3241 - Interfraction Positional Stability of a Biodegradable Balloon Rectal Spacer during Prostate SBRT

Purpose/Objective(s):

In prostate stereotactic body radiotherapy (SBRT), rectal spacers are used to maximize the distance between the prostate and the anterior rectal wall to minimize toxicity. The clinical benefit of the spacer is dependent interfraction position stability throughout the treatment course. Our study aims to quantify the translational and rotational shifts of a biodegradable balloon spacer, assessing its interfraction motion during prostate SBRT treatment course.

Materials/Methods:

A retrospective analysis of patients undergoing prostate SBRT with an implanted biodegradable balloon rectal spacer was performed. Interfractional stability was evaluated by comparing planning CT (pCT) dataset with cone-beam CT (CBCT) images from the final treatment fraction. Initially, image registration was centered on the prostate using gold fiducial markers and soft-tissue alignment. Translational shifts and rotations (pitch, yaw, roll) were quantified, with rotational corrections constrained to = 3.0° per axis. All registrations were independently reviewed by a radiation therapist and a senior radiation oncologist. Positional variations were summarized using descriptive statistics to assess the geometric reproducibility of the spacer.

Results:

Of the 49 patients initially observed, 4 were excluded from the final analysis as the balloon spacer was not identifiable on the final-fraction CBCT. For the remaining 45 patients, the mean translational shifts from planning CT to final-fraction CBCT were 0.06 ± 0.36 mm (Anterior-Posterior), 0.11 ± 0.39 mm (Cranial-Caudal), and 0.13 ± 0.23 mm (Left-Right). The total mean 3D migration was 0.490 ± 0.363 mm (95% CI: 0.382–0.597 mm). Mean rotational variations for pitch, yaw, and roll were -1.861° ± 4.873°, 1.529° ± 3.848°, and 0.630° ± 3.055°, respectively. All observed rotations remained within the pre-defined clinical threshold of = 3.0°. No spacer deformation was identified across the analyzed cohort.

Conclusion:

This is the first study to evaluate balloon spacer stability during prostate SBRT course, showing it maintains high interfractional geometric stability. Sub-millimeter translational shifts confirm that the perirectal space established at simulation remains consistent, ensuring reliable organ-at-risk sparing. Future studies should evaluate the dosimetric impact of these minimal variations on late rectal toxicity and long-term clinical outcomes.

Metric	Mean ± SD (cm)	Median (cm)	95% Confidence Interval
Lateral (X)	0.130 ± 0.235	0.075	[0.060, 0.199]
Ant-Post (Y)	0.062 ± 0.362	0.100	[-0.045, 0.169]
Sup-Inf (Z)	0.111 ± 0.396	0.075	[-0.006, 0.228]
Pitch (X-Rotation)	-1.861° ± 4.873°	-1.335°	[-3.300, -0.421]°
Roll (Y-Rotation)	0.630° ± 3.055°	0.000°	[-0.273, 1.532]°
Yaw (Z-Rotation)	1.529° ± 3.848°	0.200°	[0.392, 2.666]°