71 Sense and nonsense of yT-staging on MRI after chemoradiotherapy in rectal cancer 5 Introduction Since the introduction of organ preserving treatments such as watch-and-wait (W&W) for rectal cancer patients who respond very well to neoadjuvant treatment, response evaluation has become an increasingly important topic [1]. The aim of response evaluation in this setting is to distinguish complete (or near-complete) responders from patients with significant residual tumour to select which patients could be potential candidates for organ preservation and which patients require routine surgical resection. In most radiological staging templates it is common practice to specify the local tumour response by redefining the T stage on MRI after chemoradiotherapy (CRT); the ymrT- stage [2–5]. However, the implications of ymrT-staging are less evident than for primary T-staging at baseline [6]. Limited reports with mixed results are available on the prognostic significance of ymrT-staging to predict long-term outcomes such as overall or disease-free survival [7,8]. When confirmed endoscopically, the MRI diagnosis of a yT0-stage can help select W&W candidates [9,10]. Patients with adequate T-downstaging (to yT0-1) after neoadjuvant therapy may benefit from local excision instead of radical resection, though results from the GRECCAR2 trial have shown that patient selection based on ymrT-staging is challenging and can lead to significant undertreatment [11]. Most patients still have residual tumour after neoadjuvant therapy and require complete surgical resection. For these patients, the main role of restaging with MRI is to assess whether there is persistent invasion into adjacent organs and structures to help guide the final surgical resection strategy. Definitions outlined in the American Joint Committee on Cancer (AJCC) and Union of International Cancer Control (UICC) Tumour Node Metastasis (TNM) staging system provide no separate guidelines to classify the yT-stage after neoadjuvant treatment. Definitions used are identical to those used for baseline T-staging and based on the degree of tumour extension into and beyond the different layers of the rectal wall. However, fibrotic transformation of the tumour bed following radiotherapy greatly limits the ability of MRI to separately visualize these layers and identify areas of viable residual tumour within, resulting in overall poor reported performance of MRI for ymrT-staging [12,13]. The precise correlation between the degree of fibrotic transformation and the diagnostic accuracy of MRI for yT-restaging has not yet been clearly described. One could argue that in patients with predominant fibrosis, the performance of MRI for yT-staging will likely be lowest, while in patients with poorer degrees of tumour regression and more solid tumour remnants, MRI will be more valuable to estimate the yT-stage. This multicentre and multireader study aims to test this hypothesis and assess the value of MRI for yT-staging in relation to the degree of tumour regression at histopathology to establish if there are specific response subgroups in which ymrT-staging is more or
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