Results from 2 years of follow-up in men undergoing proton beam therapy for localized prostate cancer suggest that treatment with the rectal hydrogel spacer (SpaceOAR) provides better rectal sparing than rectal balloon immobilization.
Results from 2 years of follow-up in men undergoing proton beam therapy (PBT) for localized prostate cancer suggest that treatment with the rectal hydrogel spacer (SpaceOAR) provides better rectal sparing than rectal balloon immobilization.
The recently published findings showed that compared with rectal balloon immobilization, treatment with the hydrogel spacer significantly reduced the risk of clinically relevant (grade 2+), late rectal bleeding and was associated with a significantly lower decrease in patient-reported bowel quality of life (Int J Radiat Oncol Biol Phys Feb. 6, 2020 [Epub ahead of print]).
Co-author William J. Ellis, MD, told Urology Times, “Phase III study results showed that the hydrogel rectal spacer decreased the incidence of late rectal toxicity in men who underwent intensity-modulated radiation therapy for localized prostate cancer. Utilization of PBT is expected to grow, and this study investigated outcomes after PBT with the hydrogel spacer in a real-world setting using data that were prospectively collected over a relatively long follow-up.
“Treatments often do not perform as well in clinical practice as they do in clinical trials, and yet we found that the rectal-sparing benefit of the hydrogel spacer, particularly for reducing late rectal bleeding, was even greater than expected. These findings can hold interest for urologists who counsel patients about their treatment options for localized prostate cancer,” added Dr. Ellis, professor and vice-chair of urology, University of Washington, Seattle.
The single-institution study included data from 267 patients treated for localized, clinical stage T1-4 prostate cancer using conventionally fractionated, dose-escalated PBT from 2013 to 2018. A total of 192 men were treated with rectal balloon immobilization, and 75 men underwent placement of the rectal hydrogel spacer. Dr. Ellis and George R. Schade, MD, assistant professor of urology, University of Washington, placed all of the hydrogel spacers. Rectal balloons were inserted by radiation oncology technicians.
The incidence of late rectal bleeding and bowel quality of life were analyzed as co-primary endpoints. Grading of rectal bleeding was done retrospectively using the Common Terminology Criteria for Adverse Events. Bowel quality of life was assessed using the bowel domain of the expanded prostate cancer index composite (EPIC), which was completed at baseline and then every 6 months.
Next: 2-year rate of any bleeding higher in balloon group2-year rate of any bleeding higher in balloon group
Median follow-up for patients in the rectal balloon and hydrogel spacer groups was 19 and 22 months, respectively. The 2-year actuarial rate of any rectal bleeding was almost threefold higher in the rectal balloon group compared with the hydrogel spacer group (35% vs. 13%). The 2-year actuarial rate of grade 2+ late rectal bleeding was only 3% in those in the hydrogel spacer group versus 19% among men who had rectal balloon immobilization (p=.003). No men in the hydrogel spacer group had a grade 3 bleeding event compared with two men in the rectal balloon cohort. No patients had a grade 4+ bleeding event.
“The cumulative incidence of grade 2+ rectal bleeding among men treated with the hydrogel spacer remained relatively low and stable throughout follow-up, whereas it rose fairly steadily between 6 and 24 months in the rectal balloon cohort,” Dr. Ellis observed.
Additional analyses were done to identify variables predictive of grade 2+ rectal bleeding. In univariable analysis, a significant correlation was found only with rectal dose. In multivariable analysis, receipt of the hydrogel spacer was identified as protective. Compared with the rectal balloon immobilization group, men who had the spacer hydrogel had an 85% lower risk for having grade 2+ rectal bleeding (p=.01). The only other independent predictor of grade 2+ rectal bleeding was anticoagulation use, which increased the risk by fivefold.
The quality-of-life analysis showed that the mean EPIC-bowel domain score was similar in the patients in the rectal hydrogel spacer and rectal balloon groups at baseline (92.3 vs. 93.4). Although it was decreased from baseline in both groups at all follow-up intervals, the decrement was consistently less in the hydrogel spacer group, and the separation between groups favoring the hydrogel spacer cohort increased with lengthening follow-up.
“At 2 years, there was a 5.5-point absolute difference in the EPIC-bowel score favoring the hydrogel spacer group. The difference was statistically significant (p=.030) but also can be considered clinically significant as a minimum difference of five points has been suggested to be clinically meaningful,” Dr. Ellis said.
The study was also designed to compare dosimetric parameters for PBT in the two rectal sparing procedure groups. Analysis of those data showed that the rectal hydrogel spacer was associated with significantly improved rectal dosimetry while maintaining excellent target coverage.
Dr. Ellis noted that although some radiation oncologists place the rectal hydrogel spacer themselves, the procedure is done by urologists at the University of Washington.
“Urologists who do prostate ultrasound and are accustomed to placing fiducial markers for radiation oncologists can easily acquire the skill needed to place the hydrogel spacer,” Dr. Ellis said. “The radiation oncologists at our institution appreciate our assistance, and it has led to a mutually satisfying collaborative relationship.”