Iodine-125

Iodine-125 (¹²⁵I) 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 59.49 days and it decays by electron capture to an excited state of tellurium-125. This state is not the metastable ^125mTe, 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 ¹²⁵Te 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.^[2] Eventually, stable ground state ¹²⁵Te is produced as the final decay 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 (¹²⁵I competes with palladium-103 in such uses).^[3]

Because of its relatively long half-life and emission of low-energy photons which can be detected by gamma-counter crystal detectors, ¹²⁵I 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, 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 ¹²³I 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). ¹²⁵I 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 radiotherapy ablation 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.

When studying plant immunity, ¹²⁵I is used as the radiolabel in tracking ligands to determine which plant pattern recognition receptors (PRRs) they bind to.^[4]

¹²⁵I is produced by the electron capture decay of ¹²⁵Xe, which is an artificial isotope of xenon, itself created by neutron capture of near-stable ¹²⁴Xe (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. Because of the artificial production route of ¹²⁵I and its short half-life, its natural abundance on Earth is effectively zero.