MRI tumour regression grade 103 CHAPTER 6 The type and extent of the surgery was left to the discretion of the treating surgical oncologist. Intraoperative electron beamradiotherapywas delivered in a dose of 10-12.5 Gy when there were no clear resection margins or when there was tumour adherence to unresectable structures. Radiological and pathological assessment An MRI was performed at baseline, after finishing induction chemotherapy, and 4-6 weeks after completion of the neoadjuvant (chemo)radiotherapy and consisted of at least T2-weighted axial, coronal and sagittal planes performed on a 1.5T or 3T MRI system. MRIs were performed either in the tertiary referral hospital or in the referring hospital and were reassessed by an experienced abdomen radiologist with specific expertise in LARC and LRRC. Response was scored according to the mrTRG; mrTRG 1: low signal fibrosis only, no tumour signal, mrTRG 2: more than 75 percent fibrosis and minimal tumour signal intensity, mrTRG 3: 50 percent tumour/fibrosis, mrTRG 4: less than25 percent fibrosis, predominant tumour signal, mrTRG 5: no fibrosis.2 The radiologist was trained using mrTRG in primary tumours in a training programme, including post-neoadjuvant treatment reporting, conducted by leading experts in this field.2 To evaluate the reproducibility of the mrTRG in LRRC a second experienced abdomen radiologist, who was also trained, independently assessed all imaging using the mrTRG. The radiologists were blinded for the pathological assessment and the clinical outcomes. All specimens were revised by a specialized pathologist who was blinded to the radiological assessment as well as the clinical outcomes. On the primary assessment, in general, at least one section per centimetre maximum tumour bed diameter was sampled. The pathological response grade (pTRG) was scored according to theMandard classification; pTRG 1: complete response, pTRG 2: isolated cell nests, pTRG 3: more residual cancer cells but fibrosis still predominates, pTRG4: residual cancer outgrowing fibrosis, pTRG 5: absence of regressive changes.14 Outcomes of interest Outcomes of interest were the agreement between mrTRG and pTRG, and the interobserver radiological agreement. In addition, a subanalysis was performed to assess the agreement betweenmrTRG and pTRG in patients with a long interval versus a short interval between MRI and surgery, based on median interval values.