A Novel Model to Correlate Hydrogel Spacer Placement, Perirectal Space Creation, and Rectum Dosimetry in Prostate Stereotactic Body Radiotherapy

Radiat Oncol. 2018 Oct 1;13(1):192. doi: 10.1186/s13014-018-1135-6.


Background: The SpaceOAR hydrogel is employed to limit rectal radiation dose during prostate radiotherapy. We identified a novel parameter - the product of angle θ and hydrogel volume - to quantify hydrogel placement. This parameter predicted rectum dosimetry and acute rectal toxicity in prostate cancer patients treated with stereotactic body radiotherapy to 36.25 Gy in 5 fractions.

Methods: Twenty men with low- and intermediate-risk prostate cancer underwent hydrogel placement from 2015 to 2017. Hydrogel symmetry was assessed on the CT simulation scan in 3 axial slices (midgland, 1 cm above midgland, 1 cm below midgland). Two novel parameters quantifying hydrogel placement - hydrogel volume and angle θ formed by the prostate, hydrogel, and rectum - were measured, and the normalized product of θ and hydrogel volume calculated. These were then correlated with perirectal distance, rectum maximum 1-3 cc point doses (rDmax 1-3 cc), and rectum volumes receiving 80-95% of the prescription dose (rV80-95%). Acute rectal toxicity was recorded per RTOG criteria.

Results: In 50% of patients, hydrogel placement was symmetric bilaterally to within 1 cm of midline in all three CT simulation scan axial slices. Lateral hydrogel asymmetry < 2 cm in any one axial slice did not affect rectum dosimetry, but absence of hydrogel in the inferior axial slice resulted in a mean increase of 171 cGy in the rDmax 1 cc (p < 0.005). The perirectal distance measured at prostate midgland, midline (mean 9.1 ± 4.3 mm) correlated strongly with rV95 (R2 0.6, p < 0.001). The mean hydrogel volume and θ were 10.3 ± 4.5 cc and 70 ± 49°, respectively. Perirectal distance, rV95 and rDmax 1 cc correlated with hydrogel angle θ (p < 0.01), and yet more strongly with the novel metric θ*hydrogel volume (p < 0.001). With a median follow up of 14 months, no rectal toxicity >grade 2 was observed. Low grade rectal toxicity was observed in a third of men and resolved within 1 month of SBRT. Men who had these symptoms had higher rDmax 1 cc and smaller θ*hydrogel volume measurements.

Conclusions: Optimal hydrogel placement occurs at prostate midgland, midline. The novel parameter θ*hydrogel volume describes a large proportion of rectum dosimetric benefit derived from hydrogel placement, and can be used to assess the learning curve phenomenon for hydrogel placement.

Keywords: Dosimetry; Prostate cancer; Rectal toxicity; SpaceOAR hydrogel; Stereotactic body radiotherapy.

MeSH terms

  • Aged
  • Aged, 80 and over
  • Follow-Up Studies
  • Humans
  • Hydrogels / chemistry*
  • Male
  • Middle Aged
  • Models, Statistical*
  • Organs at Risk / radiation effects*
  • Prognosis
  • Prostatic Neoplasms / surgery*
  • Radiosurgery*
  • Radiotherapy Dosage
  • Radiotherapy Planning, Computer-Assisted / methods*
  • Radiotherapy, Intensity-Modulated / methods
  • Rectum / radiation effects*


  • Hydrogels