Discovered in 1947 by a team at Argonne National Laboratory and an independent Canadian research team, actinium-225 (Ac-225) undergoes alpha decay to francium-221 with a half-life of 10 days. For this reason, the isotope is being evaluated as an ideal candidate for use in medical applications. Alpha-emitting isotopes show great promise for cancer treatment because of their short path length and high energy transfer of radiation. Typically, the range is only that of a few cell diameters, making these isotopes highly effective at destroying cancer cells and leaving healthy cells untouched. Many radiolabeled products that incorporate Ac-225 are currently being used in numerous clinical trials for the treatment of cancers including prostate cancer, myeloid leukemia, and breast cancer.Presently, the primary source of Ac-225 comes from thorium-229 (Th-229) decay. Stores of Th-229 were considered a waste product, leftover from nuclear programs in the 1940s and 1950s. For many years, Th-229 was stored in a secure facility—until the discovery that Ac-225 and its decay daughter, Bi-213 were of great medical interest. Scientists at Oak Ridge National Laboratory (ORNL) began "milking" the stores of Th-229—a process that involves separating Ac-225 from the other isotopes that decay from thorium. The process is repeated every month, with each batch yielding millicurie quantities of Ac-225.
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