3378 - Impact of Rectal Spacer Implant Quality on Rectal Dose and Toxicity in Prostate Radiotherapy: Results from a Prospective Clinical Trial
Presenter(s)
S. H. Zimberg1, V. Agrawal2, J. T. Keane Jr3, G. Gluckman1, and J. N. Shah4; 1Advanced Radiation Centers of New York, Lake Success, NY, 2Advanced Radiation Centers Of New York, New York, NY, 3Advanced Radiation Centers of New York, Hauppauge, NY, United States, 4Advanced Radiation Centers, Lake Success, NY
Purpose/Objective(s): The biodegradable balloon rectal spacer is a valuable device for protecting the rectum during prostate radiation. This study evaluates how balloon spacer implant quality, including symmetry and positioning, affects rectal dosimetry and toxicity outcomes.
Materials/Methods: 101 men with localized prostate cancer were treated with SBRT, moderate hypofractionation, or conventional fractionation (CF) after balloon spacer implant. Spacer quality was assessed using the refined Fischer V. Symmetry Score (SYM)1 and Spacer Symmetry Score (SQS)2 on post-implant T2-weighted MRI and CT. SQS categorized the peri-rectal interspace (PRI) into four groups: 3 (>14 mm), 2 (9–14 mm), 1 (3–9 mm), and 0 (<3 mm). PRI was measured at prostate base, midgland, and apex at midline and 1 cm laterally to either side. SYM score classified symmetry as optimal (SYM-S), suboptimal (SYM-1–2), or asymmetric (SYM-3–4). Medial-lateral symmetry relative to midline was evaluated at midgland, 1 cm superior, and 1 cm inferior. QoL (EPIC-26, AUA) was assessed at screening, treatment, and follow-up. Dosimetry and toxicity were analyzed using a GEE model.
Results: 101 patients received CF (n=55), moderate hypo. (n=30), or SBRT (n=16), with a median follow-up of 95.0±15.3 days. Most spacer implants achieved SQS-3 (75.2%) and SYM-S (79.2%), with higher SQS and symmetry levels yielding lower mean rectal dose exposure at all levels (p<0.05), Table 1. Cumulative rectal toxicities were 16.3% for SYM-S, 47.1% for SYM-1-2, and 100.0% for SYM-3-4. Stratified by SQS, cumulative rectal toxicities were 22% for SQS-3, 36% for SQS-2, and 57% for SQS-1. All rectal toxicities were Grade 1, except one Grade 2 in SYM-1-2. Bowel function (EPIC score: 89.3±11.8 at baseline) declined post-treatment (80.3±20.5) but recovered at one (84.8±13.3) and three months (91.7±4.8), with similar trends for urinary and sexual QoL.
Conclusion: Spacer symmetry and positioning optimize dosimetry and reduce rectal toxicity. Optimal spacing (SYM-S and SQS-3) improved rectal dose sparing and reduced rates of rectal adverse events. Urinary, bowel, and sexual QoL declined post-treatment but recovered over time.
Abstract 3378 - Table 1: Rectal Dose (Vx% = volume of rectum receiving x% of prescribed dose)| V60% Mean ± SE | V70% Mean ± SE | V80% Mean ± SE | V90% Mean ± SE | V100% Mean ± SE | |
| SYM Model | |||||
| SYM-S (n=81; 79.2%) | 8.9 ± 0.6 | 5.1 ± 0.5 | 2.7 ± 0.3 | 1.3 ± 0.2 | 0.4 ± 0.1 |
| SYM-1 (n=9; 8.9%) | 11.5 ± 2.3 | 7.8 ± 1.6 | 5.1 ± 1.0 | 2.9 ± 0.6 | 1.0 ± 0.2 |
| SYM-2 (n=7; 6.9%) | 11.9 ± 1.3 | 8.7 ± 0.9 | 6.4 ± 0.6 | 4.5 ± 0.4 | 2.2 ± 0.2 |
| SYM-3 (n=4; 4.0%) | 21.3 ± 2.3 | 14.5 ± 2.7 | 9.9 ± 2.1 | 5.5 ± 0.8 | 2.2 ± 0.6 |
| SYM-4 (n=0; 0.0%) | N/A | N/A | N/A | N/A | N/A |
| p-value | 0.0041 | <0.001 | <0.001 | <0.001 | <0.001 |
| SQS Model | |||||
| SQS-3 (n=76; 75.2%) | 8.9 ± 0.8 | 5.2 ± 0.5 | 2.8 ± 0.3 | 1.3 ± 0.2 | 0.4 ± 0.1 |
| SQS-2 (n=11; 10.9%) | 12.9 ± 1.3 | 9.0 ± 1.2 | 5.9 ± 0.9 | 3.6 ± 0.6 | 1.3 ± 0.3 |
| SQS-1 (n=7; 6.9%) | 13.7 ± 2.4 | 10.2 ± 2.1 | 6.7 ± 2.0 | 4.7 ± 1.6 | 2.7 ± 1.3 |
| SQS-0 (n=0; 0.0%) | N/A | N/A | N/A | N/A | N/A |
| p-value | 0.002 | <0.001 | <0.001 | <0.001 | <0.001 |