Dosimetric Comparison between Hydrogel and Liquid-Filled Rectal Spacers In Proton Beam Therapy for Primary Prostate Cancer
Abstract
Purpose
Implanted spacers can improve rectal sparing in prostate radiation therapy. Several systems are commercially available, including hydrogel and liquid-filled balloon variants. Here, we perform an initial dosimetric evaluation of rectal sparing in prostate proton beam therapy (PBT) for patients implanted with hydrogel or liquid-filled balloon spacers.
Methods
Ten patients undergoing PBT for primary prostate cancer were included. The prescribed dose was 70 Gy (RBE 1.1) covering at least 98% of the prostate volume with opposed lateral beams. Five patients were implanted with a hydrogel spacer, SpaceOAR Vue (Boston Scientific). The remaining five were implanted with the BioProtect Balloon Implant System. The spacer volume was recorded. The rectal volume (in cc) receiving at least 69, 64, 59, 55, 27, and 15 Gy (RBE) (V69-V15), and the dose to the highest 7.0 cc (D7cc) were collected for analysis. Two-tailed independent samples t-tests were performed. Effect sizes were reported as Cohen’s d. Statistical significance was taken as p<0.05.
Results
All plans remained within clinical dose goals. Large, statistically significant differences were observed between implanted spacer volumes (p<0.01, d=3.0). On average, the BioProtect spacer had a 4.8 ± 1.0 cc larger volume compared to SpaceOAR Vue. Respective mean volumes were 14.1 ± 1.8 cc (range 12.5 to16.7 cc) and 9.3 ± 1.3 cc (range 7.3 to 10.7 cc). Marginal, but not statistically significant advantages in rectal V69 (p=0.35, d=0.63), V64 (p=0.32, d=0.68), V59 (p=0.44, d=0.52), V55 (p=0.37, d=0.60), V27 (p=0.63, d=0.32), V15 (p=0.55, d=0.40), or D7cc (p=0.47, d=0.48) were observed for the hydrogel spacer.
Conclusion
Though a greater volume was observed for the balloon system, both spacers achieved similar rectal sparing. However, there was large variance in most dosimetric measures due to differences in patient anatomy and rapid dose falloff. Results support clinical use of either system for rectal sparing in prostate PBT.