Hyaluronic Acid Rectal Spacer Stability during Radiation Therapy for Localized Prostate Cancer: An Intercontinental Study.

PURPOSE/OBJECTIVE(S): Despite IGRT and IMRT current techniques, rectal toxicity remains a significant problem after prostate cancer radiation therapy but implanted rectal spacers have successfully reduced toxicity rates. We are reporting the results of an intercontinental prospective rectal spacer trial performed at 13 centers in Spain, Australia, and USA. Our hypothesis was that the separation created between the prostate and anterior rectal wall was stable between the time of implant and a 3-month follow-up.

MATERIALS/METHODS: Our prospective multicenter study was approved by the corresponding IRBs and patients signed informed consent. It was performed between February - June 2021. Patients were imaged using T2 MRI immediately post-implant (MRI-1) and at 3 months (MRI-2). We analyzed the dimensions of hyaluronic acid (HA) rectal spacer inserted for low - intermediate risk prostate cancer treated with EBRT. The rectal displacement was determined by measuring the separation at 3 different levels: prostate midline, midgland +1cm (superior), and midgland -1 cm (inferior), from the posterior prostate capsule to the anterior rectal wall. The core laboratory performed the measurement for all sites. The confidence interval (CI) was computed using a Student's t-distribution.

RESULTS: A total of 136 patients randomized to the Barrigel arm underwent HA rectal spacing with 100% placement success rate. There were no device failures or surgical complications. Of these, 6 were lost to follow-up. The averages of the remaining 130 patients at the 3 perirectal distances were 13.04 mm +/- 3.1 mm and 12.79 mm +/- 3.5 mm for MRI-1 and MRI-2, respectively with an average difference of -0.17 mm +/- 3.48 mm. Additional parameters are listed on the table.

CONCLUSION: The results demonstrate the stability of the HA-created separation at the three different prostatic levels for up to 3 months. These findings show dimensional stability well within standard clinical margins. This indicates reliability for HA use in most clinics, as the results are relevant in the setting of dose escalation or ultra-hypofractionation schedules, as well as conventional fractionation.