18 Chapter 2 [2] availability of diagnostic quality primary staging MRI including at least T2-weighted sequences in three planes (sagittal, coronal, transversal). To ensure a clinically representative sample of cases, patients were selected semi-randomly so that data from all 10 study centres were represented in the study cohort with sufficient variety in terms of clinical tumour stage. MR imaging MRI exams were carried out following the local protocols of the participating study centres at the time of inclusion. From the full scan protocols, we selected 2D T2-weighted sequences in sagittal, oblique-axial (perpendicular to the tumour axis), and obliquecoronal (parallel to the tumour axis) planes as these sequences are the minimum requirement recommended for primary rectal cancer staging as outlined in recent guidelines.1 Slice thickness ranged between 3-5 mm and in-plane resolution ranged between 0.35x0.35 and 0.94x0.94 mm. Image evaluation Images were evaluated using a web-based platform (iScore) designed by one of the authors (NEK) that combines the Open Health Imaging Foundation (OHIF) DICOM viewing platform16 with customizable electronic case report forms (eCRFs). For the current study, the structured reporting template for primary rectal cancer staging published by ESGAR was converted into an eCRF, comprised of twenty staging items, including fourteen categorical staging variables and six continuous variables (listed in detail in Table 1). Readers were asked to complete the staging eCRF for each study case. For the variables cT-stage, cN-stage, MRF involvement, EMVI, and presence of sphincter invasion, a confidence level was included in the staging (see Table 1). Links to relevant background information, including TNM staging definitions and the ESGAR consensus guidelines on MRI for rectal cancer were provided in iScore.1,4,17 Readers were blinded to each other’s results and outcomes of surgery and histopathology. Statistical analysis and standard of reference Statistical analyses were performed using R statistics version 4.1.0 (2021) and IBM SPSS version 27 (2020). Group interobserver agreement (IOA) for the continuous variables was calculated using Krippendorff’s alpha. For the categorical variables the percentage agreement between study readers was calculated and grouped into items with suboptimal agreement (<60%), moderate agreement (60%-80%) and good agreement (>80%). Correlation with histopathology was only available for a minority of patients (and for only a few of the studied staging variables) since most patients underwent neoadjuvant treatment prior to surgery. As such, two rectal MRI experts (DL and LC; each with >10 years of dedicated experience in rectal MRI and with a background in rectal cancer research and teaching) staged all study cases to establish a surrogate standard of
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