A prospective trial investigating the safety and efficacy of dose-escalated radiotherapy for men with prostate cancer following insertion of a gel spacer between the prostate and rectum

Retro-prostatic hydrogel insertion (SpaceOAR®) followed by dose-escalated radiotherapy (82 Gy in 41 fractions) Hydrogel (SpaceOAR®) insertion: SpaceOAR® hydrogel is a commercially available system that is on the Australian Register of Therapeutic Goods (ID 179172) which is used to increase the separation of the rectum and prostate during radiotherapy to the prostate. The rectum is the most dose-limiting organ at risk during prostate radiotherapy, and increasing the separation between the rectum and prostate decreases the radiation dose to the rectum. This allows for dose-escalation to the prostate which may have benefits for cancer control. Prior to SpaceOAR® insertion, participants had a baseline Computed Tomography (CT) scan for the purpose of radiotherapy planning and the determination of a baseline rectal Normal Tissue Complication Probability (NTCP). Participants were instructed to have a comfortably full bladder and empty rectum for the CT scan. Following this scan, participants were scheduled to undergo insertion of the SpaceOAR hydrogel. The SpaceOAR® is a hydrogel consisting of 90% water and 10% polyethelene glycol. The SpaceOAR® is injected as a liquid between the prostate and rectum under ultrasound guidance, solidifying within seconds into a hydrogel that pushes the anterior rectal away from the high dose region that envelopes the prostate. The hydrogel is biocompatible and maintains a consistent space over a 3-6 month period, during which an eight week course of radiotherapy would take place. After 6 months, the hydrogel liquefies, allowing it to be absorbed into the body and then cleared. SpaceOAR® hydrogel insertion was performed by a consultant level Radiation Oncologist or Urologist as a day procedure under a brief general anesthetic via the trans-perineal route and with antibiotic prophylaxis. The SpaceOAR® insertion took place approximately one week prior to the post-insertion CT scan for radiotherapy planning. The retro-prostatic space was hydro-dissected under ultrasound guidance and 10ml to 15ml of SpaceOAR® hydrogel was injected into the perirectal space using a transperineal approach under real-time ultrasound guidance. While under anesthetic, three gold-seed fidicuals were also inserted into the prostate to assist with target localisation during treatment, which is the standard of care for this population of men. A record of the insertion was documented by the radiation oncologist or urologist performing the procedure in the participant's electronic medical record. Dose-Escalated Radiotherapy: Approximately one week following SpaceOAR® insertion, and as part of radiotherapy planning, patients underwent Magnetic Resonance Imaging and a radiotherapy planning CT scan to assess the success of the SpaceOAR® insertion and for a consultant radiation oncologist to delineate the prostate, target volumes and organs at risk (rectum, rectal wall, ano-rectal junction, urethra, bladder and femurs). The prescribed radiation dose to the prostate was 82Gy in 41 fractions, delivered at 5 fractions per week. If clinical dose constraints to the rectum could not be achieved, the participant was prescribed 78Gy in 39 fractions (5 per week). Participants received intensity modulated radiation therapy using 10MV photons and treatment was delivered using a Novalis Tx(TM) or Trilogy(TM) (Varian Medical Systems, USA) linear accelerator.