78 Chapter 5 The average ymrT-stage for all readers was significantly higher than the final ypT-stage at histopathology (p=0.03), with an average overstaging rate of 35%. Most cases that were overstaged on MRI concerned ypT0 and ypT1-2 tumours. Overall ymrT- /ypT-stage concordance was higher for the pTRG4-5 patients (58%) compared to the pTRG1-3 subgroup (41%), with significantly better staging performance in the pTRG4-5 subgroup (log OR = -1.0, p=0.01, see Supplement 1). Overstaging rates were higher in pTRG1-3 patients (39%) than in pTRG4-5 patients (15%; log OR = 2.7, p=0.002). When looking at the impact of reader experience, overall staging results were better for the expert readers (log OR = 0.28, p=0.02). mrTRG versus pTRG Of the 17 patients with predominant tumour (pTRG4-5), on average 34% (range 0-77%) of readers classified these patients concordantly as mrTRG4-5 on MRI; an additional 46% (range 23-86%) were classified as mrTRG3. Complete and near-complete responders Table 3 shows how well MRI was able to discriminate patients with a complete response (yT0) from patients with residual tumour (yT1-4) in relation to the pTRG. Of the 27 patients with a complete response, on average only 33% were correctly diagnosed as such on MRI and 67% were overstaged. The majority of patients with residual tumour (ypT1-4) at histopathology were correctly identified as such on MRI, with better ymrT/ ypT concordance with increasing pTRG scores (up to 90-99% in the pTRG4-5 subgroup). Twenty-two patients had tumour remnants that were limited to the bowel wall at histology (ypT1-2); these included five ypT1 cases (3 pTRG2; 2 pTRG3) and seventeen ypT2 cases (5 pTRG5, 9 pTRG3, 2 pTRG4, 1 pTRG5). Seventeen patients had a near-complete tumour regression based on histology (pTRG2), these included 3 ypT1, 5 ypT2 and 9 ypT3 cases. On average, 44% of pTRG2 cases were also classified as mrTRG2 on MRI. Table 3 Performance of MRI to differentiate between complete response and residual tumour in relation to pTRG. All readers (n=22) Experts (n=5) Non-experts (n=17) Total patients (n=90) ymrT0 ymrT1-4 ymrT0 ymrT1-4 ymrT0 ymrT1-4 pTRG1 (n=27) 33% 67% 30% 70% 34% 66% pTRG2 (n=17) 32% 68% 27% 73% 33% 67% pTRG3 (n=29) 13% 87% 3% 97% 16% 74% pTRG4 (n=15) 8% 92% 1% 99% 10% 90% pTRG5 (n=2) 5% 95% 10% 90% 3% 97%
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