Rectal spacer in radiotherapy for prostate cancer: Correlation of spacer symmetry to rectal dosimetry.

e17102 Background: Rectal spacers are used to temporarily position the anterior rectal wall away from the prostate during radiotherapy for prostate cancer, and to limit the exposure of the rectum to excessive radiation. These spacers use different insertion techniques and filling materials. Our study aims to compare and correlate the symmetry and rectal dosimetry in patients implanted with either the balloon spacer (BioProtect Balloon Implant System) or a gel spacer (SpaceOAR). Methods: A qualitative model utilizing measurements of prostate-rectal interspace from axial computed tomography scans was applied to assess 62 T1c-T4 prostate cancer patients (28 balloon, 34 gel spacer) undergoing SBRT (5/6 fractions, 36-40 Gy) or IMRT (20/28 fractions, 60-70 Gy). Spacer symmetry at midgland, 1 cm superior or inferior to midgland was scored as ‘S’. Asymmetrical placement (1 or 2cm lateral left or right) was scored as 1 or 2. Axial slices with no spacer was scored as 0. A total symmetry score (SYM) was evaluated by using a scoring of SYM-S indicating spacer symmetry in all 3 slices and scores ranging 1-to-4 representing increasing spacer asymmetry. SYM score was then evaluated relatively to rectal dosimetry at V60%-V100%. Results: For balloon and gel spacers, symmetrical distribution at midline, at 1 cm inferior and superior to midgland was observed in 16 (57.1%) and 18 (52.9%) patients, 16 (57.1%) and 15 (44.1%) patients, 10 (35.7%) and 12 (35.3%) patients, respectively. The balloon was not visible in 3 (10.7%) superior slices compared to 3 (8.8%) inferior and 6 (17.6%) superior gel spacer slices, receiving score 0. The mean rectal dose for the balloon spacer compared to the gel spacer was 9.8 and 12.9 for V60%, 5.6 and 7.9 for V70%, 3.1 and 4.5 for V80%, 1.5 and 2.0 for V90% and 0.12 and 0.17 for 100% respectively. The study results show that as the spacers became more asymmetric, the rectal dose-volume at V60%-V100% increased. Conclusions: The balloon provides a better symmetrical distribution and lower dose to the rectum compared to the gel. The potential advantages of the balloon should be further evaluated in a prospective setting.