Magnetic resonance tumour regression grade and pathological correlates in patients with rectal cancer

Background: Evidence to support the specific use of magnetic resonance tumour regression grade (mrTRG) is inadequate. The aim of this study was to investigate the pathological characteristics of mrTRG after chemoradiotherapy (CRT) for rectal cancer and the implications for surgery. Methods: Patients undergoing long-course CRT (45-50 Gy plus a booster dose of 4-6 Gy) for mid or low rectal cancer (cT3-4 or cN+ without metastasis) between 2011 and 2015 who had post-CRT rectal MRI before surgery were included retrospectively. Three board-certified experienced radiologists assessed mrTRG. mrTRG was correlated with pathological tumour regression grade (pTRG), ypT and ypN. In a subgroup of patients with mrTRG1-2 and no tumour spread (such as nodal metastasis) on MRI, the projected rate of completion total mesorectal excision (TME) if they underwent transanal excision (TAE) and had a ypT status of ypT2 or higher was estimated, and recurrence-free survival was calculated according to the operation (TME, or TAE) that patients had actually received. Results: Some 439 patients (290 men and 149 women of mean(s.d.) age 62.2(11.4) years) were analysed. The accuracy of mrTRG1 for predicting pTRG1 was 61 per cent (40 of 66), and that for ypT I or less was 74 per cent (49 of 66). For mrTRG2, these values were 22 3 per cent (25 of 112) and 36.6 per cent (41 of 112) respectively. Patients with mrTRG1 and mrTRG2 without tumour spread were ypN+ in 3 per cent (1 of 29) and 16 per cent (8 of 50) respectively. Assuming mrTRG1 or mrTRGI-2 with no tumour spread on post-CRT MRI as the criteria for TAE, the projected completion TME rate was 26 per cent (11 of 43) and 41.0 per cent (41 of 100) respectively. For the 100 patients with mrTRG1-2 and no tumour spread, recurrence-free survival did not differ significantly between TME (79 patients) and TAE (21) (adjusted hazard ratio 1.86, 95 per cent c.i. 042 to 8.18). Conclusion: Patients with mrTRG1 without tumour spread may be suitable for TAE.