SU-E-T-399: CT Image Artifacts from Brachytherapy Seed Implants: The Impact of Seed Size and Motion

Abstract

Purpose: To determine the impact of seed thickness on post-operative CT image quality, seed detection and dosimetry, with and without breathing motion. Methods: An acrylic prostate phantom mounted to a computer controlled motion platform was created for both standard (5 × 1 mm)- and small (5 × 0.5 mm)-diameter non-radioactive I-125 seeds (Oncura Inc., Arlington Heights, IL). Seed arrangement within the phantom was based on a patient brachytherapy plan. Ten low-contrast inserts, 2, 5, and 10 mm in diameter with contrast differences of 2, 12 and 29 HU, respectively, were embedded in the phantom. Volumetric CT scans were performed while the phantom exhibited vertical, longitudinal and elliptical motion cycles of amplitudes 0, 2, and 5mm. Streak artifacts were quantified by sampling the standard deviation (SD) of pixel values over regions of interest (ROI) within the phantom volume. Soft tissue contrast was quantified by calculating the contrast to noise ratio (CNR) for each low-contrast insert. Seed localization and dosimetry were assessed using VariSeed v8.0 (Varian Medical Systems, Palo Alto, CA) with prostate, rectal and urethral volumes copied from the patient plan onto the phantom. Results: Image noise was reduced with the small seeds for no motion cases (SD 49 vs. 32HU) and for 5mm amplitude cases (SD 60 vs. 37HU). CNR with the small seeds was higher by a median value of 55% for the low-contrast ROIs. Although VariSeed identified more erroneous seed positions for the small seeds, there were no clinically significant dosimetric differences to the prostate, rectal or urethral volumes. Conclusions: The use of smaller seeds for prostate brachytherapy results in improved image quality of post-operative CT scans, without hindering the ability in localizing the seeds or affecting the dose to the prostate, rectum or urethra, irrespective of motion parameters. Seeds used in this study were donated by GE Healthcare.