Dosimetric impact of hydrogel spacer use for stereotactic body radiotherapy of localised prostate cancer

Purpose: Stereotactic body radiotherapy (SBRT) of prostate cancer is associated with rectal toxicities, which can be reduced by using a hydrogel spacer. The object of this retrospective study was to show the feasibility of spacer placement under local anesthesia and utility of hydrogel spacer to reduce the dose to the rectal wall. Material and methods: We collected data from all patients with localised prostate cancer treated with SBRT (40 Gy in 5 fractions) between 2018 and 2020. A hydrogel spacer (SpaceOAR®) was placed depending on the availability of the product. We collected dosimetric data for target volumes and organs at risk. We calculated mean values, which were compared using non-parametric tests. Results: Among 35 patients, mean age was 75 years. Seventeen had a spacer placed, with a mean space created of 10 mm. No complication was reported during the intervention. High doses to the rectal wall were significantly lower in spacer group (V38: 0.39 cm³ vs. 0.72 cm³; P = 0.02). PTV were better covered in spacer group (P = 0.07). Doses to the bladder wall were similar in both groups. Conclusion: Spacer procedure under local anesthesia was well tolerated. Hydrogel spacer allowed to reduce doses to the rectum while improving PTV coverage.