Intrafraction Motion Mitigation by Rectal Spacers in Prostate Stereotactic Body Radiation Therapy: Balloon, Hydrogel, and No-Spacer Comparison.

PURPOSE: This study aimed to compare the effects of biodegradable balloon and hydrogel perirectal spacers versus no spacer on prostate motion during stereotactic body radiation therapy (SBRT).

METHODS AND MATERIALS: 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 U.S. Food and Drug Administration 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 3-dimensional 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 < .001) and hydrogel spacers by 28% (incidence rate ratio, 0.72; 95% CI, 0.55-0.93; P = .013) compared with no spacer. Balloon spacers demonstrated significantly smaller dominant oscillatory amplitudes across all spatial dimensions: antero-posterior (0.50 vs 0.73 mm, P = .005), superior-inferior (0.30 vs 0.46 mm, P = .005), and left-right (0.20 vs 0.27 mm, P = .064) versus no spacer. The 3-dimensional motion envelope volume was substantially reduced with balloon spacers (0.58 mm3) compared with no spacer (2.04 mm3, P = .002) or hydrogel (2.79 mm3, P < .001).

CONCLUSIONS

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.