Sense and nonsense of yT‐staging on MRI after chemoradiotherapy in rectal cancer

Abstract Aim The aim of this work was 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 pathological tumour regression grade (pTRG) of Mandard as a standard of reference. Method Twenty two radiologists, including five rectal MRI experts and 17 ‘nonexperts’ (general/abdominal radiologists), evaluated the ymrT stage on the restaging MRIs of 90 rectal cancer patients after chemoradiotherapy. The ymrT stage was compared with the final ypT stage at histopathology; the percentages of correct staging (ymrT = ypT), understaging (ymrT < ypT) and overstaging (ymrT > ypT) were 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 nonexperts; ymrT/ypT stage concordance was significantly higher in the pTRG4–5 subgroup (58% vs. 41% for the pTRG1–3 group; p = 0.01), with the best results for the MRI experts. Overstaging was the main source of error, especially in the pTRG1–3 subgroup (average overstaging rate 38%–44% vs. 13%–55% in the pTRG4–5 subgroup). IOA was higher for the expert versus nonexpert readers (α = 0.67 vs. α = 0.39). Conclusions ymrT‐staging is moderately accurate; accuracy is higher in poorly 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 would benefit from further clinical and endoscopic evaluation to guide final treatment planning.