Continued Benefit to Rectal Separation for Prostate RT: Final Results of a Phase III Trial

SpaceOAR®, an FDA approved hydrogel intended to create rectal-prostate space, was evaluated in a single blind Phase III trial of image-guided intensity-modulated RT (IG-IMRT, IJROBP 2015). Men (n=222) were randomized 2:1 (Spacer:Control) receiving 79.2/1.8 Gy to prostate ± seminal vesicles. We now report final results with 3-year median follow-up.

Materials and Methods: Cumulative (CTCAE v 4.0) toxicity was evaluated by log-rank test. For quality-of-life (QOL) the Expanded Prostate Cancer Index Composite (EPIC) was utilized and mean changes from baseline in EPIC domains were tested by repeated measures models. Proportions of men with minimally important differences (MID) in each domain were tested using repeated measures logistic models with pre-specified thresholds.

Results:  The 3-year incidence of rectal toxicity of grade >=1 (9.2% vs 2.0%, p=0.028) and >=2 (5.7% vs 0%, p=0.012) favored Spacer. Grade 1+ urinary incontinence was also lower in the Spacer arm (15% vs. 4%, p=0.046) with no difference in grade 2+ urinary toxicity (7% vs 7% p=0.7). From 6-months onward bowel QOL consistently favored Spacer (p=0.002) with the difference at 3-years (5.8-points, p<0.05) meeting threshold for a MID. Interestingly, Control men at 3-years had a 3.9-point greater decline in urinary QOL vs Spacer (p<0.05) but did not meet the MID threshold. At 3- years more men had a MID decline in bowel QOL for Control vs. Spacer (41% vs. 14%, p=0.002) as well as for urinary QOL (30% vs 17%, p=0.04).

Further, Control men were also more likely to see large declines (twice MID) in bowel (21% vs 5%, p=0.02) and urinary QOL (23% vs. 8%, p=0.02).

Conclusion: The benefit of a hydrogel spacer in reducing rectal dose, toxicity, and QOL declines following IG-IMRT for prostate cancer were maintained or increased with longer follow-up providing stronger evidence for the benefit of hydrogel spacer for prostate radiotherapy.

Hamstra DA, Mariados N, Sylvester J, Shah D, Karsh L, Hudes R, Beyer D, Kurtzman S, Bogart J, Hsi RA, Kos M, Ellis R, Logsdon M, Zimberg S, Forsythe K, Zhang H, Soffen E, Francke P, Mantz C, Rossi P, DeWeese T, Daignault-Newton S, Fischer-Valuck BW, Chundury A, Gay H, Bosch W, Michalski J, Continued Benefit to Rectal Separation for Prostate RT: Final Results of a Phase III Trial, International Journal of Radiation Oncology • Biology • Physics (2017), doi: 10.1016/ j.ijrobp.2016.12.024.

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