Isotopes and Radioisotopes

Isotopes

• Atoms with the same number of protons in the nucleus but different numbers of neutrons
• Most elements in the periodic table have associated isotopes, but not all nuclei of these isotopes represent stable configurations of protons and neutrons

Radioisotopes

• Isotopes of a particular element that are unstable due to their neutron-proton configuration
• Radioisotopes can spontaneously undergo changes or transformations to rectify the unstable arrangement
• Half-life (HL) is the time taken for the radioactivity to decrease by half

Medical Usage

• Radiation therapy: uses high-energy radiation to kill cancer cells or shrink tumors
• Well-oxygenated, rapidly dividing cells are sensitive to radiation damage
• Radiation can be delivered internally through infusion or implantation of radioisotopes

Radiation Therapy

• Therapeutic isotopes have long half-lives and high-energy radiation
• Radiation can be in the form of gamma rays or fast electrons (beta radiation)
• Iodine-125 (125I) is a commonly used therapeutic radioisotope
• 125I seeds are used to treat prostate cancer, with a treatment goal of delivering 145 Gy to at least 90% of the prostate volume
• Radiation safety practices include distance and time

Radiation Therapy (Cont.)

• Iodine-131 (131I) is another commonly used therapeutic radioisotope
• 131I has a half-life of 8 days and can be joined chemically with sodium to form a radioactive compound
• 131I is used to treat thyroid cancer and residual thyroid tissue after surgery
• Personnel radiation protection considerations include distance, time, and shielding

Nuclear Medicine

• Employs radioisotopes to study organ function, detect cancer spread, and treat disease
• Diagnostic techniques use short-lived radioisotopes as tracers
• Common radioisotopes used in nuclear medicine include Iodine-123 (123I) and Technetium-99m (99mTc)

Nuclear Medicine (Cont.)

• 123I is used to study thyroid gland function and has an average half-life of 13.3 hours
• 99mTc is the most commonly used radioisotope in nuclear medicine and is produced from the radioactive decay of Molybdenum-99 (99Mo)
• 99mTc can be incorporated into various compounds or biologically active substances for different tissues or organs

Positron Emission Tomography (PET) and Computed Tomography (CT)

• PET uses annihilation radiation events from the radioactive decay of unstable isotopes
• Annihilation radiation is initiated by the radioactive decay of an unstable isotope with too many protons
• Positron-emitter isotopes, such as Fluorine-18 (18F), are used in PET scanning
• 18F can be attached to a glucose molecule to form fluorodeoxyglucose (FDG), a radioactive tracer taken up by cancerous cells

PET and CT (Cont.)

• PET/CT scanners combine the functional information from PET with the anatomical information from CT
• PET scanning is an important imaging modality for detecting and monitoring cancer
• Fluorodeoxyglucose (FDG) is used to reveal the location of cancerous cells through PET scanning

Radiation Protection

• Radiation safety shielding considerations for PET/CT scanners include scattered radiation, high-energy annihilation photons, and patient prep time
• Necessary protection for technologists and personnel includes distance, time, and shielding
• A well-designed facility with necessary shielding is essential for radiation protection