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Current guidelines recommend restaging with MRI after neoadjuvant therapy for rectal cancer, but the accuracy of restaging MRI in estimating circumferential margin involvement requires additional clarification.The objective of this study was to measure the accuracy of circumferential resection margin assessment by MRI after neoadjuvant therapy and identify characteristics associated with accuracy.MRI data were retrospectively analyzed for concordance with the findings of whole-mount pathology analysis of the corresponding surgical specimens. Univariate and multivariate logistic regression analyses were performed to identify characteristics associated with accuracy.This study was conducted at a comprehensive cancer center.Included in the study were consecutive patients who underwent total mesorectal excision for rectal cancer between January 2018 and March 2020 after receiving neoadjuvant therapy and undergoing restaging with MRI.The primary outcome of this study included accuracy, sensitivity, specificity, and positive and negative predictive values for categorizing the circumferential resection margin as threatened; mean and paired mean differences were in proximity of the margin.Of the 94 patients included in the analysis, 39 (41%) had a threatened circumferential resection margin according to MRI at restaging, but only 17 (18%) had a threatened margin based on pathology. The accuracy of MRI in identifying a threatened margin was 63.8%, with margin proximity overestimated by 0.4 cm on average. In multivariate logistic regression, anterior location of the margin and tumor proximity to the anal verge were independently associated with reduced MRI accuracy.A limitation was the retrospective design at a single institution.The knowledge that MRI-based restaging after neoadjuvant therapy overestimates circumferential margin proximity may render some surgical radicality unnecessary and thereby help avoid the associated morbidity. With the recognition that MRI-based assessment of margin proximity may not be reliable for anterior margin and for distal tumors, radiologists may want to use greater caution in interpreting images of tumors with these characteristics and to acknowledge the uncertainty in their reports. See Video Abstract at http://links.lww.com/DCR/B814.ANTECEDENTES:Las pautas actuales recomiendan la re-estadificación por medio de la resonancia magnética luego de terapia neoadyuvante en los casos de cáncer de recto, pero la precisión de la reevaluación con la IRM para estimar el grado de implicación del margen circunferencial requiere aclaraciones adicionales.OBJETIVO:Medir el grado de exactitud en la evaluación del margen de resección circunferencial mediante resonancia magnética después de la terapia neoadyuvante e identificar las características asociadas con la precisión.DISEÑO:Se analizaron retrospectivamente los datos de resonancia magnética para determinar la concordancia entre los hallazgos del análisis de la pieza de anatamopatología y las muestras quirúrgicas correspondientes. Se realizó el análisis de regresión logística univariada y multivariada para identificar las características asociadas con la exactitud.AJUSTE:Centro oncológico integral.PACIENTES:Todos aquellos que se sometieron consecutivamente a una excisión total del mesorrecto por cáncer rectal entre Enero 2018 y Febrero 2020 luego de recibir terapia neoadyuvante y someterse a una re-estadificación por imágenes de resonancia magnética (IRM).PRINCIPALES MEDIDAS DE RESULTADO:La exactitud, la sensibilidad y especificidad; los valores predictivos positivos y negativos para categorizar el margen de resección circunferencial como amenazado; la diferencia media y las medias pareadas de proximidad a los margenes.RESULTADOS:De los 94 pacientes incluidos en el análisis, 39 (41%) tenían un margen de resección circunferencial amenazado según la resonancia magnética en la re-estadificación, pero solo 17 (18%) tenían un margen amenazado basado en la patología. La precisión de la resonancia magnética para identificar un margen amenazado fue del 63,8%, con la proximidad del margen sobreestimada en 0,4 cm en promedio. En la regresión logística multivariada, la ubicación anterior de los bordes de resección y la proximidad del tumor al margen anal se asociaron de forma independiente con la reducción en la precisión de la resonancia magnética.LIMITACIONES:Diseño retrospectivo en una institución única.CONCLUSIONES:El saber que la re-estadificación basada en la IRM, luego de terapia neoadyuvante sobreestima la proximidad de la lesión a los márgenes circunferenciales, hace innecesaria cierta radicalidad quirúrgica complementaria, lo que ayuda a evitar morbilidad asociada. Reconociendo que la evaluación de proximidad de los márgenes de resección basada en la resonancia magnética, no puede ser confiable en casos de márgenes anteriores y en casos de tumores distales. Los radiólogos recomiendan tener más precaución en la interpretación de imágenes de tumores con estas características y reconocen cierto desasosiego en sus informes. Consulte Video Resumen en http://links.lww.com/DCR/B814.

3509 Background: Clinical response following neoadjuvant therapy is paramount to identifying locally advanced rectal cancer (LARC) patients suitable for Watch and Wait (WW). A 3-tier schema was devised to stratify clinical response. Patients with a complete clinical response (cCR) are considered for WW, while those with an incomplete clinical response (iCR) are recommended for total mesorectal excision (TME). A near complete response (nCR) tier captures patients with significant, but not complete, response to be considered for WW. This schema’s efficacy has yet to be validated. We investigated survival and organ preservation (OP) rates based on this 3-tier clinical response assessment in patients with LARC who underwent total neoadjuvant therapy (TNT) in a prospective, multi-center clinical trial. Methods: Patients with MRI stage II and III rectal adenocarcinoma were randomized to either induction chemotherapy (FOLFOX or CAPEOX) followed by chemoradiation or chemoradiation followed by consolidation chemotherapy (FOLFOX or CAPEOX). At 8+/-4 weeks following TNT, response on digital rectal and endoscopic examinations was evaluated by the 3-tier schema. The date of this restaging clinical response assessment was used as time zero. The endpoints of rate of OP, disease-free survival (DFS), TME-free DFS, and overall survival (OS) were evaluated using the Kaplan-Meier method with differences analyzed by the log-rank test. Results: Clinical response assessments were available for 294 patients. The median time to assessment after neoadjuvant therapy was 7.9 weeks. Based on the 3-tier schema, 124 patients were categorized as cCR, 113 as nCR, and 57 as iCR. Baseline age, sex, average distance from the anal verge, clinical T classification, and clinical N classification were similar between the response groups. The table shows the 3-year rates of OP, DFS, TME-free DFS, and OS. The median follow-up was 2.36 years. Of the patients with a nCR, the 3-year TME rate was 48% compared with 21% in the cCR group. Conclusions: The 3-tier clinical response assessment has prognostic implications for OP and DFS in patients with LARC who underwent TNT. In patients with a nCR, more than half achieved OP at 3 years. This information should be utilized to counsel patients regarding their expected outcomes. Clinical trial information: NCT02008656. [Table: see text]

4008 Background: Organ preservation (OP) with a watch and wait strategy (WW) and total neoadjuvant therapy (TNT) are new treatment paradigms for patients with locally advanced rectal cancer. The safety and efficacy of WW and of TNT have not been studied prospectively. Methods: Patients with MRI stage II and III rectal adenocarcinoma were randomized to 4 months of FOLFOX or CAPEOX before (Induction) or after (Consolidation) fluorouracil or capecitabine based chemoradiotherapy (CRT). Patients were re-staged 8-12 weeks after finishing TNT with digital rectal exam, flexible sigmoidoscopy and MRI. Patients with complete or near-complete clinical response were offered WW. Those with incomplete response had total mesorectal excision. The trial was designed so that each arm served as its own single-stage study that discriminates between 3-year disease-free survival (DFS) rates of 75% (historical null) and 85%, with 86% power, and a two-sided type I error of 5%. Secondary objectives included comparing DFS, OP, and distant metastasis-free survival (DMFS) rates between the two arms using the log-rank test. Results: Of 324 patients enrolled, 307 (152 I, 155 C) are currently evaluable for the time-to-event analysis as of 2/1/2020. Median follow-up is 2.1 years; 52 DFS events were observed. Patient demographics and tumor characteristics were generally balanced across the two arms. Full compliance with systemic chemotherapy was 82% and 81% for the I- and C-arms, respectively. The median radiation dose was 5400 cGy for both arms. Table shows 3-y DFS, DMFS, and OP rates. Conclusions: A WW strategy for patients with locally advanced rectal cancer that achieve a clinical complete response to TNT results in organ preservation for a high proportion of patients without compromising survival. Up-front CRT followed by consolidation chemotherapy resulted in a numerically higher WW rate compared to induction chemotherapy followed by CRT. Clinical trial information: NCT02008656 . [Table: see text]