What's known on the subject? and What does the study add? Different spacing agents have been tested to reduce incidential radiation exposure of the rectum during radiotherapy to the prostate. These agents all had certain drawbacks; either the created space was too small or the agents used did not stay in place during radiotherapy treatment. The study describes the transperineal injection technique of a spacing agent in detail. Furthermore it shows the safety and efficacy of the spacing hydrogel used and shows that it overcomes some of the drawbacks of the previously examined spacing agents.