AVAILABLE AND POTENTIAL SHORT HALF-LIVED RADIONUCLIDE GENERATORS

Abstract

The visualization of organs, organ systems, and tumors with radionuclide imaging techniques is greatly facilitated by high counting rates. High counting rates require the administration of relatively large amounts of radioactivity. Large amounts of radioactivity are permissible in clinical studies if short-lived radionuclides are used to keep the patient-absorbed radiation dose to acceptable levels. To use short-lived nuclides it is necessary either to be physically close to a source of production or to obtain the radionuclides from a generator. Radionuclide generators make short-lived nuclides available at great distances from usual sources of production. The most desirable generator uses a long-lived parent nuclide to produce, via its decay, a short-lived daughter. The daughter nuclide may be separated by chemical or physical means, and the parent nuclide then generates a fresh supply of the daughter. The principle of the radionuclide generator is not new to radiation therapy. The separation of 3.8-day radon-222 from the 1620-year radium-226 in the preparation of radon seeds is a familiar example. The radon generator built by Failla in 19203 is still in active use at Memorial Center. Its active parent consists of 2 gm. of radium in solution. With the radon evolved from the parent, 300 radon seeds per week can be filled. Generators first came into substantial use in nuclear medicine with the development of a tellurium-132-iodine-132 system by Brookhaven National Laboratory in 1951.9, 14 The 2.3-hour half-life iodine-132 found occasional use in clinical studies in the United States but has been used to a much greater extent in Europe. From this beginning, many other generator systems have been developed, and several are now routinely and commercially available. Brookhaven National Laboratory has continued to be a prominent agency in the development of new radionuclide generators. SUMMARY Radionuclide generators make possible the clinical use of short-lived radiopharmaceuticals. Though several generator systems have been proposed, only a few actually have been developed. Of these, only one is commonplace (molybdenum-99-technetium-99m), but another is in a good state of development (tin-113-indium-113m). Clinical use of the others will depend on future investigation and development. The molybdenum-99-technetium-99m generator is available as a sterile, pyrogen-free device and is a prolific source of tech-netium-99m in the form of sodium pertechnetate. Sterility and radiochemical integrity must be checked, but this requirement should not be a detraction from the clinical advantages of the use of tech-netium-99m. Technetium generators can be operated economically only if they are eluted once a day and the eluate used gainfully. Many small institutions, however, find that they can use small generators to advantage. The tin-indium generator may also eventually come into wide-spread use. The 1.7-hour half-life of indium-113m makes possible high counting rates at modest patient absorbed radiation dose.