radioactive decay

what is nuclear pharmacy?

• specialty that uses radioactive materials to both diagnose the body and treat disease

• documents organ function and structure

• uses relatively small amounts of radioactive materials (radiopharmaceuticals) to diagnose and treat: substances localized in specific organs, bones, or tissues

what is radiation?

energy in the form of particles or electromagnetic waves

what is ionizing radiation?

radiation with sufficient energy to remove an electron from an atom/molecule

what is radioactivity?

• process by which unstable atoms spontaneously transform to new atoms and in the process emit radiation
• the new atom may be the same atom in a lower energy state

what is alpha decay?

• helium nucleus: very massive and doubly ionized
• only a hazard via ingestion/inhalation of alpha emitter
• not usually an external radiation hazard
• stopped by paper and dead layer of skin
• uranium, thorium, radon, and radon daughters

what is beta decay?

• energetic electron: singly ionized
• external hazard to skin and eyes
• internal hazard via ingestion/inhalation of beta emitter
• a 1 MeV beta can travel up to 12 feet in air and 1 cm in plastic
• phosphorus, tritium, carbon, sulfur

what is gamma decay?

• X-rays and gamma rays are photons: no charge
• external radiation hazard to deep organs and tissues
• internal hazard via ingestion/inhalation of gamma emitter
• lead (high electron density): good for shielding X and gamma rays
• Iodine 125 gamma rays (30 keV): can be easily stopped with 1/8 in of lead

what does neutron shielding material depend on?

the energy of the neutrons

what is the geiger-muller counter?

• used to detect ionizing radiation, usually beta particles and gamma rays, but certain models can detect alpha particles
• an inert gas-filled tube briefly conducts electricity when a particle/photon of radiation makes the gas conductive
• the tube amplifies this conduction by a vascade effect and outputs a current pulse
• then displayed by a needle/lamp and/or audible clicks

what are the major sources of radiation dose in the US population?

radon > internal > terrestrial > cosmic > medical x-rays > nuclear medicine > consumer products > other

how is radiation absorbed dose, effective dose, and exposure measured in the US?

rad, rem, or roentgen (R)

what is the total US average dose equivalent?

360 million rem/yr

what are the biological effects of radioactive decay?

• many groups exposed to ionizing radiation at high levels resulted in adverse effects
• somatic effects: prompt (skin burns and cataracts), delayed (cancer)
• genetic effects
• teratogenetic effects

how is cancer related to radioactive decay?

radiation can damage cells through 2 methods: production of free radicals, direct damage to the DNA

what is the risk factor for radiation dose?

• 4% increase in risk of dying of cancer for every 100 rem of dose
• normal cancer risk is 20%

what are the radiation protection basics?

• time: minimize the time that you are in contact with radioactive material to reduce exposure
• distance: keep your distance; if you double the distance, the exposure rate drops by a factor of 4
• shielding: lead, water, or concrete for gamma and X-rays; thick plastic (lucite) for beta rays

what is the external radiation inverse square law?

• radiation levels decrease as the inverse square of the distance (e.g. move back by a factor of 2, radiation levels drop to 1/4)
• applies to point sources: distance greateer than 5 times the max source dimension
  I1(R1)^2 = I2(R2)^2

what are isotopes?

atoms that have identical chemistry, same nuclear charge and atomic #, but different mass

what are radioisotopes?

• same # of protons, but varying # of neutrons
• unstable and undergo radioactive transformations
• aka radionuclides
• naturally occurring or artifically produced
• first one used in medicine: Radium

what are radiopharmaceuticals?

• a drug made with a radionuclide
• radioactive pharmaceutical agent used for diagnostic or therapeutic procedures
• regulated by the FDA as well as the Nuclear Regulatory Commission (NRC)
• may also be regulated by the State Board of Pharmacy

what is the half life of 14C?

5700 years

what is the half life of 81mKr?

13.1 sec

what is the rate of radioactive decay?

• a constant fraction of total # of undecomposed atoms present
• units: reciprocal time (sec, min, hrs)
  N = N_0 e^-𝜆t
• N_0 is # of atoms originally present when t=0

what is the rate of disintegration?

-dN/dt = 𝜆N

• N: # of undecomposed atoms at time t
• 𝜆: decay constant or the fraction disintegrating per unit of time

what is half-life of radioactive decay?

• when t1/2 occurs when N = 1/2 N0, then 1/2 N0 = N0^-𝜆t1/2 t1/2 = 0.693/𝜆

what are the units of radioactivity?

• quantity of activity is expressed in absolute units: total # of atoms disintegrating per unit time
• basic scientific unit: Curie; quantity of isotope in which 3.7 x 10^10 atoms disintegrate per second
• system international unit (basic unit for pharmacists): Becquerel (1 disintegration/sec)

what is radioisotope contrast media?

• chemical compounds containing elements of high atomic number which will stop passage of X-rays
• many contain Ba or I
• barium sulfate: agent of choice for imaging GI tract because it is insoluble in acidic gastric juices
• major side effect: constipation
• usual oral dose of Barium sulfate: 200-300g

what are radiopharmaceuticals in USP?

• the USP lists 70 official radioactive pharmaceuticals
• e.g. Technuetium-99m, strontium-89 chloride, yttrium-90, thallous-201 chloride, gallium-67 citrate, sodium iodide-123

what is sodium iodide-123?

• oral capsule
• better than I-131 in that it delivers lower radiation and better image quality
• used to diagnose and evaluate thyroid function
• emits only gamma rays
• drugs can interfere with thyroid uptake of radionuclide: corticosteroids, benzodiazepines, vitamins, expectorants, antitussives, and topicals which contain I
• all should be withheld prior to I-123 use

what is iodine-131?

• produced in a reactor
• used in diagnostic procedures involving the thyroid and treatment of thyroid disorders
• can be administered in capsule or liquid solution form
• requires special precautions to be implemented during administration

what is technetium-99m?

• possesses a relatively short half-life of 6 hrs
• offers an abundance of gamma photons for imaging without the hazard of beta particles
• can be used as a binding agent for several pharmaceuticals used for imaging
• kits are available to prepare compounds that assist in hepatobiliary imaging and ischemic heart disease
• potential use to label monoclonal antibodies
• inexpensive and easy to obtain
• easily produced by adding TcO4-99m to many choices of "cold kits"

how is technetium-99m used to make radiopharmaceuticals?

• added to a vial containing a chemical compound that binds to the radionuclide
• results in a radiopharmaceutical that will be taken up in the designated organ for imaging/analysis with a gamma camera

what is strontium-89 chloride?

• sterile, non-pyrogenic aqueous. solution for IV use and contains no preservatives
• decays by beta emission, with half-life of 51 days: beta emission is very harmful to skeletal tissue
• used exclusively for bone scans (bone tumors and metastatic lesions in bone)
• toxic: cannot give to patients with platelet counts below 60,000 and WBC counts below 2,400

what does strontium-89 chloride act as?

like calcium analogs (e.g. clears rapidly from blood stream and selectively localizes in bone mineral)

what is yttruim-90?

• trivalent radioactive metal, pure beta-emitting radionuclide
• half-life 2.68 days
• decays to form stable zirconium-90

where is yttruim-90 applied?

in radioimmunotherapy (RIT) of large solid tumors and lymphomas

what is TheraSphere?

• patented 1998
• insoluble glass microspheres used for hepatocellular carcinoma
• bond yttrium and target liver
• when injected, stay localized in the liver and minimize side effects of the radionuclide

what is thallous-201 chloride?

• sterile, non-pyrogenic solution for IV
• half-life 73.1 hrs
• K analog undergoes rapid transport to the myocardium: useful for visualization of myocardial infarction or ischemic heart disease
• often used with exercise stress testing and can differentiate between ischemic and infarcted heart tissues

what is gallium-67 citrate?

• sterile pyrogen-free aqueous solution
• behaves like the ferric ion and has half-life of 78 hrs
• can localize in viable primary and metastatic tumors and in focal sites of infection
• useful in hodgkin's disease, lymphomas, and bronchogenic carcinoma
• used for diagnosis and monitoring of fever of undetermined origin and for Pneumocystis carinii pneumonia of AIDS

what is a disadvantage of gallium-67 citrate?

considerable biological variation occurs in patients

what does the production of other radionuclides for nuclear medicine involve?

the use of a cyclotron

what is imaging equipment used for?

following administration of the radiopharmaceutical to the patient, a gamma camera is used to image the area of interest

what are gamma cameras?

• used to show how the radiopharmaceutical distributes itself throughout the body or is taken up by specifically targeted organs
• interfaced with a computer that controls data acquisition, processing, and image display
• still and dynamic images can be acquired

what is SPECT imaging?

• single photon emission computed tomography
• SPECT cameras look at a patient from many different angles and can demonstrate precise detail within the patient
• information is presented as a series of planes that correspond to certain depths within the body
• planes presented may be a series of coronal, sagittal, transverse, and/or oblique slices

what is positron emission tomography (PET)?

• has been used to study cerebral physiology since the 1970s
• imaging used for mapping regional blood flow and volume, O metabolism, bone remodeling, tumor receptor density, reporter gene expression, etc.
• extremely sensitive technique to image body chemistry
• chemical changes often occur prior to anatomic changes in the body and can be detected by PET
• radionuclides undergo positron decay usually have very short half lives

what are PET cyclotrons?

• now adjacent to pharmacies
• can be used in pharmacokinetic distribution evaluations and one can check if drug reaches a specific receptor site
• longer lived radionuclides are being investigated as radiolabels for monoclonal antibody-based PET
• uses in cancer as well as cardiovascular disorders

what are PET scanners?

• positron emission tomography
• used to study physiologic and biochemical processes within the body
• processes studied include blood flow, O, glucose and fatty acid metabolism, amino acid transport, pH, and neuroreceptor densities
• on-site cyclotron is required to produce the very short half-life PET radiopharmaceuticals

what is personal monitoring?

for any worker who usually works in a controlled area, and may receive a significant dose from occupational exposure, individual monitoring shall be undertaken where appropriate, adequate, and feasible

what are the doses from external radiation?

• thermoluminescent (TLD) or optically stimulated luminescence (OSL) dosimeter: gamma, X, and beta radiation
• film dosimeter: gamma, X, and beta radiation
• electronic dosimeter: with or without alarm
• film badge, electronic dosimeter, ring badge, TLD

what is used to assess doses from internal radiation?

• urinalysis
• whole body monitor: gamma-emitting radioisotopes
• thyroid monitoring: iodine radioisotopes

what is record keeping?

• employers and licensees shall maintain exposure records for each monitored worker
• records are to be maintained as required by the Regulatory Body
• information is confidential and must be kept secure
• access to records shall be provided to the relevant worker, relevant employer, Regulatory Body, and health surveillance professionals