Background: Baseline rectal MRI is routinely used for tumor staging, selecting the initial treatment and prognostication of patients with rectal adenocarcinoma. However, associations of baseline clinical and MRI variables with organ preservation (OP) in a prospective randomized trial have not been reported. Methods: Patients with MRI staged clinical stage II or III rectal adenocarcinoma treated in a prospective phase II clinical trial were randomized to either induction chemotherapy (FOLFOX or CAPEOX) followed by chemoradiation or chemoradiation followed by consolidation chemotherapy (FOLFOX or CAPEOX) and recommended total mesorectal excision (TME) or watch-and-wait (WW) based on clinical response. The primary outcome was OP, defined as the time from randomization to the clinical decision of TME or last follow-up, whichever occurred first. Clinical variables included: age, sex, race, tumor grade, radiation dose and treatment arm. Radiological variables (evaluated by baseline MRI prior to randomization) included: distance from the anal verge, tumor length, clinical T stage, clinical N stage, relationship to the mesorectal fascia, presence of extramural vascular invasion (EMVI) and mucinous radiographic findings (>50%). Associations of OP with clinical and radiological variables were assessed utilizing the Cox regression model in univariate and multivariate settings. The final multivariate model was chosen using backward selection. Results were analyzed by intention to treat. Results: Case report forms (CRFs) from baseline MRI were available for 289 of 324 randomized patients (89%). Patients with CRFs differed from those without CRFs in race, distance of the tumor from the anal verge and clinical T stage. Median follow-up was 3.4 years. A total of 156 of 324 patients were recommended TME during the study period. The 3-year OP rate was 47%. Independent predictors of OP in the multivariate analysis can be seen in the table. Conclusions: Although the patients included in this analysis may not be fully representative of the entire cohort, our data shows that several radiological variables associated with unfavorable tumor biology – tumor involvement of the mesorectal fascia, presence of EMVI and involved mesorectal nodes – were negatively associated with organ preservation. This information will be useful in selecting rectal cancer patients for WW and should be used for patient stratification in future clinical trials. Clinical trial information: NCT02008656.

¹HR = Hazard Ratio, ²CI = Confidence Interval.