Perirectal spacers have been shown to reduce rectal exposure to radiation therapy, reduce toxicity, and improve the quality of life of men receiving radiation therapy for prostate cancer. However, first-generation spacing products also carry a small risk of rectal fistulas, which may result in Grade IV complications. The FDA has recently approved a next-generation perirectal spacer constrained by an absorbable vicryl balloon, which may reduce the risk of severe injury, improve spacing quality, allow for repositioning, and provide more consistent space than other approved spacers. The risk of rectal injury during the procedure may be nullified by the fact that blunt dissection is used for placement and the balloon is filled with sterile normal saline alone. We report our initial experience of using this new device in patients undergoing LDR brachytherapy. The patients were enrolled in an IRB-approved prospective registry. Ten patients were implanted by a physician experienced with implantation of hydrogel and hyaluronic acid-based spacers. The ten patients were the first ten patients attempted by the physician with the new balloon implant device. The Rectal Spacing Quality Score (SQS) was evaluated using the method established by Grossman et al.1 The SQS is scored as an integer from 0 (worst) to 2 (best). A validated AI contouring tool derived the prostate and rectal contours to prevent bias (MIM Protége), and was authenticated by the author. The spacing height (mm) was measured in 3 axial planes: 6 mm caudal to the base, at the midgland, and 6 mm cranial to the apex. These three axial locations had spacer heights measured from the posterior edge of the prostate gland to the anterior edge of the rectum. In each axial plane, the distance was evaluated at the midline of the rectum and 1 cm to the right and left of the midline. Post-implant dosimetry of the rectum for D1cc, D3CC, and V100% is reported. The median age was 66.5 years. The mean ± SD of the prostate volume was 32 ± 16 cc. Four patients received LDR monotherapy prescribed at 125 Gy with PD-103, and six received LDR as a boost with PD-103 delivered before the external beam course. The mean, minimum, and maximum spaces achieved in each of the three axial sections are summarized in Table 1. Nine (90%) patients achieved an SQS score of 2, whereas 1 (10%) achieved a score of 1. None of the patients had a prescription dose touching the rectum in post-implant dosimetry (v100%=0% for all). The mean (min-max) D1cc% and D3cc% for the rectum were 21% (8-36%) and 2.5% (1%-24%), respectively. Two patients reported transient grade 1 rectal discomfort on the day of implantation, which resolved the following day. No Grade 2 or higher toxicities were reported. Absorbable balloon spacers achieve superior spacing compared to a historical cohort of non-constrained spacers as measured by the SQS score.1 In this study, 90% of patients had the ideal SQS score (SQS=2), compared to only 43% in the historical comparator. Balloon spacers appear to have a minimum learning curve to achieve optimal space given that these patients represent the first ten attempted by an experienced clinician with this new product. Although the mean spaces exceeded those routinely achieved by other spacing products, patients reported minimal discomfort. Balloon spacers appear to be safe and effective in brachytherapy. [ABSTRACT FROM AUTHOR]