Geometry of hydrogel rectal spacer placement and risk of MRI-identified rectal wall infiltration.

312 Background: The use of rectal spacers in the management of localized prostate cancer treated with definitive radiotherapy has become ubiquitous in recent years. However, pre-treatment MRIs often identify varying degrees of hydrogel involvement within the rectal wall. In the present study, we evaluate the geometry of spacer placement and its association with radiological rectal wall infiltration. Methods: We identified all patients who underwent hydrogel rectal spacer placement in preparation for 5-fraction prostate SBRT from 1/2020 to 9/2021. Two specialty trained body radiologists evaluated all MRIs independently. Scans were evaluated for the following spacer parameters: spacer thickness, prostate-rectal distance, symmetry, and degree of rectal wall infiltration. Prostate-rectal distance was measured at the level of the prostatic apex, midgland, and base. Symmetry of the rectal spacer was measured using right or left lateralization from midgland. Degree of rectal wall invasion was categorized as follows: none, muscularis, submucosal, and intraluminal. Results: A total of 336 patients underwent MRI following hydrogel rectal spacer placement from 1/2020 to 9/2021. Patients were excluded from MRI if they had AICD/pacemaker, foreign body, or patient refusal. In those patients with any rectal wall invasion, gel thickness as measured at the base (11 vs. 10 mm, p = 0.02), midgland (14 vs. 10 mm, p < 0.001), and apex (12 vs. 8 mm, P < 0.001) was significantly larger than those patients without invasion. This translated into significantly larger distances between the posterior aspect of the prostate and anterior aspect of the rectum at the level of the apex (12 vs. 8 mm, p < 0.001) and midgland (13 vs. 11, p < 0.001), but not at the base (14 vs. 14 mm, p = 0.5). There was no association seen with asymmetrical spacer placement and rectal wall invasion (p = 0.7). Subgroup analysis of patients with more extensive invasion into the muscularis or submucosa confirmed significantly larger gel thickness at all prostate levels, as well as a larger prostate-rectal distance at the level of the apex and midgland. Significant associations remained consistent with both independent radiological evaluations. Conclusions: Hydrogel spacer rectal wall infiltration was associated with increased axial gel thickness, specifically at the level of the prostatic midgland and apex. Rectal wall infiltration was not associated with lateralization of gel. Rectal infiltration may be a result of surplus hydrogel placed particularly in the region where the potential space between the prostate and the rectum is limited.