Selenium-75 is a radioactive isotope of the element selenium with a half-life of approximately 119.78 days. It is used in various applications, including industrial radiography, nuclear medicine, and research.
Selenium-75 is typically produced by bombarding natural selenium-74 with neutrons in a nuclear reactor, resulting in the formation of selenium-75 through the nuclear reaction selenium-74(n,γ)selenium-75. Once produced, selenium-75 can be incorporated into radiopharmaceuticals for imaging studies or used as a radiation source for industrial purposes.
In nuclear medicine, selenium-75 has been used in the past for imaging studies, particularly for the evaluation of blood volume and red blood cell mass. However, its use in this field has decreased over time due to the availability of other more commonly used radioisotopes with better imaging characteristics.
In industrial radiography, selenium-75 is used as a gamma radiation source for non-destructive testing of materials, such as welds, pipelines, and structural components. Its high energy gamma radiation allows for deep penetration and high-quality imaging of thick materials.
Overall, selenium-75 plays a role in various applications that require gamma radiation sources, such as industrial radiography and certain research studies. Its properties make it useful for specific applications where its characteristics are advantageous.
Properties:
Selenium-75 (75Se) is a gamma emitter (γ, 58.9% at 264 keV, 58.3% at 136 keV, 25.0% at 279 keV and 17.2% at 121 keV) decaying through an electron capture process (96% at 463 keV) into stable 75As with a half-life of 119.8 days.
Manufacturing:
75Se is produced in a reactor based on the following reaction [74Se(n, γ)75Se]. It can also be produced as a no carrier added radionuclide following irradiation in a 30 MeV cyclotron [75As(p,n)75Se].
Source and availability:
GE Healthcare is the only GMP-quality provider of this radionuclide for radiolabeling. 75Se is also available from JSC Isotope, Russia.
75Se is also an alternative to 192Ir for use in gamma radiography sealed sources.
Derivatives:
75Se is only available as a label for cholesterol analogues used as a tracer, 75Se-Tauroselcholic acid sold by GE Healthcare (SeHCAT) for the diagnosis of bile acid malfunction (BAM). The long half-life limits its applications and general use. In the 1960s, there were some attempts to use 75Se-selenomethionine to count platelets, but this approach did not transform into a widely marketed product. 75Se-Selenocholesterol (Scintadren®) and 75Se-Selenomethionine (Sethotope®) have been withdrawn from the market.
Price:
As with all reactor-produced radionuclides, the manufacturing price remains quite low and 75Se must be considered as an affordable isotope (a few EUR/mCi).
Issues:
- The long half-life of this radionuclide is its disadvantage and limits its use. This is one of the rare radionuclides with a half-life higher than 100 days that is still accepted for use in hospitals (the trend is to limit the use of radionuclides with a half-life shorter than 100 days in humans, to limit waste elimination issues). This radionuclide is expected to disappear completely from the market within the next years.
Comments:
The interest in 75Se came from its availability, as this radionuclide is mainly used in sealed sources for non-medical applications (gamma radiography). However, its half-life excludes it from future use as a radionuclide for the development of radiotherapeutics.