Abstract: A method of fabricating a radioactive core to be integrated within a source wire for use in radiation treatment of tumors within the body of a patient utilizes a thin elongate fiber of substantially pure iridium encapsulated in a straight quartz tube substantially transparent to the flux of a beam of radiation. The iridium fiber is oriented lengthwise along the axis of the tube, so that the fiber is restrained from curling. The tube and the fiber therein are irradiated with a beam directed normal to the axis of the tube while rotating the tube about the axis, to render the fiber radioactive to a predetermined level of activity. The method is adapted to produce a generally cylindrical Ir-192 source having a longitudinal axis of symmetry, a length of about one centimeter, a diameter of about 0.35 millimeter, a weight of about 20 milligrams and a radioactivity level of at least approximately 10 curies.