Permanent Prostate Brachytherapy Placement: Imaging Assessment of Seed Positioning, Spacer Use, and Complications

Low-dose-rate (LDR) brachytherapy with iodine 125 (¹²⁵I) seeds is an effective treatment of localized prostate cancer, delivering targeted doses to the prostate while minimizing radiation exposure to adjacent tissues. ¹²⁵I seeds implanted under transrectal US guidance provide tailored radiation distribution through loose or stranded configurations. Loose seeds allow flexible placement but carry a higher migration risk than stranded seeds. Optimal placement strategies involve targeting the peripheral zone near the prostate capsule to achieve adequate dose coverage for potential extracapsular extensions, with careful consideration to minimize toxicity to adjacent organs. Seed migration, primarily to the lungs, typically occurs through venous pathways due to the extensive periprostatic venous plexus. Less common migration sites include the heart, vertebral venous plexus, kidneys, liver, and testicular veins. Migration mechanisms involve retrograde venous flow and, in rare cases, arterial pathways through pulmonary arteriovenous malformations or intracardiac shunts. These seed migrations are typically incidental findings at imaging and rarely necessitate intervention because of the gradual radioactive decay of the seeds. Hydrogel spacers composed of polyethylene glycol are increasingly used during brachytherapy to reduce rectal toxicity by creating a separation between the prostate and rectum. Although these spacers are generally effective, complications such as asymmetric distribution, intraprostatic or intrarectal injection, and fistula formation can compromise radiation safety and efficacy. These complications can be identified at postprocedural imaging. Accurate imaging assessment is crucial for evaluating seed placement, detecting seed migration, and identifying complications related to hydrogel spacers, ultimately contributing to improved patient outcomes in prostate cancer treated with LDR brachytherapy. ^©RSNA, 2025 Supplemental material is available for this article.