Comparison between MRI and pathology in the assessment of tumour regression grade in rectal cancer

Background: Limited data exist regarding the correlation between MRI tumour regression grade (mrTRG) and pathological TRG (pTRG) in rectal cancer. Methods: mrTRG and pTRG were compared in rectal cancer patients from two phase II trials (EXPERT and EXPERT-C). The agreement between radiologist and pathologist was assessed with the weighted k test while the Kaplan-Meier method was used to estimate survival outcomes. Results: One hundred ninety-one patients were included. Median time from completion of neoadjuvant treatment to pre-operative MRI and surgery was 4.1 weeks (interquartile range (IQR): 3.7-4.7) and 6.6 weeks (IQR: 5.9-7.6), respectively. Fair agreement was found between mrTRG and pTRG when regression was classified according to standard five-tier systems (kappa = 0.24) or modified three-tier systems (kappa = 0.25). Sensitivity and specificity of mrTRG 1-2 (complete/good radiological regression) for the prediction of pathological complete response was 74.4% (95% CI: 58.8-86.5) and 62.8% (95% CI: 54.5-70.6), respectively. Survival outcomes of patients with intermediate pathological regression (pTRG 2) were numerically better if complete/good regression was also observed on imaging (mrTRG 1-2) compared to poor regression (mrTRG 3-5) (5-year recurrence-free survival 76.9% vs 65.9%, P = 0.18; 5-year overall survival 80.6% vs 68.8%, P = 0.22). Conclusions: The agreement between mrTRG and pTRG is low and mrTRG cannot be used as a surrogate of pTRG. Further studies are warranted to assess the ability of mrTRG to identify pathological complete responders for the adoption of non-operative management strategies and to provide complementary prognostic information to pTRG for better risk-stratification after surgery.