Radioactive seeds are routinely implanted to treat cancers, such as prostate cancer. The radioactive elements iodine-125, which has a 60-day half-life and emits a 31 keV x-ray, and palladium-103, which has a 17-day half-life and emits a 21 keV x-ray, both emit only soft x-rays from electron capture decay and are widely used in seeds for this type of radiotherapy.
Co-pending U.S. patent application Ser. No. 09/047,728 teaches a method of ion-implanting the precursor isotope Xe-124 into an aluminum pellet and subsequently activating the Xe-124 in a nuclear reactor to produce I-125 embedded beneath the surface of the aluminum pellet. Using this method for a palladium seed, however, would be extremely expensive because palladium which has been extensively enriched for Pd-102, which is the preferred activatable isotope, is costly. For example, 78% enriched Pd-102 (e.g., having 78% Pd-102) is available from Oak Ridge National Laboratory at a cost of approximately $867,000 per gram. Pd-104 and Pd-105 provide the balance of the palladium in this material, and it is substantially free of Pd-106, Pd-108, and Pd-110. The presence of Pd-108 and Pd-110 in particular is undesirable because, upon activation, these isotopes tend to form long-lived gamma-emitting isotopes.
U.S. Pat. No. 4,702,228 to Russell teaches using a substantially enriched separated isotope (i.e., enriched to a minimum of 50% Pd-102) to fabricate radioactive interstitial implants emitting radiation from Pd-103. The disadvantage of using this highly enriched, separated Pd-102 is that it is extremely expensive and is difficult to obtain in sufficient quantities. Russell teaches that this highly enriched isotope is necessary because natural palladium contains only about 1% Pd-102 and the remainder of the palladium absorbs much of the x-ray intensity generated by the radioactive Pd-103. However, the Russell design uses two spherical pellets approximately 0.6 mm in diameter in which the enriched palladium is distributed uniformly throughout the volume of the pellet. This design contributes to self-absorption because radiation from palladium atoms located away from the surface of the pellets must travel to the surface of the pellet without being absorbed along the way.