Date Approved

8-3-2026

Graduate Degree Type

Project

Degree Name

Medical Dosimetry (M.S.)

Degree Program

School of Interdisciplinary Health

First Advisor

Sarah Johnson

Second Advisor

Kristen Vu

Academic Year

2025/2026

Abstract

Purpose: Hydrogel rectal spacers reduce rectal dose during prostate radiotherapy, but their effect on bladder wall dose is not well characterized, particularly with volumetric modulated arc therapy with simultaneous integrated boost (VMAT-SIB). This study evaluated the effect of spacer placement on whole and regional bladder wall dose.

Methods: Twenty-nine patients treated with VMAT-SIB (70 Gy in 28 fractions) were analyzed retrospectively: 9 with and 20 without a spacer. The bladder wall was defined as the whole bladder minus a 3-mm inner margin. Primary endpoints were D0.03 cc and D1 cc; secondary endpoints included D2 cc, D3 cc, D10 cc, volume-based metrics, and regional substructures outside the PTV70 Prostate. Groups were compared using the Wilcoxon rank-sum test with exact Monte Carlo estimation.

Results: The primary endpoints (D0.03 cc, D1 cc) did not differ significantly. Among whole bladder wall endpoints, only D10 cc was significantly lower in the spacer group (35.8 vs 53.1 Gy; P = .014). No significant differences were observed in whole-bladder dose metrics (Bladder V40, V65, and V70 Gy). Regional analysis outside the PTV70 Prostate showed significant reductions in D1 cc (P = .045), D2 cc (P = .013), D3 cc (P = .005), and D10 cc (P = .011). Rectal dose volume metrics were significantly reduced at all endpoints with a spacer.

Conclusions: Hydrogel rectal spacer placement was associated with reduced intermediate-dose exposure to regional bladder wall substructures while preserving reduction in rectal dose. Regional bladder wall assessment revealed localized dosimetric differences not apparent with whole-bladder metrics.

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