Purpose-To characterize the effect of a prostate-rectum spacer on dose to rectum during external beam radiotherapy for prostate cancer, and to assess for factors correlated with rectal dose reduction.Materials and methods-Fifty-two patients at 4 institutions were enrolled onto a prospective pilot clinical trial. Patients underwent baseline scans, then were injected with perirectal spacing hydrogel and re-scanned. IMRT plans were created on both scans for comparison. Objectives were to establish rates of creation of ≥7.5mm of prostate-rectal separation, and decrease in rectal V70 of ≥25%. Multiple regression analysis was performed to evaluate associations between pre-vs. postinjection changes in rectal V70 and changes in plan conformity, rectal volume, bladder volume, bladder V70, PTV volume, as well as post-injection mid-gland separation, gel volume, gel thickness, length of PTV/gel contact, or gel left-to-right symmetry.Results-Hydrogel resulted in ≥ 7.5mm prostate-rectal separation in 95.8% of patients; 95.7% had decreased rectal V70 of ≥ 25%, with mean reduction of 8.0 Gy. There were no significant differences in pre-and post-injection prostate, PTV, rectal, and bladder volumes. Plan conformities were significantly different pre-vs. post-injection (P = 0.02); plans with worse conformity indexes post-injection compared to pre-injection (n=13) still had improvements in rectal V70. In multiple regression analysis, greater post-injection reduction in V70 was associated with decreased relative post-injection plan conformity (P=0.01). Reductions in V70 did not significantly vary by institution, despite significant inter-institutional variations in plan conformity. There were no significant relationships between reduction in V70 and the other characteristics analyzed.
Conflict of Interest NotificationThis work was supported by funding from Augmenix Inc., including provision of research materials (hydrogel), funding for data monitoring support, and research-related travel costs. Drs. Song, DeWeese, and Ford have also received consulting fees from Augmenix Inc. Conclusions-Injection of hydrogel into prostate-rectal interface resulted in dose reductions to rectum for > 90% of patients treated. Rectal sparing was statistically significant across a range of 10-75 Gy, and was demonstrated within the presence of significant inter-institutional variability in plan conformity, target definitions, and injection results.
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