PURPOSE: To compare the effects of biodegradable balloon and hydrogel perirectal spacers versus no spacer on prostate motion during SBRT. METHODS: We analyzed 341 SBRT fractions with continuous ultrasound tracking in 70 patients treated for localized prostate cancer. Patients received a balloon spacer (118 fractions), a hydrogel spacer (88 fractions), or no spacer (135 fractions) based on institutional availability and FDA approval timeline. Motion was quantified using Poisson regression analysis of clinically meaningful deviation events (≥2-3 mm for ≥3 seconds), Fast Fourier Transform analysis of dominant oscillatory amplitudes, and 3D motion envelope volumes calculated from covariance-derived ellipsoids. RESULTS: At the 3 mm threshold, balloon spacers reduced deviation events by 53% (incidence rate ratio 0.47; 95% CI, 0.31-0.69; P < 0.001) and hydrogel spacers by 28% (IRR 0.72; 95% CI, 0.55-0.93; P = 0.013) compared to no spacer. Balloon spacers demonstrated significantly smaller dominant oscillatory amplitudes across all spatial dimensions: anteroposterior (0.50 vs 0.73 mm, P = 0.005), superior-inferior (0.30 vs 0.46 mm, P = 0.005), and left-right (0.20 vs 0.27 mm, P = 0.064) versus no spacer. The 3D motion envelope volume was substantially reduced with balloon spacers (0.58 mm³) compared to no spacer (2.04 mm³, P = 0.002) or hydrogel (2.79 mm³, P < 0.001). CONCLUSION: In this retrospective SBRT cohort, rectal spacing, particularly with a biodegradable balloon, was associated with less intrafraction prostate motion. These findings suggest a potential motion-stabilizing effect of spacer design.
Tward et al. (Fri,) studied this question.