81 Sense and nonsense of yT-staging on MRI after chemoradiotherapy in rectal cancer 5 of tumours that were downstaged to ≤ypT3 at histopathology, were still falsely staged as T4 on MRI. Based on these data, restaging MRI reports should be interpreted with caution and surgeons should also critically take the findings of the baseline MRI into account when planning their resections. Moreover, feedback and correlation between radiology and final pathology should be promoted so that radiologists become more aware of their performance and limitations, and learn from their errors. Previous reports with smaller numbers of study readers have shown relatively low IOA for ymrT-staging after neoadjuvant treatment, with kappas in the range of 0.20-0.41 [24,27]. In our current study, IOA was assessed in a large group of readers including both dedicated rectal MRI experts as well as more general and abdominal radiologists. Interestingly, though overall IOA was in line with previous reports, agreement was substantially better for the more expert radiologists who also showed consistently better concordance with histopathology, stressing the importance of experience level and dedicated radiological training in restaging rectal cancer. There are some limitations to our study design in addition to its retrospective nature and the relatively low number of study subjects, especially in the TRG4-5 subgroup, which constituted only 19% of the study group.The pathology reports did not include a subclassification in case of pT3 disease. As such, we had to simplify the mrT-staging for correlation with histopathology. Our cohort dates back as far as 2010, which entails that some scans were acquired with outdated study protocols leading to inherent variation in image quality. Performing an in-depth analysis of this variation was beyond the scope of this paper, but we acknowledge that it may have introduced potential bias. Readers evaluated the T2-weighted and DWI concordantly, which does not allow subanalyses to investigate the benefit and limitations of these two separate techniques. Previous reports have shown that addition of DWI can improve the differentiation between complete responders and patient with residual tumour in case of fibrosis, but there is no solid evidence that DWI also has added value for further yT-staging [28]. This remains a question to be addressed by future studies. Finally, our study findings are only applicable to solid (non-mucinous) adenocaricnomas. As mucinous tumours exhibit distinctly different signal and response patterns, investigating the value of ymrT-staging in this subgroup was beyond the scope of the current project. In conclusion, this study shows that MRI is mainly valuable to establish the presence of residual tumour and has reasonable accuracy – especially in more experience hands – for yT-staging in poor responding patients with predominant solid tumour and limited fibrosis after CRT. In good responders who show predominant fibrosis, ymrT-staging is highly inaccurate and results in substantial overstaging. As such we would advise radiologists to shift their focus from detailed ymrT-stage assessment to detecting gross residual and progressive disease, and identifying any potential candidates for organ-preservation who benefit from further clinical and endoscopic evaluation to guide final treatment planning.
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