eP211 - Bayesian Analysis of Factors Impacting Concordance of MRI and Histopathology in Determining Rectal Tumor Proximity to the Anterior Peritoneal Reflection

Purpose/Background: Rectal protocol MRI has become a crucial component of rectal cancer staging in recent years. Of the tumor characteristics described by MRI, tumor position relative to the anterior peritoneal reflection (aPR) is of particular value, playing a critical role along with tumor stage in determining the stratification of patients to either preoperative neoadjuvant therapy or upfront resection. There is a paucity of data, however, regarding the concordance of this radiologic description and final anatomic pathology.Methods/Interventions: A single center retrospective cohort study was performed examining rectal cancer patients diagnosed between 2020 and 2023. A total of 115 patients underwent surgery for rectal cancer during the study period. 78 of these patients had available data regarding tumor position relative to the aPR both on preoperative MRI and final pathology, which were included in the analysis. Tumor position relative to the aPR was recorded on both rectal protocol MRI and final pathology as “above”, “straddles” or “below”. Positive predictive value of MRI was determined in its ability to predict tumor position relative to the aPR on pathology. Patient demographics and tumor characteristics were collected and examined for significance with respect to their influence on the concordance of MRI and final pathology utilizing a Bayesian logistic regression model.Results/Outcomes: Among the 78 patients examined, 65.3% (n=51) were found to have concordant MRI and pathology reported tumor position relative to the aPR. 34 tumors were identified as below the aPR on MRI, of which, 26 had concordant final pathology (PPV = 76%). 18 patients had tumors reported above the aPR on MRI. Of these, only 10 (n=55.5%) were found to, in fact, be above the aPR on final pathology with 8 (n=44.4%) found to straddle the aPR (PPV = 55.5%). Among tumors identified above the aPR on MRI, controlling for tumor height, BMI, neoadjuvant status and tumor size on MRI, sex was found to be significantly associated with discordant final pathology on Bayesian logistic regression. Specifically, male patients were more likely to demonstrate discordant tumor position (95% credible interval -15.61, -2.35).Conclusion/

Discussion: In this series, tumors reported above the aPR on MRI demonstrated the highest discordance with tumor position on final pathology. These data suggest a limitation of MRI in correctly identifying tumor relationship to the aPR, particularly in the instance of higher tumors, specifically in male patients. We surmise that this limitation may be a result of MRI’s ability to exclusively identify the peritoneal reflection on its anterior surface, leaving the remainder of its circumference in question, ultimately risking inappropriate treatment stratification.Bayesian Logistic Regression of Factors Influencing MRI and Pathology Concordance