Iodine-125

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 59.392 days and it decays by electron capture to an excited state of tellurium-125. This state is not the metastable Te, but a much shorter-lived excited state that decays either by (7% chance) emitting a gamma ray with energy of 35 keV, or more likely (93% chance), undergoing internally conversion and ejecting an electron (of lower energy than 35 keV). The resulting electron vacancy leads to emission of characteristic X-rays (27–32 keV) and Auger electrons (50 to 500 eV). In either case stable ground state Te is the product.

In medical applications, 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 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-countercrystal detectors, I is a preferred isotope for taggingantibodies in radioimmunoassay and other gamma-counting procedures involving proteins outside the body. The same properties of the isotope make it useful for brachytherapy, and for certain nuclear medicine scanning procedures, in which it is attached to proteins (albumin or fibrinogen), and where a half-life longer than that provided by I is required for diagnostic or lab tests lasting several days.

Iodine-125 can be 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). I is useful for glomerular filtration rate (GFR) testing in the diagnosis or monitoring of patients with kidney disease. Iodine-125 is used therapeutically in brachytherapy treatments of tumors. For radiotherapyablation of tissues that absorb iodine (such as the thyroid), or that absorb an iodine-containing radiopharmaceutical, the beta-emitteriodine-131 is the preferred isotope.

When studying plant immunity, I is used as the radiolabel in tracking ligands to determine which plant pattern recognition receptors (PRRs) they bind to.

I is produced by the electron capture decay of Xe, which is an artificial isotope of xenon, itself created by neutron capture on nearly-stable Xe (it undergoes double electron capture with a half-life orders of magnitude larger than the age of the universe), which makes up around 0.1% of naturally occurring xenon.