Recently, a randomized trial (NCT04189913) reported that a hyaluronic acid (HA) rectal spacer reduced acute grade 2+ gastrointestinal (GI) toxicity for hypofractionated radiation therapy (RT). However, 26.5% of patients who received the spacer experienced a minimally clinically important difference (MCID; 5 points) in EPIC bowel quality-of-life (QOL). We evaluated whether characteristics of the spacer implant, particularly apical separation, were associated with change in bowel QOL at 3-months and acute grade 1+ GI toxicity. We conducted a secondary analysis of 136 patients randomized to receive the HA spacer. The post-spacer implant structure sets and treatment plans were analyzed. The mid-plane (MP) was defined as the prostate center-of-mass. Four horizontal planes were defined at the superior (MP +1 cm), mid-gland, inferior (MP - 1 cm), and apex of the prostate. Separations between the prostate and anterior rectal wall at midline were computed at each plane in a custom Python programming environment. Implant symmetry was computed based on a previously published method (Fischer-Valuck, PRO, 2017). The volume of rectum receiving > = 30 Gy (rV30) was extracted from DVHs; rV30 has been associated with bowel frequency, fecal incontinence, and rectal pain for hypofractionated RT (Wilkins, IJROBP, 2020). First, we evaluated whether any of the 4 separation or symmetry variables were associated with rV30. Then, we evaluated whether significant spacing variables, rV30, and baseline bowel QOL were predictive of the change in bowel QOL at 3-months using multivariate linear regression. Finally, we evaluated whether significant spacing variables and rV30 were predictive of acute grade 1+ GI toxicity (21 events) within 3-months, utilizing multivariate logistic regression. The mean (standard deviation) superior, mid-gland, inferior, and apex separations were 15.6 (SD 6.0), 12.7 (3.7), 11.2 (3.7), and 9.7 (4.0), respectively. 130 of 136 (95.6%) had a symmetry score of 1 (symmetric). Apical separation was the only variable significantly associated with rV30 (r = -0.32; p < 0.01). On multivariate analysis, apical separation (0.41/mm; p = 0.01) was significantly associated with the change in bowel QOL, after adjusting for baseline bowel score (p = 0.0002) and rectum V30 (p = 0.50). Mean (SD) changes in bowel QOL were 0.01 (5.9) and -3.7 (8.1) for apical separations > = 10 vs <10 mm, respectively. Respective percentages of patients with a bowel MCID were 14.8% and 36.6% (p = 0.006). However, apical separation was not associated with increased odds of experiencing grade 1+ GI toxicity (p = 0.98), when adjusted for rectum V30 (odds ratio 1.04; p = 0.04). Increased apical separation may be associated with improved EPIC bowel QOL at 3-months for patients who received a HA rectal spacer prior to hypofractionated RT. This finding is clinically important, because HA can be deliberately injected into the perirectal space at the level of the prostate apex.