What is "iodine-125"

n. light radioactive isotope of iodine with a half-life of 60 days; used as a tracer in thyroid studies and as a treatment for hyperthyroidism

Iodine-125 (I) is a radioisotope of iodine which has uses in biological assays, nuclear medicine imaging and in radiation therapy as brachytherapy to treat a number of conditions, including prostate cancer, uveal melanomas, and brain tumors. It is the second longest-lived radioisotope of iodine, after iodine-129.

Its half-life is 59.43 days and it decays by electron capture to an excited state of tellurium-125. This state is not the metastable Te-125m, but rather a lower energy state that decays immediately by gamma decay with a maximum energy of 35 keV. Some of the excess energy of the excited Te-125 may be internally converted ejected electrons (also at 35 keV), or to x-rays (from electron bremsstrahlung), and also a total of 21 Auger electrons, which are produced at the low energies of 50 to 500 electron volts. Eventually, stable nonradioactive ground-state Te-125 is produced, as the final decay product.

The internal conversion and Auger electrons cause little damage outside the cell which contains the isotope atom. The X-rays and gamma rays are of low enough energy to deliver a higher radiation dose selectively to nearby tissues, in "permanent" brachytherapy where the isotope capsules are left in place (I-125 competes with palladium-103 in such uses).

Because of its relatively long half-life and emission of low-energy photons which can be detected by gamma-counter crystal detectors, I-125 is a preferred isotope for tagging antibodies in radioimmunoassay and other gamma-counting procedures involving proteins outside the body. The same properties of the isotope make it useful for brachytherapy (as noted), and for certain nuclear medicine scanning procedures, in which it is attached to proteins (albumin or fibrinogen), and where a longer half-life than provided by I-123 is required for test lasting several days.

Iodine-125 has been used in scanning/imaging the thyroid, but iodine-123 is preferred for this purpose, due to better radiation penetration and shorter half-life (13 hours). For radiotherapy killing of tissues that absorb iodine (such as the thyroid) or that absorb an iodine-containing radiopharmaceutical, the beta-emitter iodine-131 is the preferred isotope; iodine-125 is used therapeutically (to kill tissue) only in brachytherapy.

I is created by the electron capture decay of Xe, which is a synthetic isotope of xenon, itself created by neutron capture of the slightly radioactive Xe, which occurs naturally with an abundance of around 0.1%. Because of the synthetic production route of I and its short half-life, the natural abundance is effectively zero.