The term radioisotope has historically been used to refer to all radiopharmaceuticals, and this usage remains common. Technically, however, many radiopharmaceuticals incorporate a radioactive tracer atom into a larger pharmaceutically-active molecule, which is localized in the body, after which the radionuclide tracer atom allows it to be easily detected with a gamma camera or similar gamma imaging device. An example is fludeoxyglucose in which fluorine-18 is incorporated into deoxyglucose. Some radioisotopes (for example gallium-67, gallium-68, and radioiodine) are used directly as soluble ionic salts, without further modification. This use relies on the chemical and biological properties of the radioisotope itself, to localize it within the body.
Production of a radiopharmaceutical involves two processes:
The production of the radionuclide on which the pharmaceutical is based.
The preparation and packaging of the complete radiopharmaceutical.
Radionuclides used in radiopharmaceuticals are mostly radioactive isotopes of elements with atomic numbers less than that of bismuth, that is, they are radioactive isotopes of elements that also have one or more stable isotopes. These may be roughly divided into two classes:
Those with excess neutrons in the nucleus to those required for stability are known as proton-deficient, and tend to be most easily produced in a nuclear reactor, the majority of radiopharmaceuticals are based on proton deficient isotopes, with technetium-99m being the most commonly used medical isotope, and therefore nuclear reactors are the prime source of medical radioisotopes.
Those with fewer neutrons in the nucleus to those required for stability are known as neutron-deficient, and tend to be most easily produced using a proton accelerator such as a medical cyclotron.
Because radiopharmeuticals require special licenses and handling techniques, they are often kept in local centers for medical radioisotope storage, often known as radiopharmacies. A radiopharmacist may dispense them from there, to local centers where they are handled at the practical medicine facility.
A list of nuclear medicine radiopharmaceuticals follows. Some radioisotopes* are used in ionic or inert form without attachment to a pharmaceutical, these are also included. There is a section for each radioisotope with a table of radiopharmaceuticals using that radioisotope. The sections are ordered alphabetically by the English name of the radioisotope. Sections for the same element are then ordered by atomic mass number.
125I is a gamma emitter with a long half-life of 59.4 days (the longest of all radioiodines used in medicine). Iodine-123 is preferred for imaging, so I-125 is used diagnostically only when the test requires a longer period to prepare the radiopharmaceutical and trace it, such as a fibrinogen scan to diagnose clotting. I-125's gamma radiation is of medium penetration, making it more useful as a therapeutic isotope for brachytherapy implant of radioisotope capsules for local treatment of cancers.
131I is a beta and gamma emitter. It is used both to destroy thyroid and thyroid cancer tissues (via beta radiation, which is short-range), and also other neuroendocrine tissues when used in MIBG. It can also be seen by a gamma camera, and can serve as a diagnostic imaging tracer, when treatment is also being attempted at the same time. However iodine-123 is usually preferred when only imaging is desired.
99Tc is a gamma emitter. It is obtained on-site at the imaging center as the soluble pertechnetate which is eluted from a technetium-99m generator, and then either used directly as this soluble salt, or else used to synthesize a number of technetium-99m-based radiopharmaceuticals.
Notes for guidance on the clinical administration of radiopharmaceuticals and use of sealed radioactive sources. Administration of radioactive substances advisory committee. March 2006. Produced by the Health Protection Agency.