MRI tumour regression grade 113 CHAPTER 6 Discussion This retrospective study aimed to investigate the correlation between the mrTRG and pTRG in patients with LRRC after treatment with induction chemotherapy and chemo(re)irradiation. A fair to moderate agreement between mrTRG and pTRG was observed, suggesting that the predictive value for pTRG is limited. Moreover, the interobserver agreement between the two radiologists was fair tomoderate, indicating low reproducibility. However, there was a good agreement between the radiological assessment and pathology when the interval between MRI and surgery is short (≤7 weeks) and when assessed by the lead radiologist, mrTRG can safely predict good responders (PPV 95 percent). Radiological evaluation of LRRC is often difficult due to postoperative changes in anatomy, previous radiotherapy, and the presence of fistula and/or abscesses. This hampers not only the initial assessment, but also makes evaluation of the mrTRG score more difficult. Despite those difficulties, the agreement between mrTRG and pTRG in this study (k = 0.30 and k = 0.25 for the lead radiologist and the other abdomen radiologist respectively) was comparable with the literature on LARC (k = 0.24).3 Surgery forLRRCgenerally involves resectionofmultipleorgansaswell as soft tissue, bony andvascularresections, resultingincomplexproceduresandthenecessityofreconstructive surgery. This isassociatedwithahighpostoperativemorbidityrateandan impairedquality of life.10,15,16 Recently, it was reported that patients with LRRC with a pathological complete response have excellent long-term survival.13 Preoperative prediction of the pathological response potentially provides an opportunity to adopt a non-operative treatment strategy inpatientswith a clinical complete response, whichmay be very valuable in the light of the complexity and impact on quality of life of LRRC surgery. To select patients with a clinical complete response, a high PPV is especially important, as a false-positive prediction can lead to undertreatment with possible disastrous consequences. In the present study, the mrTRGhad aPPV for a good response of 95percent when assessedby the lead radiologist; underestimation of the presence of residual tumour occurred in only one patient. This suggests that themrTRG score has the potential to safely predict good responders. However, in the present study overstaging was, as in LARC, much more frequent: in 17 percent of the patients the presence of residual tumour was overestimated when using mrTRG.17 In LARC, endoscopy and a digital exam may aid in assessing the response.18 However, in LRRC, these diagnostic modalities are usually not sufficient due to the location and/or extent of the tumour and decisions therefore have to be made solely based on the assessment of the MRI. An MR grading system incorporating T2-weigthed
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