117 Predicting response to chemoradiotherapy in rectal cancer via visual morphologic assessment and staging on baseline MRI 7 Introduction Recently, there has been a shift towards more organ-preserving treatments for rectal cancer. Patients with advanced tumors who show clinical evidence of a complete response (CR) after neoadjuvant chemoradiotherapy (CRT) may be entered into a watchand-wait (W&W) program while patients with small tumor remnants may be cured with local treatment options such as transanal excision instead of major resection[1-3]. In addition, there are ongoing trials (such as the STAR-TREC trial) investigating the benefit of giving chemoradiotherapy to early-stage tumors with the aim of achieving organ preservation[4]. According to current guidelines, these early tumors are typically managed with direct surgery. These developments have urged the need to accurately monitor response after CRT, but have also given rise to an increased interest to predict treatment response before the start of CRT. If we could differentiate at baseline which patients are likely to respond well and which patients will be non-responders, this could aid in the selection of patients who would be good candidates for CRT while avoiding unnecessary side effects in anticipated non-responders. Pre-treatment knowledge of the anticipated treatment response could also help to optimize neoadjuvant treatment strategies further. Several studies have investigated the potential role of imaging and image biomarkers as pre-treatment predictors of response[5-9]. So far, these studies have mainly focused on functional imaging techniques such as diffusion-weighted imaging (DWI) and dynamic contrast enhanced (DCE) MRI, and on multiparametric imaging models developed using artificial intelligence (AI) methods such as radiomics[10-14]. Interestingly, several of these reports have shown that basic tumor descriptors such as the T- and N-stage, morphology and volume were amongst the variables showing the best potential to predict response[15-16].This indicates that visual morphologic interpretation by radiologists is not only crucial for staging but could also be helpful to render predictors of treatment response. Van Griethuysen et al. were one of the first to develop a method to estimate the likelihood of response based solely on radiologists’ visual interpretation and staging of baseline MRI scans[17]. They showed that a confidence scoring system taking into account the size, signal and shape of the tumor, T- and N-stage, mesorectal fascia (MRF) involvement and extramural vascular invasion (EMVI) could predict the chance of achieving a good or complete response to CRT on baseline MRI with areas under the curve (AUCs) of 0.67-0.83, when assessed by two expert radiologists. To the best of our knowledge, visual morphologic response prediction methods such as the one proposed by van Griethuysen have not yet been tested evaluated by larger groups of readers and/or using multicenter MRI data.
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