Radionuclides and Radiopharmaceuticals

• A radionuclide is an unstable atom that undergoes radioactive decay to achieve stability.
• The radiation emitted by radionuclides can be used in medical imaging and therapy.
• Radiopharmaceuticals, which incorporate radioactive molecules, are agents approved for human use in these applications.
• They portray physiology, biochemistry, or pathology without causing significant physiological effects.
• Radiopharmaceuticals are also known as "radiotracers" due to their use in subpharmacological doses to "trace" processes in the body.
• The chapter covers general principles of clinically used radionuclides and radiopharmaceuticals.
• It also presents their production, radiolabeling, and quality assurance processes.

Production of Radionuclides

• Naturally occurring radionuclides are heavy, toxic elements with long half-lives (over 1000 years).
• Examples include uranium, actinium, thorium, radium, and radon.
• Most of these natural radionuclides are not used in nuclear medicine.
• Radionuclides for clinical use are commonly produced artificially.
• Table 4.1 lists physical properties of single-photon-emitting radionuclides.
• These are used in medical imaging with a gamma camera.
• Tables 4.2 and 4.3 list dual-photon positron-emitting agents and radionuclides for therapy, respectively.
• Appendix 2 is a periodic table of the elements.
• Medical isotope production involves nuclear fission or neutron activation in a nuclear reactor.
• It also involves charged-particle bombardment in a particle accelerator (cyclotron).
• Another method is the decay of a radioactive parent in a radionuclide generator.
• Table 4.4 outlines production methods.
• Various production reactions can be annotated in equation form.
• This includes the reaction type, involved particles, initial isotope, and final product.

Box 4.1 Key Terms

• Radionuclide: An unstable isotope transitioning to greater stability via radioactive decay.
• Radiopharmaceutical: An FDA-approved radioactive agent for imaging or therapy.
• Activity: The rate of decay, measured in curies (3.7 × 10^10 decays per second) or becquerels (1 decay per second; 1 mCi = 37 MBq).
• Half-life (T1/2): The time for half the radioactive atoms in a sample to decay.
• Equilibrium: A steady state relationship in a contained parent/daughter pair where the parent's half-life is longer than the daughter's; used in generators.
• Carrier-free: A radiopharmaceutical free from contamination by other isotopes (stable or radioactive) of the same element.
• Carrier molecule: A substance radiolabeled to assess a physiological parameter or cellular function, improving properties like localization.
• Specific activity: The concentration of the radionuclide per unit volume or weight; high specific activity is optimal.
• Radiative abundance: Likelihood of desired emissions resulting from radioactive decay, also known as radiation yield.

Fission & Neutron Activation

• When a heavy nuclide like uranium-235 is bombarded with a neutron, a neutron is captured.
• Instead of stabilizing through radioactive decay, the atom undergoes nuclear fission and splits.
• This results in two smaller nuclides with masses ranging from 72 to 161.
• It also releases energy and multiple neutrons.
• These high-energy neutrons cause further fission reactions, creating a chain reaction.
• The process can be controlled within a nuclear reactor.
• Fission daughters include radionuclides like molybdenum-99, iodine-131, xenon-133, and cesium-137.
• Radionuclides produced this way are typically carrier-free.
• In neutron activation, stable target material is exposed to thermal neutrons in a reactor.
• Neutron activation happens if the target atom captures a neutron.
• The resulting unstable atom stabilizes via gamma-ray emission and/or β- decay.
• Besides the desired daughter product, contaminants (other isotopes of the element) are also created.
• It can be difficult to separate the daughter, so the product is not carrier-free.
• Neutron-rich radionuclides stabilize through β- decay.
• Isotopes like P-32, Sr-89, and Sm-153 are created this way.
• Mo-99 and I-131 can also be produced this way, but unlike fission production, they are not carrier-free.
• The third production method involves bombarding the target material with charged particles (protons, deuterons, alpha).