PP10 Presentation Time: 11:00 AM: Assessment of NASHA Spacer Symmetry for Prostate Radiation Therapy

Abstract

<h3>Purpose</h3> NASHA (non-animal stabilized hyaluronic acid) is a spacer that can increase the separation between the prostate and anterior rectal wall prior to prostate radiation therapy (RT). Since there is no pre-defined time limit for NASHA spacer injection, physicians can precisely distribute the product within the peri-rectal space. Furthermore, the spacer is highly visible on T2-weighted MR, and generally visible on CT with optimized view settings. We conducted a randomized controlled trial evaluating whether a NASHA spacer improves rectal dosimetry among patients receiving hypofractionated RT (NCT 04189913). The purpose of the study is to evaluate the symmetry of a NASHA spacer among all patients who received the device on that study. <h3>Materials and Methods</h3> We analyzed data for 131 of 136 patients, who were randomized to and received a NASHA spacer on trial (5 datasets were unavailable at the time of analysis). All patients had pre-implant and post-implant CT/MR simulation. We developed an automated system for evaluating symmetry based on a previously published method (Fischer-Valuck, PRO, 2017). Briefly, prostate, rectum and spacer contours that had been prospectively validated by a core laboratory were downloaded. From the prostate center-of-mass, we defined axial slices at mid-gland, 1-cm superior, and 1-cm inferior. For each axial slice, we calculated the center of the spacer (spacer width center) along the transverse axis. Axial slice symmetry scores were assigned based on the distance between the spacer width center and the prostate midline (1 = distance within 1 cm of midline (symmetric), 2L/2R = distance between 1-2 cm from midline, 3L/3R = distance more than 2 cm from midline). Patient-level symmetry scores were assigned based on the number of slices with an asymmetric distribution (1 = none, 2 = 1 of 3 slices with axial score of 2L/2R, 3 = 2 of 3 slices with axial score of 2L/2R, 4 = all 3 slices with axial score of 2L/2R). <h3>Results</h3> The percentages of patients with an axial slice symmetry score of 1 (symmetric) at mid-gland, superior, and inferior planes, were 97%, 98%, and 98%, respectively (see Figure). Following the Fischer rubric, planes without gel were omitted (N=2 mid-gland, N=2 superior and N=9 inferior planes). The percentage of patients with patient-level symmetry scores of 1, 2, 3 and 4, were 95.4%, 2.3%, 1.5%, and 0.8% respectively. <h3>Conclusions</h3> Over 95% of NASHA spacers placed in this prospective trial demonstrated patient-level symmetry. These results suggest that NASHA may also improve rectal dosimetry for prostate brachytherapy. Further study is ongoing to correlate the dosimetry and clinical outcomes to implant quality for these patients.