Iodine-125

<h2 id="production">Production</h2>
125I is a reactor-produced radionuclide and is available in large quantities. Its production involves the two following <a href="/facts/Nuclear_reaction/202yClUr">nuclear reactions</a>:

124Xe (n,γ) → 125mXe (57 s)   → 125I       (t½ = 59.4 d)
124Xe (n,γ) → 125gXe (19.9 h) → 125I       (t½ = 59.4 d)
The irradiation target is the <a href="/facts/Primordial_nuclide/lFAUqHDv">primordial nuclide</a> 124Xe, which is the target isotope for making 125I by <a href="/facts/Neutron_capture/elsFpM12">neutron capture</a>. It is loaded into irradiation capsules of the zirconium alloy <a href="/facts/Zircaloy/1mrpJjWL">zircaloy-2</a> (a <a href="/facts/Corrosion/lIC7H4we">corrosion</a> resisting <a href="/facts/Alloy/I7w7WQWR">alloy</a> transparent to <a href="/facts/Neutron/oRCM1geb">neutrons</a>) to a pressure of about 100 <a href="/facts/Bar_(unit)/wwFWTTkT">bar</a> (~ 100 <a href="/facts/Atmosphere_(unit)/aTbYALII">atm</a>). Upon irradiation with <a href="/facts/Thermal_neutron/UMKR4XgJ">slow neutrons</a> in a <a href="/facts/Nuclear_reactor/8xqjIbge">nuclear reactor</a>, several <a href="/facts/Isotopes_of_xenon/Yz78R52h">radioisotopes of xenon</a> are produced. However, only the decay of 125Xe leads to a radioiodine: 125I. The other xenon radioisotopes decay either to stable <a href="/facts/Xenon/9E3XR69R">xenon</a>, or to various <a href="/facts/Isotopes_of_caesium/m5AYV9tp">caesium isotopes</a>, some of them radioactive (a.o., the long-lived <a href="/facts/Caesium-135/m5AYV9tp">135Cs</a> (t½ = 1.33 Ma) and <a href="/facts/Cesium-137/NrY5Tb6j">137Cs</a> (t½ = 30 a)).
Long <a href="/facts/Neutron_activation/FXwq3HUs">irradiation</a> times are disadvantageous. Iodine-125 itself has a <a href="/facts/Neutron_capture/elsFpM12">neutron capture</a> <a href="/facts/Neutron_cross_section/gAkJmXXA">cross section</a> of 900 <a href="/facts/Barn_(unit)/0yCEtKJ9">barns</a>, and consequently during a long irradiation, part of the 125I formed will be converted to 126I, a <a href="/facts/Beta_decay/vxTBlckp">beta-emitter</a> and <a href="/facts/Positron_emission/psWp4bJT">positron-emitter</a> with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 12.93 days,<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> 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 produced xenon-125 (t½ = 17 hours) to decay to iodine-125.
To isolate radio-iodine, the irradiated capsule is first cooled at low temperature (to <a href="/facts/Condensation/yvmkI08A">condense</a> the free iodine gas onto 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 a dilute <a href="/facts/Sodium_hydroxide/xjtdYtq7">NaOH</a> solution to collect iodine as <a href="/facts/Solubility/kLE9h3Jb">soluble</a> <a href="/facts/Iodide/JyVRGWMX">iodide</a> (I−) and <a href="/facts/Hypoiodite/XBDsmBKr">hypoiodite</a> (IO−), according to the standard <a href="/facts/Disproportionation/67QMkXrV">disproportionation</a> reaction of <a href="/facts/Halogen/YeFgWHeA">halogens</a> in <a href="/facts/Alkaline/s6NqPGSP">alkaline</a> solution. Any <a href="/facts/Caesium/dXKN1S5k">caesium</a> atom present immediately <a href="/facts/Redox/Pyd5RVcj">oxidizes</a> 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 <a href="/facts/Ion_exchange/C2DPoGqg">cation-exchange</a> column, which exchanges Cs+ for another non-radioactive <a href="/facts/Cation/wrbwhF3z">cation</a> (e.g., Na+). The radioiodine (as <a href="/facts/Anion/wrbwhF3z">anion</a> I− or IO−) remains in solution as a mixture iodide/hypoiodite.

<h2 id="availability-and-purity">Availability and purity</h2>
Iodine-125 is commercially available in dilute <a href="/facts/Sodium_hydroxide/xjtdYtq7">NaOH</a> solution as 125I-iodide (or the <a href="/facts/Hypohalite/YUeU1wgI">hypohalite</a> sodium <a href="/facts/Hypoiodite/XBDsmBKr">hypoiodite</a>, 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 = 12.93 d)<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a> is unavoidable due to the <a href="/facts/Neutron_capture/elsFpM12">neutron capture</a> noted above. The 126I tolerable content (which is set by the unwanted isotope interfering with <a href="/facts/Dosimetry/pnl0fMxH">dose calculations</a> in <a href="/facts/Brachytherapy/YCxeYI8L">brachytherapy</a>) lies at about 0.2 atom % (<a href="/facts/Mole_fraction/nW4EepmJ">atom fraction</a>) of the total iodine (the rest being 125I).

<h2 id="producers">Producers</h2>
As of October 2019, there were two producers of iodine-125, the <a href="/facts/McMaster_Nuclear_Reactor/Gc4Lo4sn">McMaster Nuclear Reactor</a> in <a href="/facts/Hamilton%2c_Ontario/wnN4KHHf">Hamilton</a>, <a href="/facts/Ontario/HUkxk5Rf">Ontario</a>, Canada; and a VVR-SM research reactor in Uzbekistan.<a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> The McMaster reactor is presently the largest producer of iodine-125, producing approximately 60 per cent of the global supply in 2018;<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> 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.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>
In November 2019, the research reactor in Uzbekistan shut down temporarily in order to facilitate repairs. The temporary shutdown threatened the global supply of the radioisotope by leaving the McMaster reactor as the sole producer of iodine-125 during the period.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a><a class="footnote-ref" id="fnref:10" href="#fn:10">10</a>
Prior to 2018, the <a href="/facts/National_Research_Universal_reactor/AFph0rAY">National Research Universal</a> (NRU) reactor at <a href="/facts/Chalk_River_Laboratories/aHQ9MQYv">Chalk River Laboratories</a> in <a href="/facts/Deep_River%2c_Ontario/wuRX3vM3">Deep River</a>, Ontario, was one of three reactors to produce iodine-125.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a> However, on March 31, 2018, the NRU reactor was permanently shut down ahead of its scheduled decommissioning in 2028, as a result of a government order.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a><a class="footnote-ref" id="fnref:13" href="#fn:13">13</a> The Russian nuclear reactor equipped to produce iodine-125, was offline as of December 2019.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a>

<h2 id="decay-properties">Decay properties</h2>
The detailed decay mechanism to form the stable daughter nuclide tellurium-125 is a multi-step process that begins with <a href="/facts/Electron_capture/EIccoV1u">electron capture</a>, which produces a tellurium-125 nucleus in an excited state with a half-life of 1.6 ns. The excited tellurium-125 nucleus may undergo <a href="/facts/Gamma_decay/5Drf913J">gamma decay</a>, emitting a gamma <a href="/facts/Photon/mFNhAEzA">photon</a> at 35.5 keV, or undergo <a href="/facts/Internal_conversion/mVkoudEe">internal conversion</a> to emit an <a href="/facts/Electron/L0KBo5cW">electron</a>. The electron vacancy from internal conversion results in a cascade of <a href="/facts/Electron_excitation/66pkjfmB">electron relaxation</a> as the core electron hole moves toward the <a href="/facts/Valence_electron/mXPA15VP">valence orbitals</a>. The cascade involves many <a href="/facts/Characteristic_X-ray/Qw75UEha">characteristic X-rays</a> and <a href="/facts/Auger_effect/a0SVoK1r">Auger transitions</a>. In the case the excited tellurium-125 nucleus undergoes gamma decay, a different electron relaxation cascade follows before the nuclide comes to rest. Throughout the entire process an average of 13.3 electrons are emitted (10.3 of which are <a href="/facts/Auger_electron/a0SVoK1r">Auger electrons</a>), most with energies less than 400 eV (79% of yield).<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a> 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 <a href="/facts/DNA/nB89Iauz">DNA</a>, which is not the case for present radiopharmaceuticals which use 125I as the radioactive label nuclide.<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a> Rather, cellular damage results from the gamma and characteristic X-ray photons.
As with other radioisotopes of iodine, accidental iodine-125 uptake in the body (mostly by the <a href="/facts/Thyroid/bjovTJVu">thyroid</a> gland) can be blocked by the prompt administration of stable <a href="/facts/Iodine-127/HxgH1yJD">iodine-127</a> in the form of an iodide salt.<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a><a class="footnote-ref" id="fnref:18" href="#fn:18">18</a> <a href="/facts/Potassium_iodide/6023uFnx">Potassium iodide</a> (KI) is typically used for this purpose.<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a>
However, unjustified <a href="/facts/Self-medication/wkD9fMtg">self-medicated</a> preventive administration of stable KI is not recommended in order to avoid disturbing the normal <a href="/facts/Thyroid/bjovTJVu">thyroid function</a>. Such a treatment must be carefully dosed and requires an appropriate KI amount <a href="/facts/Medical_prescription/4jt1T3mY">prescribed</a> by a specialised physician.

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Isotopes_of_iodine/HxgH1yJD">Isotopes of iodine</a></li>
<li><a href="/facts/Iodine-123/t3hBrGLV">Iodine-123</a></li>
<li><a href="/facts/Iodine-129/oJDlGf5H">Iodine-129</a></li>
<li><a href="/facts/Iodine-131/cMvRbKfK">Iodine-131</a></li>
<li><a href="/facts/Iodine_in_biology/45UtNjGN">Iodine in biology</a>
<ul><li><a href="/facts/2%2c5-Dimethoxy-4-iodoamphetamine/gUnTooCA">2,5-Dimethoxy-4-iodoamphetamine</a> (DOI), often labeled with 125I</li></ul></li></ul>
<h2 id="notes-and-references">Notes and references</h2>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">"Iodine-125 | Oncology Medical Physics". Retrieved 2025-02-17. <a href="https://oncologymedicalphysics.com/iodine-125/" target="_blank">https://oncologymedicalphysics.com/iodine-125/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">I-125 vs. Pd-103 for permanent prostate brachytherapy accessed June 22, 2010. <a href="http://www.prostate-cancer.com/brachytherapy/cancer-treatments/tools-of-brachytherapy.html" target="_blank">http://www.prostate-cancer.com/brachytherapy/cancer-treatments/tools-of-brachytherapy.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Boutrot, Freddy; Zipfel, Cyril (2017-08-04). "Function, Discovery, and Exploitation of Plant Pattern Recognition Receptors for Broad-Spectrum Disease Resistance". Annual Review of Phytopathology. 55 (1). Annual Reviews: 257–286. doi:10.1146/annurev-phyto-080614-120106. ISSN 0066-4286. PMID 28617654. <a href="https://doi.org/10.1146%2Fannurev-phyto-080614-120106" target="_blank">https://doi.org/10.1146%2Fannurev-phyto-080614-120106</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
 <a href="https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf" target="_blank">https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
 <a href="https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf" target="_blank">https://www-nds.iaea.org/amdc/ame2020/NUBASE2020.pdf</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">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. <a href="https://www.thestar.com/news/gta/2019/12/30/shortages-expected-as-mcmaster-becomes-the-worlds-only-supplier-of-medical-isotope-used-to-treat-prostate-cancer.html" target="_blank">https://www.thestar.com/news/gta/2019/12/30/shortages-expected-as-mcmaster-becomes-the-worlds-only-supplier-of-medical-isotope-used-to-treat-prostate-cancer.html</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">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. <a href="https://www.ourcommons.ca/Content/Committee/421/FINA/Brief/BR10007576/br-external/McMasterUniversity-e.pdf" target="_blank">https://www.ourcommons.ca/Content/Committee/421/FINA/Brief/BR10007576/br-external/McMasterUniversity-e.pdf</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Hemsworth, Wade (6 December 2019). "McMaster helps solve world shortage of cancer-treatment isotopes". Brighter World. McMaster University. <a href="https://brighterworld.mcmaster.ca/articles/mcmaster-helps-solve-world-shortage-of-cancer-treatment-isotopes/" target="_blank">https://brighterworld.mcmaster.ca/articles/mcmaster-helps-solve-world-shortage-of-cancer-treatment-isotopes/</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">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. <a href="https://www.thestar.com/news/gta/2019/12/30/shortages-expected-as-mcmaster-becomes-the-worlds-only-supplier-of-medical-isotope-used-to-treat-prostate-cancer.html" target="_blank">https://www.thestar.com/news/gta/2019/12/30/shortages-expected-as-mcmaster-becomes-the-worlds-only-supplier-of-medical-isotope-used-to-treat-prostate-cancer.html</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Hemsworth, Wade (6 December 2019). "McMaster helps solve world shortage of cancer-treatment isotopes". Brighter World. McMaster University. <a href="https://brighterworld.mcmaster.ca/articles/mcmaster-helps-solve-world-shortage-of-cancer-treatment-isotopes/" target="_blank">https://brighterworld.mcmaster.ca/articles/mcmaster-helps-solve-world-shortage-of-cancer-treatment-isotopes/</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">"Medical Isotope Production @ McMaster – Nuclear". Retrieved 3 November 2019. <a href="https://nuclear.mcmaster.ca/products-services/medical-isotopes/" target="_blank">https://nuclear.mcmaster.ca/products-services/medical-isotopes/</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">"Something borrowed, something new". Nuclear Engineering International. Compelo. 21 May 2019. Retrieved 15 June 2019. <a href="https://www.neimagazine.com/features/featuresomething-borrowed-something-new-7218068/" target="_blank">https://www.neimagazine.com/features/featuresomething-borrowed-something-new-7218068/</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">"National Research Universal". Canadian Nuclear Laboratories. Archived from the original on 20 June 2019. Retrieved 15 June 2019. <a href="https://web.archive.org/web/20190620035929/http://www.cnl.ca/en/home/facilities-and-expertise/nru/default.aspx" target="_blank">https://web.archive.org/web/20190620035929/http://www.cnl.ca/en/home/facilities-and-expertise/nru/default.aspx</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">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. <a href="https://www.thestar.com/news/gta/2019/12/30/shortages-expected-as-mcmaster-becomes-the-worlds-only-supplier-of-medical-isotope-used-to-treat-prostate-cancer.html" target="_blank">https://www.thestar.com/news/gta/2019/12/30/shortages-expected-as-mcmaster-becomes-the-worlds-only-supplier-of-medical-isotope-used-to-treat-prostate-cancer.html</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Pomplun, E.; Booz, J.; Charlton, D. E. (1987). "A Monte Carlo simulation of Auger cascades". Radiation Research. 111 (3): 533–552. Bibcode:1987RadR..111..533P. doi:10.2307/3576938. ISSN 0033-7587. JSTOR 3576938. PMID 3659286. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">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. <a href="http://jnm.snmjournals.org/cgi/pmidlookup?view=long&pmid=1460515" target="_blank">http://jnm.snmjournals.org/cgi/pmidlookup?view=long&pmid=1460515</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">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. <a href="/wiki/OSTI_(identifier)" target="_blank">/wiki/OSTI_(identifier)</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">Michigan State University (October 2013). Radiation safety manual, Environmental Health & Safety, see I-125, p. 81. <a href="https://ehs.msu.edu/_assets/docs/rad/msu-rad-safety-man.pdf" target="_blank">https://ehs.msu.edu/_assets/docs/rad/msu-rad-safety-man.pdf</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">"NCRP Report 161 Management of persons contaminated with radionuclides – National Council on Radiation Protection and Measurements (NCRP) – Bethesda, MD". ncrponline.org. 29 May 2015. Retrieved 3 November 2019. <a href="https://ncrponline.org/publications/reports/ncrp-report-161/" target="_blank">https://ncrponline.org/publications/reports/ncrp-report-161/</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
</ol>

Iodine-125 open-in-new

Iodine-125