Background: To report on our primary experience with the placement of a hydrogel spacer following stereotactic body radiation therapy (SBRT) in low- and intermediate-risk prostate cancer patients and assess its impact on dosimetry as well as acute toxicity. Methods: A total of 70 patients treated with SBRT (total dose of 36.25 Gy) in 5 fractions were included. Hydrogel spacers were inserted in 53 patients along with gold fiducial markers. For dosimetry, we trisected the rectum on the sagittal image of magnetic resonance imaging and defined it as the upper rectum (UR), middle rectum (MR), and lower rectum (LR). We compared the dose to each part of the rectum with and without hydrogel spacer using dose volume histograms. Genitourinary (GU) and gastrointestinal (GI) toxicity assessments were conducted until 6 months of follow-up visits. Results: The median volume of the hydrogel spacer was 12.3 mL. Overall, the hydrogel spacer could significantly reduce the rectal dose in the middle-to-high-dose region (V20–V35). The rectum doses at the UR and MR were significantly lower in the spacer group in the middle to high dose region (V20–V35); the dose at the LR was significantly lower in the spacer group in the high-dose region (V30–V35). There was no grade ≥ 3 toxicity observed, but grade 2 toxicity of GU and GI occurred in 17.1% and 1.4% of the patients, respectively. Conclusion: Hydrogel spacers could contribute to rectal dose reduction, especially in high dose regions, by creating a prostate–rectum distance.
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