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Iodine-125, 125I
Namesiodine-125, I-125, radioiodine
Nuclide data
Natural abundance0
Half-life59.49 days[1] ± 0.13
Parent isotopes125Xe
Decay products125Te
Decay modes
Decay modeDecay energy (MeV)
electron capture0.035 (35 keV)
Isotopes of iodine
Complete table of nuclides

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.

125I is produced by the electron capture decay of 125Xe, which is an artificial isotope of xenon, itself created by neutron capture of stable 124Xe, which occurs naturally with an abundance of around 0.1%. Because of the artificial production route of 125I and its short half-life, its natural abundance on Earth is effectively zero.


125I is a reactor-produced radionuclide and is available in large quantities. Its production follows the two reactions:

124Xe (n,γ) → 125mXe (57 s) → 125I (59.4 d)
124Xe (n,γ) → 125gXe (19.9 h) → 125I (59.4 d)

The irradiation target is natural xenon gas containing 0.0965 atom % (mole fraction) of the primordial nuclide 124Xe, which is the target isotope for making 125I by neutron capture. It is loaded into irradiation capsules of the zirconium alloy zircaloy-2 (a corrosion resisting alloy transparent to neutrons) to a pressure of about 100 bar (about 100 atm). Upon irradiation with slow neutrons in a nuclear reactor, several radioisotopes of xenon are produced. However, only the decay of 125Xe leads to a radioiodine: 125I. The other xenon radioisotopes decay either to stable xenon, or to various caesium isotopes, some of them radioactive (a.o., the long-lived 135Cs and 137Cs).

Long irradiation times are disadvantageous. Iodine-125 itself has a neutron capture cross section of 900 barns, and consequently during a long irradiation, part of the 125I formed will be converted to 126I, a beta-emitter and positron-emitter with a half-life of 13.1 days, which is not medically useful. In practice, the most useful irradiation time in the reactor amounts to a few days. Thereafter, the irradiated gas is allowed to decay for three or four days to eliminate short-lived unwanted radioisotopes, and to allow the newly created xenon-125 (half-life 17 hours) to decay to iodine-125.

To isolate radioiodine, the irradiated capsule is first cooled at low temperature (to collect free iodine gas on the capsule inner wall) and the remaining Xe gas is vented in a controlled way and recovered for further use. The inner walls of the capsule are then rinsed with dilute NaOH solution to collect iodine as soluble iodide (I) and hypoiodite (IO), according to the standard disproportionation reaction of halogens in alkaline solutions. Any caesium atom present immediately oxidizes and passes into the water as Cs+. In order to eliminate any long-lived 135Cs and 137Cs which may be present in small amounts, the solution is passed through a cation-exchange column, which exchanges Cs+ for another non-radioactive cation. The radioiodine (as anion I or IO) remains in solution as iodide/hypoiodite.

Availability and purity[edit]

Iodine-125 is commercially available in dilute NaOH solution as 125I-iodide (or the hypohalite sodium hypoiodite, NaIO) . The radioactive concentration lies at 4 to 11 GBq/ml and the specific radioactivity is >75 GBq/µmol (7.5 × 1016 Bq/mol). The chemical and radiochemical purity is high. The radionuclidic purity is also high; some 126I (t1/2 = 13.1 d) is unavoidable due to the neutron capture noted above. The 126I tolerable content (which is set by the unwanted isotope interfering with dose calculations in brachytherapy) lies at about 0.2 atom % (atom fraction) of the total iodine (the rest being 125I).


As of October 2019, there were two producers of iodine-125, the McMaster Nuclear Reactor in Hamilton, Ontario, Canada; and a research reactor in Uzbekistan.[4] The McMaster reactor is presently the largest producer of iodine-125, producing approximately 60 per cent of the global supply in 2018;[5] with the remaining global supply produced at the reactor based in Uzbekistan. Annually, the McMaster reactor produces enough iodine-125 to treat approximately 70,000 patients.[6]

In November 2019, the research reactor in Uzbekistan shut down its research reactor temporarily in order to facilitate repairs. The resulting temporary shut down threatened the global supply of the radioisotope, as it left the McMaster reactor as the only producer of iodine-125 during that period.[4][6]

Prior to 2018, the National Research Universal (NRU) reactor at Chalk River Laboratories in Deep River, Ontario, was one of three reactors to produce iodine-125.[7] However, on March 31, 2018, the NRU reactor was permanently shut down ahead of its scheduled decommissioning in 2028, as a result of government direction to end operations.[8][9] A nuclear reactor in Russia is also equipped to iodine-125, although the reactor is offline as of December 2019.[4]

Decay properties[edit]

The detailed decay mechanism to form the stable daughter nuclide tellurium-125 is a multi-step process that begins with electron capture. This is followed by a cascade of electron relaxation as the core electron hole moves toward the valence orbitals. The cascade involves many Auger transitions, each of which cause the atom to become increasingly ionized. The electron capture produces a tellurium-125 nucleus in an excited state with a half-life of 1.6 ns, which undergoes gamma decay emitting a gamma photon or an internal conversion electron at 35.5 keV. A second electron relaxation cascade follows the gamma decay before the nuclide comes to rest. Throughout the entire process an average of 13.3 electrons are emitted (10.3 of which are Auger electrons), most with energies less than 400 eV (79% of yield).[10] The internal conversion and Auger electrons from the radioisotope have been found in one study to do little cellular damage, unless the radionuclide is directly incorporated chemically into cellular DNA, which is not the case for present radiopharmaceuticals which use 125I as the radioactive label nuclide.[11]

As with other radioisotopes of iodine, accidental iodine-125 uptake in the body (mostly by the thyroid gland) can be blocked by the prompt administration of stable iodine-127 in the form of an iodide salt.[12][13] Potassium iodide (KI) is typically used for this purpose.[14]

However, unjustified self-medicated preventive administration of stable KI is not recommended in order to avoid to disturb the normal thyroid function. Such a treatment must be carefully dosed and requires an appropriate KI amount prescribed by a specialised physician.

See also[edit]

Notes and references[edit]

  1. ^ "Radionuclide half-life measurements data". NIST. 6 September 2009. Retrieved 3 November 2019.
  2. ^ Comparison of radiotoxicity of radioiodine isotopes[permanent dead link] accessed 6/22/10
  3. ^ I-125 vs. Pd-103 for permanent prostate brachytherapy accessed June 22, 2010.
  4. ^ a b c Frketich, Joanna (30 December 2019). "Shortages expected as McMaster becomes the world's only supplier of medical isotope used to treat prostate cancer". Toronto Star. Torstar Corporation. Retrieved 12 February 2020.
  5. ^ McMaster University (2019). "Written Submission for the Pre-Budget Consultations in Advance of the 2019 Budget" (PDF). House of Commons of Canada. p. 5. Retrieved 11 June 2019.
  6. ^ a b Hemsworth, Wade (6 December 2019). "McMaster helps solve world shortage of cancer-treatment isotopes". Brighter World. McMaster University.
  7. ^ "Medical Isotope Production @ McMaster – Nuclear". Retrieved 3 November 2019.
  8. ^ "Something borrowed, something new". Nuclear Engineering International. Compelo. 21 May 2019. Retrieved 15 June 2019.
  9. ^ "National Research Universal". Canadian Nuclear Laboratories. Retrieved 15 June 2019.
  10. ^ Pomplun, E.; Booz, J.; Charlton, D. E. (1987). "A Monte Carlo simulation of Auger cascades". Radiation Research. 111 (3): 533. doi:10.2307/3576938. ISSN 0033-7587. JSTOR 3576938.
  11. ^ Narra V.R., Howell R.W., Harapanhalli R.S., Sastry K.S., Rao D.V. (December 1992). "Radiotoxicity of some iodine-123, iodine-125 and iodine-131-labeled compounds in mouse testes: implications for radiopharmaceutical design". J. Nucl. Med. 33 (12): 2196–201. PMID 1460515.CS1 maint: uses authors parameter (link)
  12. ^ Harper, P.V.; Siemens, W.D.; Lathrop, K.A.; Brizel, H.E., & Harrison, R.W. (1961). "Iodine-125". Proc. Japan Conf. Radioisotopes. 4th. OSTI 4691987.CS1 maint: uses authors parameter (link)
  13. ^ Michigan State University (October 2013). Radiation safety manual, Environmental Health & Safety, see I-125, p. 81.
  14. ^ "NCRP Report 161 Management of persons contaminated with radionuclides – National Council on Radiation Protection and Measurements (NCRP) – Bethesda, MD". Retrieved 3 November 2019.