A new brachytherapy seed design for improved ultrasound visualization

In permanent brachytheraphy treatment of prostate cancer, typically 80 to 100 small radioactive seeds are implanted guided by transrectal ultrasound (TRUS) imaging. Analysis of clinical ultrasound and X-ray images show that only one third of implanted conventional seeds are visible with TRUS imaging, and that seed visibility is highly dependent on the angular orientation of individual seeds to the ultrasound beam. To facilitate ultrasound imaging, a new seed design, EchoSeed™, has been developed with a modified outer seed surface shape. The new seed is essentially identical to OncoSeed™ (Amersham Health) with respect to radiation dosimetry and outer dimensions. A simulation model of the seed ultrasound reflection properties based on diffraction theory was developed to optimize the seed design. The model was verified by in-vitro measurements of the angular dependency of the seed backscattered intensity. Prototypes were further evaluated by imaging of seeds implanted in excised dog prostates. While conventional seeds were only detectable over the tissue background for rotational angles of ±10°, EchoSeeds were visible over the entire investigated range of ±60°. The new design was further evaluated in an in-vivo dog prostate model. Ultrasound identified 99.2% (relative to fluoroscopy) of the implanted prototype EchoSeeds compared to 36.7% of the conventional seeds. The numbers identified by CT were close to 100% for both seed types.