<jats:title>Abstract</jats:title><jats:sec> <jats:title>Background</jats:title> <jats:p>The aim of this study was to clarify the association between intrafractional prostate shift and hydrogel spacer.</jats:p> </jats:sec><jats:sec> <jats:title>Methods</jats:title> <jats:p>Thirty-eight patients who received definitive volumetric modulated arc therapy (VMAT)-stereotactic body radiation therapy (SBRT) for prostate cancer with prostate motion monitoring in our institution in 2018–2019 were retrospectively evaluated. In order to move the rectum away from the prostate, hydrogel spacer (SpaceOAR system, Boston Scientific, Marlborough, the United States) injection was proposed to the patients as an option in case of meeting the indication of use. We monitored intrafractional prostate motion by using a 4-dimensional (4D) transperineal ultrasound device: the Clarity 4D ultrasound system (Elekta AB). The deviation of the prostate was monitored in each direction: superior-inferior, left–right, and anterior–posterior. We also calculated the vector length. The maximum intrafractional displacement (MID) per fraction for each direction was detected and mean of MIDs was calculated per patient. The MIDs in the non-spacer group and the spacer group were compared using the unpaired t-test.</jats:p> </jats:sec><jats:sec> <jats:title>Results</jats:title> <jats:p>We reviewed 33 fractions in eight patients as the spacer group and 148 fractions in 30 patients as the non-spacer group. The superior MID was 0.47 ± 0.07 (mean ± SE) mm versus 0.97 ± 0.24 mm (<jats:italic>P</jats:italic> = 0.014), the inferior MID was 1.07 ± 0.11 mm versus 1.03 ± 0.25 mm (<jats:italic>P</jats:italic> = 0.88), the left MID was 0.74 ± 0.08 mm versus 0.87 ± 0.27 mm (<jats:italic>P</jats:italic> = 0.55), the right MID was 0.67 ± 0.08 mm versus 0.92 ± 0.21 mm (<jats:italic>P</jats:italic> = 0.17), the anterior MID was 0.45 ± 0.06 mm versus 1.16 ± 0.35 mm (<jats:italic>P</jats:italic> = 0.0023), and the posterior MID was 1.57 ± 0.17 mm versus 1.37 ± 0.22 mm (<jats:italic>P</jats:italic> = 0.56) in the non-spacer group and the spacer group, respectively. The max of VL was 2.24 ± 0.19 mm versus 2.89 ± 0.62 mm (<jats:italic>P</jats:italic> = 0.19), respectively.</jats:p> </jats:sec><jats:sec> <jats:title>Conclusions</jats:title> <jats:p>Our findings suggest that maximum intrafractional prostate motion during VMAT-SBRT was larger in patients with hydrogel spacer injection in the superior and anterior directions. Since this difference seemed not to disturb the dosimetric advantage of the hydrogel spacer, we do not recommend routine avoidance of the hydrogel spacer use.</jats:p> </jats:sec>