MRI tumour regression grade 101 CHAPTER 6 Introduction In patients with locally advanced rectal cancer (LARC), the MRI based tumour regression grade (mrTRG), a five-tier imaging-based scoring systembased on the ability to distinguish between tumour and fibrosis, has proven to be reproducible amongst radiologists with a good interobserver agreement.1,2 Moreover, mrTRG has proven to be a prognostic factor for disease-free (hazard ratio (HR) 3.28; 95 percent CI 1.228.80) and overall survival (HR 4.40; 95 percent CI 1.65-11.7) in these patients, although the agreement between mrTRG and pathological tumour regression grade (pTRG) appeared suboptimal.2,3 It is unknownwhethermrTRGcan be used in the treatment decisionmaking for patients presenting with locally recurrent rectal cancer (LRRC). LRRC requires intensive neoadjuvant treatment comprising chemo(re)irradiation followed by extensive surgery.4–8 The goal of surgery is to achieve a resection with clear resection margins, as this is the most important prognostic factor for local recurrence-free and overall survival.9–11 Previous studies from our group showed that the addition of induction chemotherapy to the neoadjuvant treatment in patients with LRRC enhances tumour response.12,13 In addition, it was demonstrated that pTRG is an independent predictive variable for long-term oncological outcomes in LRRC patients.13 Obviously, pTRG can only be obtained postoperatively, and thus does not offer the opportunity to adapt treatment strategies. In that perspective, mrTRGmay bemore suitable in the decisionmaking process, as it provides an opportunity to consider non-operative therapy in case of a clinical complete response. Therefore, this study aimed to investigate the agreement between mrTRG and pTRG in a retrospective cohort of patients with LRRC treated with induction chemotherapy and chemo(re)irradiation. Also interobserver agreement between radiologists for mrTRG assessment was evaluated.