Same-day versus delayed simulation imaging after placement of a perirectal hydrogel spacer for prostate radiotherapy

Introduction Placement of a perirectal hydrogel spacer has been demonstrated to reduce the risk rectal toxicity from prostate radiation. Practices vary regarding timing CT simulation after placement, and ideal schedule remains unknown. Methods Thirty patients with localized adenocarcinoma underwent transrectal ultrasound-guided placement an iodinated SpaceOAR™ prior radiotherapy. Per evolving practice, 15 completed same-day returned for 1–2 weeks later. Hydrogel volume, distance, air-void dosimetry per NRG GU005 were compared between simulation, 1st fraction Cone-Beam-CT (CBCT), final CBCT. Results occurred 8.8 ± 2.4 days in delayed group, no significant difference interval groups ( p = 0.165). Greater observed de-creases volume (0.57 cc vs. 0.04 cc, 0.0002), distance at both mid-gland (1.32 mm 0.17 mm) tallest point (2.40 seen on 1st-fraction CBCT group 0.0039; 0.0002). recalculated CBCT, five (D3 D50%) versus one (D50%) dose parameters exceeded groups, respectively, 10 had relative increase ≥ 20%. Conclusion Due anatomic changes following scanning may introduce unintended variability time