2230 - The Clear Advantage of Rectal Spacing when Treating Prostate Cancer with Proton Therapy, A Preliminary Report Comparing Rectal Spacers

Purpose/Objective(s):

Dose escalation in prostate cancer radiotherapy is limited by rectal toxicity. While rectal spacers reduce bowel toxicity in photon EBRT, their benefit in proton therapy remains unclear. Further, proton therapy may result in high-dose exposure to the anterior rectal wall due to lateral penumbra with conventional opposed lateral beam arrangements. We hypothesize that the greater spacing that is potentially achievable with the balloon spacer will provide greater dosimetric benefits in proton therapy compared to SpaceOAR, with future analysis on gastrointestinal and genitourinary toxicity outcomes.

Materials/Methods:

This interim report of a prospective study includes 15 patients with localized prostate cancer treated at a single institution with proton therapy. A balloon spacer was inserted in 7 patients, and SpaceOAR Vue in 8 patients based on institutional contracts. Post-insertion computed tomography (CT) scans were fused with post spacer insertion MRI’s for treatment planning for a modified hypofractionated regimen (70 Gy to the prostate and 50.4 Gy to the seminal vesicles in 2.5/1.8 Gy for 28 fractions) with 4 mm margins around the CTV. Dosimetric parameters were compared between the two spacer groups.

Results:

Dosimetric outcomes for the bladder and rectum are summarized in Table 1. The rectal dose was lower with the balloon spacer, with V45 of 6.29% compared to 10.28% in the gel group, and V65 at 1.34% versus 2.68%. The balloon spacer group also demonstrated lower mean bladder doses across all evaluated parameters, with V45 of 11.76% compared to 18.86% in the gel group. Similarly, V55 and V65 were lower in the balloon group (8.19% and 4.84%, respectively) than in the gel group (14.03% and 8.39%). Variability in dose distribution, as indicated by standard deviations, was greater in the gel group for both bladder and rectal parameters.

Conclusion: This ongoing prospective study indicates that the balloon spacer provides superior dosimetric benefits over SpaceOAR Vue in proton therapy for prostate cancer, with consistently lower bladder and rectal doses. Further data collection and analysis, including long-term gastrointestinal and genitourinary toxicity outcomes, are underway to refine the clinical implications of spacer selection in proton therapy.

Abstract 2230 - Table 1

Dosimetric Parameter	Balloon (Mean ± SD)	SpaceOAR (Mean ± SD)
Bladder V45 < 45%	11.76% ± 4.77%	18.86% ± 7.91%
Bladder V55 < 25%	8.19% ± 4.02%	14.03% ± 6.15%
Bladder V65 < 15%	4.84% ± 2.90%	8.39% ± 3.60%
Bladder V65 < 10cc	1.07 ± 1.04	1.53 ± 0.87
Rectum V45 < 45%	6.29% ± 4.83%	10.28% ± 4.16%
Rectum V55 < 25%	2.94% ± 2.36%	6.03% ± 2.96%
Rectum V65 < 15%	1.34% ± 1.43%	2.68% ± 1.67%