Radioactive therapeutic seeds include a seed capsule, a radioactive isotope, and a radiopaque marker. According to one embodiment of the invention, the radiopaque marker is a silver wire coil and the isotope is coated on the surface of the coil. According to a first method of the invention, the therapeutic dosage of the seed is adjusted during manufacture by expanding or contracting the coil before it sized and deposited in the capsule. According to second embodiment, the radiopaque marker is a silver rod and the isotope is coated on a radiotransparent coil which is placed over the rod. According to a third embodiment, the isotope is coated on a plurality of rings which are stacked on a radiopaque rod. According to a second method of the invention, the therapeutic dosage of the seed is adjusted during manufacture by stacking a mixture of radio-inactive rings with radioactive rings on the radiopaque rod before the capsule is sealed.