70 Chapter 5 Abstract Aim To investigate the value of rectal cancer T-staging on MRI after chemoradiotherapy (ymrT-staging) in relation to the degree of fibrotic transformation of the tumour bed as assessed using the pathologic tumour regression grade (pTRG) of Mandard as a standard of reference. Methods Twenty-two radiologists, including 5 rectal MRI experts and 17 ‘non-experts’ (general/ abdominal radiologists), evaluated the ymrT-stage on the restaging MRIs of 90 rectal cancer patients after chemoradiotherapy. The ymrT-stage was compared to the final ypT-stage at histopathology; the percentage of correct staging (ymrT=ypT), understaging (ymrT<ypT) and overstaging (ymrT>ypT) was calculated and compared between patients with predominant tumour at histopathology (pTRG4-5) and patients with predominant fibrosis (pTRG1-3). Interobserver agreement (IOA) was computed using Krippendorff’s alpha. Results Average ymrT-/ypT-stage concordance was 48% for the experts and 43% for the non-experts; ymrT/ypT-stage concordance was significantly higher in the pTRG4-5 subgroup (58% vs 41% for the pTRG1-3 group; p=0.01), with best results for the MRI experts. Overstaging was the main source of error, especially in the pTRG1-3 subgroup (average overstaging rate 38-44% versus 13-55% in the pTRG4-5 subgroup). IOA was higher for the expert versus non-expert readers (α=0.67 vs α=0.39). Conclusions • ymrT-staging is moderately accurate; accuracy is higher in poor responding patients with predominant tumour but low in good-responders with predominant fibrosis, resulting in significant overstaging. • Radiologists should shift their focus from ymrT-staging to detecting gross residual (and progressive) disease, and identifying potential candidates for organ-preservation who benefit from further clinical and endoscopic evaluation to guide final treatment planning.
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