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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Selection of radionuclides for radiopharmaceuticals

Problem

One of the benefits of using radiopharmaceuticals is the non-invasive diagnosis and therapy of cancer. Correspondingly, there are therapeutic or diagnostic radiopharmaceuticals, each of which possesses certain characteristics that allow for their optimal performance. A radiopharmaceutical preparation contains a radionuclide in the following forms: as an element in atomic or molecular form, as an anion, or included in or attached to organic molecules.
There are three methods of producing radionuclides. The first method is through irradiation of a target element with neutrons in nuclear reactors. Either neutron beams are used to produce radioisotopes with excessive numbers of neutrons or the fission products are isolated and purified. The second method involves placing a parent radionuclide into a generator which facilitates the transport over long distances into hospitals and the extraction of the daughter nuclide. Their use has proven to be generally cost-efficient, especially in the case of Technetium-99m. The final method is through irradiation with positive charged baryons in a cyclotron. Radionuclides prepared via this route are often beta-positive emitters, but some decay by electron capture.
There are a multitude of existing radionuclides, which are radioactive isotopes of existing elements, but only a select few offer any therapeutic value. To be of any therapeutic value, the radionuclide must have an optimal half-life, an emission with an energy appropriate to its use, and a cost-effective method of synthesis. The optimal half-life should be short enough to minimize the radiation dose to the patient but long enough to perform the diagnostic procedure. When a radionuclide undergoes decay, it can release an alpha, beta, or gamma particle, and the decay mode, along with energy production, are two key factors in determining the scope of the radionuclide’s usage.
RadionuclideHalf-lifeParent NuclideHalf-lifeRadiationProduction
start superscript, start text, 99, m, end text, end superscriptTc6.0 hrstart superscript, start text, 99, end text, end superscriptMo2.74 dγGenerator
start superscript, start text, 68, end text, end superscriptGa67.6 minstart superscript, start text, 69, end text, end superscriptKr32 msβ+ (56%)Generator
start superscript, start text, 18, end text, end superscriptF110 minstart superscript, start text, 22, end text, end superscriptSi29 msβ+ (97%)Cyclotron
start superscript, start text, 153, end text, end superscriptSm1.95 dstart superscript, start text, 153, end text, end superscriptBa80 msβ‾, γReactor
start superscript, start text, 223, end text, end superscriptRa11.4 dstart superscript, start text, 239, end text, end superscriptPu24125 yαGenerator
start superscript, start text, 131, end text, end superscriptI8.0 dstart superscript, start text, 131, end text, end superscriptCd68 msβ‾Reactor
Which of the following is equal to the total number of nucleons and electrons in technetium-99?
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