KEY POINTS
- This retrospective study included 198 patients receiving MR-guided prostate SBRT between October 2023 and March 2025. Each patient had six MRI scans — one simulation plus imaging at all five treatment fractions — providing more than 1,000 serial datasets for spacer analysis.
- Median prescription was 40 Gy in five fractions, with an optional 5-Gy boost to MRI-guided biopsy-proven dominant intraprostatic lesions. Mean time from simulation to fraction 1 was 19.9 days and to the final fraction 31.1 days.
- An institutional nnU-Net model automatically segmented the spacer on T2-weighted MRI and achieved Dice similarity >0.98 on withheld validation cases before being applied to the full serial dataset.
- Among 14 longitudinal shape features, only flatness remained significant after false-discovery correction, increasing by 0.003 units per 10 days (95% CI 0.001–0.005; q=0.019). The direction represented a very small shift toward a rounder configuration.
- Volume, surface area, compactness, major dimensions and other geometric measures showed no significant longitudinal change after multiple-testing correction, supporting overall morphological stability throughout the approximately one-month observation period.
- Spacer volume appeared 10.62% larger on final-treatment MRI than at simulation (95% CI 8.68–12.56%; p<0.001), corresponding to 1.19 cm³. The authors considered this more likely related to differences in MRI acquisition than true biological expansion, because longitudinal volume modelling did not demonstrate progressive growth.
- In the ten patients with the greatest increases and decreases in flatness, changes in rectal D0.03cm³, D1cm³, D30.15Gy, V24Gy, V10Gy and mean dose were all small and not statistically significant. Importantly, every fraction underwent adaptive replanning, which may have mitigated any dosimetric consequence of anatomical change.
CLINICAL TAKEAWAY
A polyethylene-glycol rectal spacer can reasonably be assumed to retain its rectal-sparing geometry throughout a typical five-fraction prostate SBRT course. Small morphological changes occur, but these data do not support routine concern about early spacer resorption; surrounding rectal and bladder anatomy remains the more relevant target for daily adaptation.