capecitabine-based chemotherapy), availability of a diagnostic quality restaging MRI (+ primary staging MRI for comparison) including – in line with current guideline recommendations for restaging [2] – T2-weighted sequences in 3 orthogonal planes and a DWI sequence, and availability of a final response outcome consisting of either histopathological tumour regression grade (TRG) after surgery or ≥2 years clinical follow-up in case of watch-and-wait treatment to confirm a persistent complete response. Patients were selected semi-randomly from a larger study cohort taking into consideration that data of all 10 study centres had to be represented and ensuring a clinically representative patient sample. Due to the retrospective nature of the study informed consent was waived. MRI imaging The restaging MRIs were performed according to the local protocols of the participating centres at the time of inclusion. These included 2D T2-weighted sequences in sagittal, axial (perpendicular to the tumour axis), and coronal (parallel to the tumour axis) planes with a slice thickness ranging between 3-5 mm and in-plane resolution ranging between 0.35x0.35 and 0.94x0.94 mm. DWI series were acquired in the same axial plane as the axial T2W series and with at least one high b-value (b600-1200) and a slice thickness ranging from 2.7-8.0 mm. Apparent Diffusion Coefficient (ADC) maps calculated from the DWI series were also routinely available. Image evaluation Twenty-two radiologists from 14 different countries participated as study readers, including five ‘experts’ (dedicated abdominal radiologists, specialized in oncologic and/or GI imaging, each with ≥10 years dedicated experience in reporting, research and teaching of rectal MRI), and seventeen ‘non-experts’ (seven abdominal radiolo-
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