123 Predicting response to chemoradiotherapy in rectal cancer via visual morphologic assessment and staging on baseline MRI 7 Standard of reference The main outcome of this study was to predict a (near-)complete response, which was defined as the absence of viable cancer cells, or presence of only rare or small clusters of residual cancer cells at histopathology after surgery. The primary standard of reference in the patients that had undergone surgery was the histopathological Mandard tumor regression grade (TRG), where a (near-)complete response was defined as TRG 1-2 [20]. In patients undergoing W&W, a sustained clinical complete response with a local regrowth-free follow-up period of ≥2 years was considered a surrogate endpoint of a complete response (TRG1). Statistical analysis Statistical analyses were performed using R statistics version 4.1.0 (2021) and IBM SPSS version 27 (2020). The scores from the 22 radiologists were averaged for each patient in order to produce a probability of response that was then used to compute Receiver Operator Characteristics (ROC) curves and calculate mean areas under the curve (AUC) for each scoring method. Optimal cut-off values for the 5-point and 4-point scores were derived from the ROC-curves to calculate sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and overall accuracy to predict a (near-)complete response (being the positive study outcome). Results were separately analyzed for the five MRI-experts versus the 17 less experienced readers and mixed model linear regression was used to assess the impact of reader experience on the diagnostic accuracy figures of each scoring method. To account for the repeated measurements of each patient, a patient-level random intercept was used. A logistic regression was performed to analyze the possibility of an association between the diagnosis accuracy and the interval between completion of CRT and final surgery/entry into a W&W program. To do so, the proportion of correct diagnoses for each patient and method was computed across all readers. This proportion was then used as response and interval between completion of CRT and final surgery/entry into a W&W program was used as a covariate. P-values <0.05 were considered statistically significant. Group interobserver agreement (IOA) was calculated using Krippendorff’s alpha (α). Results Patient characteristics Table 2 shows the baseline characteristics of the 90 study patients. Fifty-two patients (58%) were male; median age was 65 years (range 41-82). Forty-four patients (49%) were (near-)complete responders, including 27 (30%) complete responders (21 after surgery; 6 clinical complete responders undergoing W&W). Mean time interval between completion of CRT and surgery (or inclusion into a W&W program) was 11±2.5 weeks.
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