Hydrogel Spacer Distribution Within the Perirectal Space in Patients Undergoing Radiotherapy for Prostate Cancer: Impact of Spacer Symmetry on Rectal Dose Reduction and the Clinical Consequences of Hydrogel Infiltration Into the Rectal Wall

Pract Radiat Oncol. May-Jun 2017;7(3):195-202. doi: 10.1016/j.prro.2016.10.004. Epub 2016 Oct 17.


Purpose: Hydrogel prostate-rectum spacers, biomaterials placed between the prostate and rectum, continue to gain interest as a method to reduce or limit rectal dose during dose escalated prostate cancer radiation therapy. Because the spacer is initially injected into the perirectal space as a liquid, the final distribution can vary. The purpose of this study was to evaluate hydrogel spacer (SpaceOAR system) implantation and distribution from a recent prospective randomized control trial and correlate spacer symmetry with rectal dose reduction as well as rectal wall infiltration (RWI) to acute and late toxicity.

Methods and materials: T2-weighted magnetic resonance imaging sets of 149 patients enrolled in a prospective clinical trial who received transperineal spacer injection were assessed for hydrogel spacer midline symmetry and RWI using a semiqualitative scoring system. Symmetry was then correlated to rectal dose reduction using a Student t test (1-tailed, paired), whereas a Fisher exact test was used to correlate RWI with acute and late rectal toxicity. All patients had control treatment plans created before spacer injection.

Results: Hydrogel spacer was symmetrically placed at midline for 71 (47.7%) patients at the prostate midgland as well as 1 cm superior and inferior to midgland. The remaining 78 (50.9%) patients had some level of asymmetry, with only 2 (1.3%) having far lateral distribution (ie, >2 cm) of hydrogel spacer. As the hydrogel spacer became more asymmetric, the level of rectal dose reduction relative to their control plans decreased. However, all but the most asymmetrical 1.3% had significant rectal dose reduction (P < .05). Rectal wall hydrogel spacer infiltration was seen in 9 (6.0%) patients. There was no correlation between RWI and procedure-related adverse events or acute/late rectal toxicity.

Conclusions: Significant reduction of rectal dose can still be achieved even in the setting of asymmetric hydrogel spacer placement. RWI does not correlate with patient complications.

Publication types

  • Multicenter Study
  • Randomized Controlled Trial

MeSH terms

  • Humans
  • Hydrogel, Polyethylene Glycol Dimethacrylate / adverse effects
  • Hydrogel, Polyethylene Glycol Dimethacrylate / therapeutic use*
  • Male
  • Organ Sparing Treatments / methods*
  • Organs at Risk
  • Prostatic Neoplasms / radiotherapy*
  • Radiotherapy Dosage
  • Rectum / drug effects*
  • Rectum / radiation effects*


  • Hydrogel, Polyethylene Glycol Dimethacrylate