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The vaginal cuff is a common site of tumor recurrence following surgery for endometrial cancer, and adjuvant vaginal cuff brachytherapy (VCB) is indicated to reduce the risk of local recurrence. Following hysterectomy, many patients have redundant tissue at the lateral apex of the vaginal cuff ("dog ears"). This extra tissue may extend irregularly beyond the typical cylindrical dose distribution of radiation delivered during single channel VCB and it is hypothesized that they are a potential site of local treatment failure even after VCB. The effects of the presence and size of vaginal cuff "dog ears" on tumor recurrence is not yet fully understood. The aim of this study is to evaluate the prevalence and size of extra tissue at the vaginal apex, and whether these factors predict disease recurrence in patients treated with adjuvant VCB for endometrial cancer. We retrospectively reviewed the medical records of 219 patients with early-stage endometrial cancer treated with image-guided single channel VCB at our institution from 2012 - 2021 with available brachytherapy planning CT simulation images. "Dog ears" were defined as the presence of additional soft tissue extending at least 10 mm from the apex of the vaginal cuff perpendicular to the surface of the brachytherapy applicator as defined on a coronal or axial CT simulation. Patient characteristics and outcomes including stage, size and laterality of dog ears, presence of air gaps, and patterns of failure were collected. Vaginal cuff recurrence (VCR) failure free survival was calculated from time of first brachytherapy to VCR and was estimated by the product limit method of Kaplan and Meier. Fisher's exact test was used to compare categorical variables and a multivariate logistic regression model was developed to evaluate the association of overall treatment failure with various factors of interest. We found that 57.5% of patients (n = 126) met our criteria for having "dog ears," which, on average, extended 23.8 mm (SD = 6.4) from the brachytherapy applicator. In total, 13 patients (5.9%) developed a VCR, 12 (5.5%) developed a regional recurrence, and 16 (7.3%) developed distant recurrence. There was no statistically significant difference in VCRs between patients with dog ears and those without dog ears (7.1% vs 4.3%, p=0.56). The various surgical approaches (abdominal, laparoscopic, transvaginal) did not have an increased propensity to result in dog ears (p=0.74). The 3-year probability of VCR failure-free survival is 86%. On multivariate analysis (MVA), no factors predicted for overall treatment failure including the presence of dog ears, maximum dimension of dog ears, FIGO stage, presence of air gaps at the applicator surface on post insertion CT images, or brachytherapy duration. There was a trend toward increased risk of recurrence with higher grade histology on MVA (p=0.0805). In this investigation of a single institution experience with single-channel image-guided VCB, the presence of "dog ears" at the vaginal apex is prevalent; however, VCR remains low with distant metastases being the predominant site of failure. While dog ears appear to have little impact on overall recurrence patterns, these may be a factor to consider when planning adjuvant brachytherapy including choosing alternate applicators. [ABSTRACT FROM AUTHOR]