Radioactive therapeutic seeds include a substantially cylindrical capsule containing a radioactive isotope and, preferably, a radiopaque marker. According to one embodiment, the capsule is made of titanium or stainless steel, the isotope is deposited on the surface of a hollow cylinder and a titanium or stainless steel tube is disposed inside the hollow cylinder. The capsule is sealed by swaging the end of the capsule against the inner tube and welding the swaged end and tube. The inner tube facilitates swaging and protects the contents of the capsule from the heat of welding. In alternate embodiments, a radiopaque marker is disposed inside the inner tube or the cylinder bearing the radioisotope is radiopaque. According to other embodiments, a gripping stem is installed on one or both ends of a capsule, a capsule is made of very thin material with rotary swaged ends welded shut and a supporting structure inside the capsule, a meltable isotope mixture is used to coat the interior of a capsule, a capsule is assembled from two halves which are placed over an interior tube and the halves are butt welded, a capsule is made of a borosilicate or elemental silicon glass tube with ends melted to seal the capsule. Bioabsorbable spacers are provided for use with seeds having gripping/linking stems so that seeds are linked together.