118 Chapter 7 This study therefore aims to evaluate the visual response prediction method of van Griethuysen et al. in a multicenter study setting and to compare it to two simplified adaptations of the same scoring system in terms of diagnostic performance and reproducibility amongst a large inter-national group of radiologists with varying levels of expertise. Methods Patient selection This retrospective diagnostic study was conducted as part of an ongoing institutional review board approved multicenter project focused on MRI for rectal tumor risk and response assessment, including the imaging and clinical outcome data of 1037 rectal cancer patients from ten centers in the Netherlands acquired between 2010-2018. For the current study, we identified from this cohort patients fulfilling the following inclusion criteria: [a] biopsy-proven non-mucinous rectal adenocarcinoma, [b] neoadjuvant treatment consisting of “routine” long course CRT (50.0-50.4 Gy with concurrent capecitabine based chemotherapy), [c] availability of diagnostic quality primary staging MRI including at least T2-weighted sequences in three planes (sagittal, coronal, transversal), and [d] availability of a final response outcome (histology after surgery or ≥2 years clinical follow-up in case of W&W treatment). From this group we semi-randomly selected a sample of n=90 patients to be included in the current study cohort, taking into consideration that data of all 10 study centers had to be represented and ensuring a clinically representative sample in terms of response outcomes to allow meaningful statistical analyses. This semi-random (selective) approach was chosen, because two of the ten centers are referral centers for W&W, which could have otherwise resulted in relative overrepresentation of complete responders in the cohort. Because of the retrospective nature of the study, informed consent was waived. MR imaging All MRIs were performed according to the local protocols of the participating centers at the time of inclusion. From the full protocols, we selected for this study the 2D T2-weighted spin echo sequences in sagittal, oblique-axial (perpendicular to the tumor axis), and oblique-coronal (parallel to the tumor axis) planes, in line with the minimal requirements for primary rectal cancer staging as outlined in recent guidelines[18]. Slice thickness ranged between 3-5 mm and in plane resolution between 0.35x0.35 and 0.94x0.94 mm.
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