MRI restaging of rectal cancer: The RAC (Response–Anal canal–CRM) analysis joint consensus guidelines of the GRERCAR and GRECCAR groups

For a long time, surgical strategies for locally advanced rectal cancers (LARCs) were mainly determined on the basis of findings at baseline staging magnetic resonance imaging (MRI) examination [1]. However, after external chemoradiotherapy (ECRT), most patients demonstrate variable degrees of tumor response, including complete response in 4–31% of them [2,3]. MRI excels, in conjunction with endoscopy, in identifying poor responder who may be referred to consolidation therapy and complete responders who can potentially undergo organ sparing treatment [4,5]. In addition, MRI helps redefine surgical strategy as downstaging and retraction from previous involved structures such as mesorectal fascia (MRF) or sphincter involvement, may alter the initial surgical plan. As such, MRI restaging after neoadjuvant therapy (NAT) has become a critical issue to define tailored therapies and propose a more personalized approach [6], [7], [8]. However, the evaluation of the tumor response after NAT is challenging to assess, especially for non-expert radiologists [9]. Interpretation of MRI examination after ECRT is well-known to be hampered by difficulties in discerning fibrosis from residual disease. Different MRI interpretation and classification systems have been suggested focusing on specific morphological patterns on T2-weighted (T2W) images (including MR tumor regression grade [mrTRG]) and/or signal patterns on diffusion-weighted imaging (DWI) to assess response after ECRT. In order to improve accuracy of radiology reports, the European Society of Gastrointestinal Abdominal Radiology (ESGAR) and the Society of Abdominal Radiology have published/updated guidelines regarding rectal MRI staging and restaging [10,11]. However, both guidelines lack practical tumor response descriptions and assessments. Particularly, the recent concept of “near-complete response” driven by the observation that a significant proportion of patients with a very good but incomplete response at first assessment (i.e., six to eight weeks after ERCT) may convert into a complete response if given a longer interval and that re-assessment should be more detailed [12]. It was recently pointed out that the terminology, criteria and features used to describe a near CR present wide variations [13]. As such, the aim of this joint paper was to propose recommendations for rectal cancer MRI restaging in line with clinical requirements. Our intent was first to present a practical resource for radiologists with some tips for high quality interpretation at MRI restaging and second improve reproducibility in reporting among radiologists for / or ‘involved in’ GRECCAR multicenter clinical trials.

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