Iodine-125
Encyclopedia
Iodine-125 is a radioisotope of iodine
which has uses in biological assays, nuclear medicine
imaging and in radiation therapy
as brachytherapy
to treat prostate cancer
and brain tumor
s. It is the second longest-lived radioisotope of iodine, after iodine-129
.
Its half-life
is around 59 days and it decays by electron capture
to an excited state of tellurium-125. This state is not the metastable Te-125m, 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 Te-125 may be internally converted
ejected electrons (also at 35 keV), or to x-rays (from electron bremstrahlung), and also a total of 21 Auger electrons, which are produced at the low energies of 50 to 500 electron volts. Eventually, stable nonradioactive ground-state Te-125 is produced, as the final decay product.
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-125 competes with palladium-103 in such uses).
Because of its relatively long half-life, and emission of low-energy photons which nevertheless activate gamma-counter crystal detectors, I-125 is the 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 (as noted), and for certain nuclear medicine scanning procedures, in which it is attached to proteins (albumin or fibrinogen), and where a longer half-life than provided by I-123 is required, in order to follow the isotope during the several days of test.
Iodine-125 has been sometimes been 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). For radiotherapy killing 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; iodine-125 is used therapeutically (to kill tissue) only in brachytherapy
.
125I is created by the electron capture
decay of 125Xe, which is a synthetic isotope of xenon
, itself created by neutron capture
of the slightly radioactive 124Xe, which occurs naturally with an abundance of around 0.1%. Because of the synthetic production route of 125I and its short half-life, the natural abundance is effectively zero.
124Xe (n,γ)→ 125mXe(57s)→125I (59,4 d)
124Xe (n,γ)→ 125gXe(19,9h)→125I (59,4 d)
The irradiation target is natural xenon
gas
containing 0.0965% 124Xe, which is the target isotope for making I-125 by neutron capture. It is loaded into capsules of the zirconium alloy zircaloy-2
(a very nonreactive alloy transparent to neutrons) to a pressure of about 100 bars (about 100 atmospheres). Upon irradiation with slow neutrons in a nuclear reactor
, several radionuclides of xenon are produced. Only the decay of 125Xe leads to a radioiodine, and this is 125I, however. The other radioxenon isotopes decay either to stable xenon
, or to various cesium isotopes, some of them radioactive.
Long irradiations 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 dispose of short-lived unwanted isotopes, 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 (to collect free iodine gas on the capsule sides) and the remaining Xe gas is allowed to escape. The inner walls of the capsule are then rinsed with dilute NaOH solution to collect iodine as soluble iodide
and hypoiodite OI-, according to the standard disproportionation
reaction of halogen
s in alkaline solutions. Any cesium 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 OI- ) remains in solution as iodide/hypoiodite.
sodium hypoiodite, NaOI) . The radioactive concentration lies at 4 to 11 GBq/ml and the specific radioactivity is >75GBq/µmol. The chemical and radiochemical purity is high. The radionuclidic purity is also high; some 126I (t1/2=13.1d) is unavoidable due to the neutron capture noted above. The I-126 tolerable content (which is set by the unwanted isotope interfering with dose calculations in brachytherapy) lies at about 0.2% atom fraction of the total iodine (the rest being I-125).
's Reactor
and the National Research Universal
(NRU) reactor.
The detailed decay mechanism is electron capture
to form the nearly-stable nuclide tellurium-125. This is followed by gamma decay at 35.5 keV energies noted, or else internal conversion
electron emission, followed by an average of 21 Auger electron
s emitted at very low energies (50-500 eV). The internal conversion and Auger electrons from the radioisotope have been found in one study to do little cellular damage, unless the radionuclide is incorporated chemically directly into cellular DNA, which is not the case for present radiopharmaceuticals which use I-125 as the radioactive label nuclide.
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
which has uses in biological assays, nuclear medicine
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
imaging and in radiation therapy
Radiation therapy
Radiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control...
as brachytherapy
Brachytherapy
Brachytherapy , also known as internal radiotherapy, sealed source radiotherapy, curietherapy or endocurietherapy, is a form of radiotherapy where a radiation source is placed inside or next to the area requiring treatment...
to treat prostate cancer
Prostate cancer
Prostate cancer is a form of cancer that develops in the prostate, a gland in the male reproductive system. Most prostate cancers are slow growing; however, there are cases of aggressive prostate cancers. The cancer cells may metastasize from the prostate to other parts of the body, particularly...
and brain tumor
Brain tumor
A brain tumor is an intracranial solid neoplasm, a tumor within the brain or the central spinal canal.Brain tumors include all tumors inside the cranium or in the central spinal canal...
s. It is the second longest-lived radioisotope of iodine, after iodine-129
Iodine-129
Iodine-129 is long-lived radioisotope of iodine which occurs naturally, but also is of special interest in the monitoring and effects of man-made nuclear fission decay products, where it serves as both tracer and potential radiological contaminant....
.
Its half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
is around 59 days and it decays by electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
to an excited state of tellurium-125. This state is not the metastable Te-125m, but rather a lower energy state that decays immediately by gamma decay with a maximum energy of 35 keV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
. Some of the excess energy of the excited Te-125 may be internally converted
Internal conversion
Internal conversion is a radioactive decay process where an excited nucleus interacts with an electron in one of the lower atomic orbitals, causing the electron to be emitted from the atom. Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but...
ejected electrons (also at 35 keV), or to x-rays (from electron bremstrahlung), and also a total of 21 Auger electrons, which are produced at the low energies of 50 to 500 electron volts. Eventually, stable nonradioactive ground-state Te-125 is produced, as the final decay product.
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-125 competes with palladium-103 in such uses).
Because of its relatively long half-life, and emission of low-energy photons which nevertheless activate gamma-counter crystal detectors, I-125 is the preferred isotope for tagging antibodies in radioimmunoassay
Radioimmunoassay
Radioimmunoassay is a very sensitive in vitro assay technique used to measure concentrations of antigens by use of antibodies...
and other gamma-counting procedures involving proteins outside the body. The same properties of the isotope make it useful for brachytherapy (as noted), and for certain nuclear medicine scanning procedures, in which it is attached to proteins (albumin or fibrinogen), and where a longer half-life than provided by I-123 is required, in order to follow the isotope during the several days of test.
Iodine-125 has been sometimes been 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). For radiotherapy killing 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; iodine-125 is used therapeutically (to kill tissue) only in brachytherapy
Brachytherapy
Brachytherapy , also known as internal radiotherapy, sealed source radiotherapy, curietherapy or endocurietherapy, is a form of radiotherapy where a radiation source is placed inside or next to the area requiring treatment...
.
125I is created by the electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
decay of 125Xe, which is a synthetic isotope of xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
, itself created by neutron capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
of the slightly radioactive 124Xe, which occurs naturally with an abundance of around 0.1%. Because of the synthetic production route of 125I and its short half-life, the natural abundance is effectively zero.
Production
125I is reactor-produced radionuclide and is available in large quantities. Its production follows the reaction:124Xe (n,γ)→ 125mXe(57s)→125I (59,4 d)
124Xe (n,γ)→ 125gXe(19,9h)→125I (59,4 d)
The irradiation target is natural xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
containing 0.0965% 124Xe, which is the target isotope for making I-125 by neutron capture. It is loaded into capsules of the zirconium alloy zircaloy-2
Zircaloy
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance...
(a very nonreactive alloy transparent to neutrons) to a pressure of about 100 bars (about 100 atmospheres). Upon irradiation with slow neutrons in a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
, several radionuclides of xenon are produced. Only the decay of 125Xe leads to a radioiodine, and this is 125I, however. The other radioxenon isotopes decay either to stable xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
, or to various cesium isotopes, some of them radioactive.
Long irradiations are disadvantageous. Iodine-125 itself has a neutron capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
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 dispose of short-lived unwanted isotopes, 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 (to collect free iodine gas on the capsule sides) and the remaining Xe gas is allowed to escape. The inner walls of the capsule are then rinsed with dilute NaOH solution to collect iodine as soluble iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
and hypoiodite OI-, according to the standard disproportionation
Disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
reaction of halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
s in alkaline solutions. Any cesium 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 OI- ) remains in solution as iodide/hypoiodite.
Availability and purity
Iodine-125 is commercially available in dilute NaOH solution as 125I-iodide (or the hypohaliteHypohalite
A hypohalite is an oxyanion containing a halogen in oxidation state +1.This includes hypoiodite, hypobromite, hypochlorite, and hypofluorite....
sodium hypoiodite, NaOI) . The radioactive concentration lies at 4 to 11 GBq/ml and the specific radioactivity is >75GBq/µmol. The chemical and radiochemical purity is high. The radionuclidic purity is also high; some 126I (t1/2=13.1d) is unavoidable due to the neutron capture noted above. The I-126 tolerable content (which is set by the unwanted isotope interfering with dose calculations in brachytherapy) lies at about 0.2% atom fraction of the total iodine (the rest being I-125).
Producers
The two largest producers of iodine-125 medical isotopes are in Canada. They are McMaster UniversityMcMaster University
McMaster University is a public research university whose main campus is located in Hamilton, Ontario, Canada. The main campus is located on of land in the residential neighbourhood of Westdale, adjacent to Hamilton's Royal Botanical Gardens...
's Reactor
McMaster Nuclear Reactor
The McMaster Nuclear Reactor is a 5MWth pool-type reactor located on the campus of McMaster University, in Hamilton, Ontario.-Description:...
and the National Research Universal
National Research Universal Reactor
The National Research Universal reactor, located in Chalk River, Ontario, is one of Canada’s national science facilities. It is a multipurpose science facility that serves three main roles....
(NRU) reactor.
Physical Data
- Element: Iodine
- Z: 53
- A: 125
- Atomic Mass:
- Density:
- Physical state: Solid at room temperature
- Isotopic abundance: 0%
- Radioactive: Yes
- T(1/2): 59.4 days
- Decay: Electron captureElectron captureElectron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
to 125Te - Emissions: Gamma-rays at 35.5 keV. 7% emitted, 93% internally converted to:
- 27.0 keV (113% abundance relative to 7% gamma emission)
- 31.0 keV (26%)
- 27-32 keV (14%)
- Half-value layer: 0.025 mm Pb
The detailed decay mechanism is electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
to form the nearly-stable nuclide tellurium-125. This is followed by gamma decay at 35.5 keV energies noted, or else internal conversion
Internal conversion
Internal conversion is a radioactive decay process where an excited nucleus interacts with an electron in one of the lower atomic orbitals, causing the electron to be emitted from the atom. Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but...
electron emission, followed by an average of 21 Auger electron
Auger electron
The Auger effect is a physical phenomenon in which the transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in...
s emitted at very low energies (50-500 eV). The internal conversion and Auger electrons from the radioisotope have been found in one study to do little cellular damage, unless the radionuclide is incorporated chemically directly into cellular DNA, which is not the case for present radiopharmaceuticals which use I-125 as the radioactive label nuclide.