| General | |
|---|---|
| Symbol | 125I |
| Names | iodine-125, 125I, I-125, radioiodine |
| Protons(Z) | 53 |
| Neutrons(N) | 72 |
| Nuclide data | |
| Natural abundance | synth |
| Half-life(t1/2) | 59.392±0.008 d[1] |
| Isotope mass | 124.9046306(15)[2]Da |
| Parent isotopes | parent_mass125Xe |
| Decay products | decay_mass125Te |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| electron capture | 0.035 (35keV) |
| Isotopes of iodine Complete table of nuclides | |
Iodine-125 (125I) is aradioisotope ofiodine which has uses inbiological assays,nuclear medicine imaging and inradiation therapy as brachytherapy to treat a number of conditions, includingprostate cancer,uveal melanomas, andbrain tumors. It is the second longest-lived radioisotope of iodine, afteriodine-129.
Itshalf-life is 59.392 days and it decays byelectron capture to an excited state oftellurium-125. This state is not the metastable125mTe, but rather a lower energy state. The excited125Te may (7% chance) undergogamma decay with a maximum energy of 35keV. More often (93% chance), the excited125Te undergoesinternally conversion and ejects an electron (< 35 keV). The resulting electron vacancy leads to emission ofcharacteristic X-rays (27–32 keV) and a total of 21Auger electrons (50 to 500 eV).[3] Eventually, stable ground state125Te 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 withpalladium-103 in such uses).[4]
Because of its relatively long half-life and emission of low-energy photons which can be detected bygamma-countercrystal detectors,125I is a preferred isotope fortaggingantibodies inradioimmunoassay and other gamma-counting procedures involvingproteins 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 orfibrinogen), and where a half-life longer than that provided by123I is required for diagnostic or lab tests lasting several days.
Iodine-125 can be used inscanning/imaging thethyroid, but iodine-123 is preferred for this purpose, due to betterradiation penetration and shorter half-life (13 hours).125I is useful forglomerular filtration rate (GFR) testing in the diagnosis or monitoring of patients withkidney disease. Iodine-125 is usedtherapeutically inbrachytherapy treatments oftumors. Forradiotherapyablation of tissues that absorb iodine (such as the thyroid), or that absorb an iodine-containingradiopharmaceutical, thebeta-emitteriodine-131 is the preferred isotope.
When studyingplant immunity,125I is used as theradiolabel in trackingligands to determine whichplant pattern recognition receptors (PRRs) they bind to.[5]
125I is produced by theelectron capture decay of125Xe, which is an artificial isotope ofxenon, itself created byneutron capture of near-stable124Xe (it undergoesdouble 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 of125I and its short half-life, itsnatural abundance on Earth is effectively zero.
125I is a reactor-produced radionuclide and is available in large quantities. Its production involves the two followingnuclear reactions:
The irradiation target is theprimordial nuclide124Xe, which is the target isotope for making125I byneutron capture. It is loaded into irradiation capsules of the zirconium alloyzircaloy-2 (acorrosion resistingalloy transparent toneutrons) to a pressure of about100bar(~ 100atm). Upon irradiation withslow neutrons in anuclear reactor, severalradioisotopes of xenon are produced. However, only the decay of125Xe leads to a radioiodine:125I. The other xenon radioisotopes decay either to stablexenon, or to variouscaesium isotopes, some of them radioactive (a.o., the long-lived135Cs (t½ = 1.33 Ma) and137Cs (t½ = 30 a)).
Longirradiation times are disadvantageous. Iodine-125 itself has aneutron capturecross section of 900barns, and consequently during a long irradiation, part of the125I formed will be converted to126I, abeta-emitter andpositron-emitter with ahalf-life of 12.93 days,[1] 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 (tocondense 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 diluteNaOH solution to collect iodine assolubleiodide (I−) andhypoiodite (IO−), according to the standarddisproportionation reaction ofhalogens inalkaline solution. Anycaesium atom present immediatelyoxidizes and passes into the water as Cs+. In order to eliminate any long-lived135Cs and137Cs which may be present in small amounts, the solution is passed through acation-exchange column, which exchanges Cs+ for another non-radioactivecation (e.g., Na+). The radioiodine (asanion I− or IO−) remains in solution as a mixture iodide/hypoiodite.
Iodine-125 is commercially available in diluteNaOH solution as125I-iodide (or thehypohalite sodiumhypoiodite, NaIO). The radioactive concentration lies at 4 to11 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; some126I(t1/2 = 12.93 d)[1] is unavoidable due to theneutron capture noted above. The126I tolerable content (which is set by the unwanted isotope interfering withdose calculations inbrachytherapy) lies at about0.2 atom % (atom fraction) of the total iodine (the rest being125I).
As of October 2019, there were two producers of iodine-125, theMcMaster Nuclear Reactor inHamilton,Ontario, Canada; and a VVR-SM research reactor in Uzbekistan.[6] The McMaster reactor is presently the largest producer of iodine-125, producing approximately 60 per cent of the global supply in 2018;[7] 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.[8]
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.[6][8]
Prior to 2018, theNational Research Universal (NRU) reactor atChalk River Laboratories inDeep River, Ontario, was one of three reactors to produce iodine-125.[9] 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.[10][11] The Russian nuclear reactor equipped to produce iodine-125, was offline as of December 2019.[6]
The detailed decay mechanism to form the stable daughter nuclide tellurium-125 is a multi-step process that begins withelectron capture, which produces a tellurium-125 nucleus in an excited state with a half-life of 1.6 ns. The excited tellurium-125 nucleus may undergogamma decay, emitting a gammaphoton at 35.5 keV, or undergointernal conversion to emit anelectron. The electron vacancy from internal conversion results in a cascade ofelectron relaxation as the core electron hole moves toward thevalence orbitals. The cascade involves manycharacteristic X-rays andAuger transitions. 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 areAuger electrons), most with energies less than 400 eV (79% of yield).[12] 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 cellularDNA, which is not the case for present radiopharmaceuticals which use125I as the radioactive label nuclide.[13] 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 thethyroid gland) can be blocked by the prompt administration of stableiodine-127 in the form of an iodide salt.[14][15]Potassium iodide (KI) is typically used for this purpose.[16]
However, unjustifiedself-medicated preventive administration of stable KI is not recommended in order to avoid disturbing the normalthyroid function. Such a treatment must be carefully dosed and requires an appropriate KI amountprescribed by a specialised physician.