3412 - Rectal Spacing in Large-to-Very-Large Prostates: Balloon vs. Hydrogel Spacer Quality and OAR Dose

Purpose/Objective(s): Rectal spacers have demonstrated decreased toxicity across rectal, urinary and sexual domains. Large prostate glands increase prostate-rectum interface extent and coverage demands, potentially limiting spacer effectiveness. We compared an inflatable biodegradable balloon spacer and a PEG hydrogel spacer in patients with large glands (>75 cc), focusing on spacer symmetry, spacer geometry, and organs at risk (OAR) dose.

Materials/Methods: We retrospectively identified 112 consecutive patients (56 balloon and 56 hydrogel) with MRI prostate volume >75 cc treated with prostate radiotherapy. A Spacer Quality Score (SQS: 0-3; 3 optimal) was assigned based on (i) space creation (mm), defined by AP prostate-rectum interface (PRI) separation at base, mid-gland, and apex, and (ii) symmetry, assessed at each level by measurements at the rectal midline and 1.0 cm laterally on each side (right/left). Spacer geometry was quantified using Feret diameter (maximum craniocaudal [CC] dimension), flatness ratio (width/depth; lateral spread vs thickness across PRI), and elongation ratio (length/width; CC extent relative to width). Dosimetry was compared for OAR, including rectum and penile bulb (V80-V100).

Results: Overall mean gland volume was 109.6 cc (range 75.0-229.7), with no significant difference between cohorts (p=0.23). Balloon spacers improved prostate-rectum separation and symmetry versus hydrogel, including greater mid-gland PRI separation (15.1 vs 10.6 mm; p<0.001) and a more favorable SQS distribution (global p<0.001), with SQS=3 (optimal) in 73.2% vs 7.1% of hydrogel cases. Balloon spacers demonstrated improved geometry in all categories (Feret diameter, flatness, elongation; all p<0.001) and reduced rectal and penile bulb high dose volumes (V80-V100) compared with hydrogel (Table).

Conclusion: In large-to-very-large prostates, a pre-formed volume-confined balloon spacer achieved greater symmetry and improved geometry versus injectable hydrogel and was associated with lower rectal and penile bulb dose. These findings inform spacer selection in challenging anatomy and motivate correlation with toxicity and patient-reported outcomes in future studies.