Radioactive therapeutic seeds include a substantially radiotransparent cylindrical capsule provided with a radioactive isotope, a radiopaque marker, and an engagement structure adapted to engage tissue surrounding the seeds at the implant site. The engagement structure is also preferably adapted to releasably engage the inside of a delivery needle prior to implantation. The engagement structure is preferably adapted to prevent axial longitudinal movement and axial rotation of a seed about its longitudinal axis when the seed is implanted. According to a preferred embodiment of the invention, the engagement structure includes three spring elements which are biased away from the seed capsule and which provide positive fixation in the tissue, and also frictionally engage the inside of the needle.