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This paper describes a new 3-D ultrasound robotic prostate brachytherapy system. It uses a stationary 3-D ultrasound probe rigidly fixed to a robotic needle insertion mechanism. The novelty of the system is its ability to track prostate motion intraoperatively to allow the dose planning and needle trajectories or depths to be adapted to take into account these motions. Prostate tracking is done using a fast 3-D ultrasound registration algorithm previously validated for biopsy guidance. The 7-degree-of-freedom (7-DOF) robot and ultrasound probe are calibrated together with an accuracy of 0.9 mm, allowing the needles to be precisely inserted to the seed targets chosen in the reference ultrasound image. Experiments were conducted on mobile, deformable synthetic prostate phantoms using a prototype laboratory system. Results showed that, with prostate motions of up to 7 mm, the system was able to reach the chosen targets with less than 2-mm accuracy in the needle insertion direction. This measured accuracy included extrinsic measurement errors of up to 1.1 mm. A preliminary cadaver feasibility study was also described in preparation for more realistic experimentation of the system.