Low rectal toxicity after dose escalated IMRT treatment of prostate cancer using an absorbable hydrogel for increasing and maintaining space between the rectum and prostate: Results of a multi-institutional phase II trial

Purpose: To evaluate the safety and efficacy of an absorbable hydrogel when injected between the rectum and prostate to reduce rectal radiation toxicity in adult men undergoing Intensity Modulated Radiotherapy (IMRT) for treatment of low and intermediate risk prostate cancer. Methods: This prospective, non-randomized, multi-center, single arm, open-label study included 52 men with a confirmed diagnosis of prostate cancer. They received transperineal injection of the hydrogel and 3-5 days after injection the simulation scans. All patients received IMRT (78 Gy delivered, 2 Gy per fraction). Space stability was evaluated by using MRI or CT. Gastrointestinal (GI) and genitourinary (GU) toxicity was assessed using RTOG/EORTC scoring system and proctoscopy after 12 months. The median follow up time was 12 months. Results: Hydrogel application was straight forward using brachytherapy equipment and techniques, with minimal patient discomfort. Six patients (12%) experienced acute GI Grade 2 toxicity, with no patients experiencing Grade 3 or 4 toxicity. In addition, no patients had early late GI toxicity ≥ Grade 2 after 12 months. The gel was stable during the course of radiotherapy and was not detectable in MRI after 9-12 months due to absorption in 42 of 43 patients. Conclusion: These data demonstrated that the hydrogel is a safe method to displace the rectal wall away from the prostate therefore substantially reducing toxicity to the rectum.