Prostate–Rectum Spacing from Apex to Base and Its Impact on Organs-At-Risk Dosimetry in Prostate Cancer SBRT

Stereotactic body radiation therapy (SBRT) for localized prostate cancer delivers high doses per fraction, making dose constraints for the rectum and other organs at risk critical during treatment planning. This study evaluated the association between prostate–rectum separation, achieved with a biodegradable balloon rectal spacer at different anatomical levels, and corresponding organ-at-risk dose patterns. Thirty-three patients underwent transperineal balloon spacer implantation followed by SBRT to 36.25 Gy in five fractions. Prostate–rectum separation at the apex, mid-gland, and base were measured on CT and/or MRI and categorized as <10 mm, 10–14 mm, or ≥14 mm. Rectal dose–volume parameters and mean doses to the rectum, bladder, and penile bulb were assessed using linear regression analyses and group comparisons at 14 mm separation. Mean prostate–rectum separation was 16.6 mm overall, with minimal high-dose rectal exposure observed. Increasing separation was associated with reduced rectal dose–volume parameters at the apex and mid-gland, while greater base separation corresponded primarily to lower bladder mean dose. Increased apical separation was also associated with reduced penile bulb mean dose. No acute gastrointestinal toxicity was observed, and genitourinary toxicity was limited to low-grade events. These findings indicate that prostate–rectum separation varies by anatomical level and is associated with distinct organ-at-risk dose relationships in prostate SBRT.