119 Predicting response to chemoradiotherapy in rectal cancer via visual morphologic assessment and staging on baseline MRI 7 Image evaluation MRIs were assessed by twenty-two radiologists from 14 different countries, including five rectal MRI-experts (each with ≥10 years’ dedicated experience in rectal MRI and rectal cancer research) and 17 abdominal radiologists or general radiologists with a specific interest in abdominal imaging. The 17 abdominal/general radiologists had a median of 6 years’ experience in reading rectal MRI (range 1.5-21 years) with an estimated median of 100 (range 50-250) rectal MRI cases read on a yearly basis. Study readers were recruited via an open call to members of the European Society of Gastrointestinal and Abdominal Radiology (ESGAR), in specific those with an interest in rectal imaging. Image evaluation was performed using an in-house developed web-based platform (iScore) that was designed by one of the authors (N.E.K.) and incorporates the Open Health Imaging Foundation (OHIF) DICOM viewing platform[19]. Study readers were asked to review the baseline MRIs of the 90 study cases using electronic case report forms (eCRFS) that were embedded into iScore. These eCRFs included three different scoring methods designed to estimate the likelihood that patients would achieve a complete or near-complete response to chemoradiotherapy based on the overall tumor risk profile. The first scoring method was the 5-point confidence score published by van Griethuysen et al. that is based on a combination of tumor size, signal heterogeneity, shape (regular/irregular), T-stage, N-stage, EMVI and MRF invasion [17]. The second scoring method was a simplified 4-point adaptation, taking into account only MRF invasion, high risk T-stage, EMVI and N-stage. The final scoring method was a further simplified, dichotomized (2-point score) adaptation. Full details of the three scoring methods are provided in Table 1 and supporting images are provided in Figure 1 and Figure 2. A visual representation of the scoring setup in iScore including the full eCRFS is provided in Supplement 1. Readers were asked to indicate for each individual case whether they found the respective scoring methods easy, moderately easy/difficult, or difficult to apply. Finally, after completion of all cases they were asked to give an overall indication of which scoring method they would prefer to use in their daily clinical practice. Readers were blinded to each other’s scorings and to the final response outcomes.
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