Radiation Biology Study Notes

RADIATION BIOLOGY

## Instructor Information

  • James Lawrence Aduche, RRT, RSO, MSRT (ip)

  • Review Lecturer License No: 0023749

    SOURCES OF RADIATION EXPOSURE

  • Radiation: Transfer of energy through space.

    Classification of Radiation:

  • Ionizing Radiation: Capable of ionization.

    • Ionization: Removal of an electron from an atom.

    • Occurs when an x-ray passes close to an orbital electron, transferring sufficient energy to remove it from the atom.

    • Ion pair: Orbital electron and atom.

    • Ion: An atom or molecule with a net electric charge due to the loss or gain of one or more electrons.

    Particulate Ionizing Radiation:

  • Subatomic particles capable of ionization include electrons, protons, and other nuclear fragments when in motion with sufficient kinetic energy.

    Types of Particulate Radiation:

  • Alpha Particles:

    • Equivalent to a helium nucleus consisting of 2 protons and 2 neutrons.

    • Mass: Approximately 4 amu.

    • Charge: Carries 2 units of positive charge.

    • Emission: Comes from the nuclei of heavy elements.

    • Cannot be emitted by light elements due to insufficient energy.

    • Characteristics:

      • Quickly loses energy and has a very short range in matter.

      • Travels approximately 5 cm in air and less than 100 micrometers in soft tissue.

      • Generally harmless as energy is deposited in superficial skin layers.

      • Can irradiate local tissue only when deposited in the body.

      • Used in therapeutic applications.

  • Beta Particles:

    • An electron emitted from the nucleus of a radioactive atom.

    • Characteristics:

      • Light particles with an atomic mass number of 0.

      • Carry a charge of either -1 (electron) or +1 (positron).

      • May traverse 10 to 100 cm in air and approximately 1 to 2 cm of soft tissue.

  • X-rays:

    • Produced outside the nucleus in electron shells.

    • Bremsstrahlung Radiation: 90% of x-rays; produced by the acceleration or deceleration of charged particles after passing through electric and magnetic fields of a nucleus.

      • Effects the angle of x-rays due to the nucleus.

    • Characteristic Radiation: 10%; occurs when outer-shell electrons fill a vacancy in the inner shell of an atom, releasing x-rays characteristic to each element.

  • Gamma Radiation:

    • Emitted from the nucleus of a radioisotope, often associated with alpha or beta emissions.

    • Produced in diagnostic imaging systems and emitted spontaneously from radioactive materials.

    • Application: Nuclear medicine and radiotherapy.

    SOURCES OF IONIZING RADIATION

  • Natural Environment:

    • Annual dose: Approximately 3 mSv.

      • Cosmic Rays: Particulate and electromagnetic radiation emitted by the sun and stars.

      • Terrestrial Radiation: Results from deposits of uranium, thorium, and other radionuclides.

      • Internally Deposited Radionuclides: Potassium-40, naturally metabolized.

      • Radon: Largest source of natural environmental radiation; alpha-emitting radioactive gas from uranium decay, posing the most significant exposure risk via the lungs.

  • Man-Made Ionizing Radiation:

    • Annual dose: Approximately 3.2 mSv.

      • Diagnostic X-ray: Largest man-made source of ionizing radiation.

      • Nuclear Power Generation: Includes all research applications.

      • Industrial Sources: Various consumer items that emit radiation.

    CLASSIFICATION OF HUMAN EXPOSURE TO RADIATION

  • 1. Industrial Exposure: 0.1%

  • 2. Occupational Exposure: 0.1%

  • 3. Consumer Exposure: 2%

  • 4. Terrestrial Exposure: 3%

  • 5. Internal Exposure: 5%

  • 6. Space Exposure: 5%

  • 7. Conventional Radiography/Fluoroscopy: 5%

  • 8. Nuclear Medicine: 12%

  • 9. CT Scans: 24%

  • 10. Radon and Thoron: 37%

    RADIOBIOLOGY

  • Definition: Combines physics and biology principles to study the action of ionizing radiation on biological tissues and living organisms.

  • Mechanism: Studies biological effects produced by energy absorption in small volumes, corresponding to single cells or parts of cells.

    HUMAN RADIATION RESPONSE

  • ALARA Principle: Stands for “As Low As Reasonably Achievable” – results in the greatest medical benefit with the lowest risk to patients and radiation workers.

  • Effects of X-ray on Humans: Interaction at the atomic level leads to ionization.

    • Ionization: Removal of an electron from an atom.

    • Excitation: Addition of energy by raising the energy of electrons via x-rays.

    Molecular Changes from Deposited Energy:

  • Ionization can lead to:

    1. Breakage of a molecule.

    2. Change in chemical binding properties.

    3. Relocation of the atom within the molecule.

  • Consequences of Molecular Changes:

    • Cells may be deficient in energy production, growth, and metabolism.

    • Abnormal molecules may function improperly over time.

    DNA as the Radio-sensitive Target Molecule:

  • Deposited energy causes molecular changes; it can lead to cell damage and death depending on the extent of the ionization.

  • Humans experience both early (deterministic effects) and late (stochastic effects) radiation responses.

    Early Radiation Effects:

  • Deterministic Effects: Occur within minutes or days after exposure. Severity increases with larger doses.

  • Categories of Early Effects include:

    1. Acute Radiation Syndrome:

      • Hematologic syndrome

      • Gastrointestinal syndrome

      • Central nervous system syndrome

    2. Local tissue damage (skin, gonads, extremities)

    3. Hematologic depression

    4. Cytogenetic damage

    Late Radiation Effects:

  • Stochastic Effects: Observed months or years after exposure. Severity increases with dose received.

  • Late effects include:

    1. Leukemia

    2. Other malignant diseases (e.g., bone, lung, thyroid, breast cancer)

    3. Local tissue damage

    4. Shortening of lifespan

    5. Genetic damage (cytogenetic damage, doubling dose, genetically significant dose)

    Effects of Fetal Irradiation

  • Consequences of-in-utero exposure include:

    1. Prenatal death

    2. Neonatal death

    3. Congenital malformations

    4. Childhood malignancy

    5. Diminished growth and development

    COMPOSITION OF THE HUMAN BODY

  • Atomic Composition:

    • Hydrogen (60%)

    • Oxygen (25.7%)

    • Carbon (10.7%)

    • Nitrogen (2.4%)

    • Calcium (0.2%)

    • Phosphorus (0.1%)

    • Sulfur (0.1%)

    • Trace elements (0.8%)

  • Molecular Composition:

    • Water (80%)

    • Proteins (15%)

    • Lipids (2%)

    • Carbohydrates (1%)

    • Nucleic Acids (1%)

    • Other (1%)

    CELL THEORY

  • Histories and Milestones:

    1. Robert Hooke (1665) - First named the cell.

    2. Anton Van Leeuwenhoek (1673) - Accurately described cells from microscopic observations.

    3. Schneider & Schwann (1838) - Showed that cells are the basic units in all plants & animals.

    4. Watson & Crick (1953) - Described the molecular structure of DNA.

    5. Human Genome Project (2000) - Further explored cell structures and functions.

    MOLECULAR & TISSUE COMPOSITION

  • Macromolecules: Large molecules such as proteins, lipids, carbohydrates, and nucleic acids.

  • Principal Organic Molecules:

    • Proteins

    • Lipids

    • Carbohydrates

    FUNCTIONS OF MOLECULES

  • Proteins: Long chains that provide structure (muscles), enzymes, hormones, and antibodies.

    • Protein Synthesis uses typically 22 amino acids.

    • General Formula: $Cn{Hn}{On}{Nn}{T_n}$ (where n signifies the sequence).

  • Enzymes: Important molecules that catalyze biochemical reactions.

  • Hormones: Regulatory molecules produced and secreted by endocrine glands:

    • Includes pituitary, adrenal, thyroid, parathyroid, pancreas and gonads.

  • Antibodies: Defense mechanisms against infections.

  • Lipids: Organic macromolecules composed mainly of carbon, hydrogen, and oxygen.

    • General Formula: $Cn{Hn}{O_n}$

    • Types:

      1. Glycerol – 1 molecule

      2. Fatty Acid – Up to 3 molecules.

  • Carbohydrates: Molecules specified to fuel cellular metabolism.

    • Includes sugars (monosaccharides & disaccharides).

    • Chemical formulas: $C6{H{12}O6}$ (for glucose) and $(C6{H{10}O5})_n$ (for polysaccharides).

    NUCLEIC ACIDS

  • DNA:

    • Control center for the cell containing genetic info.

    • Structure: double-helix consisting of nucleotides with sugar (deoxyribose), base (adenine, thymine, cytosine, guanine).

  • RNA: Plays crucial roles in protein synthesis and regulation of genes.

    CELLULAR STRUCTURE

  • Nucleus: Contains DNA, RNA, protein, and water.

  • Cytoplasm: Contains numerous molecular components except DNA.

  • Mitochondria: Energy generators of the cell.

  • Ribosomes: Sites of protein synthesis; essential for cellular function.

  • Lysosomes: Contain enzymes to digest cellular debris.

    MITOSIS AND MEIOSIS

  • Mitosis: Process by which cells replicate.

    • Four stages (PMAT): Prophase, Metaphase, Anaphase, Telophase.

    • Results in two identical daughter cells.

  • Meiosis: Cell division that results in four gametes with half the number of chromosomes (23 for humans).

    • Consists of two divisions; interphase not included in the second division.

  • Radio sensitivity: Affected by the cell's maturity and function.

    TISSUES AND ORGANS

  • Cells: Groupings of cells form tissues, which in turn form organs.

  • Structural Types:

    • Parenchymal: Functional tissue of an organ.

    • Stromal: Supportive tissue, typically connective.

  • Radiosensitivity: Determined by organ function, cell maturation, and inherent sensitivity of cell types.

    FUNDAMENTAL PRINCIPLES OF RADIOBIOLOGY

  • Law of Bergonie and Tribondeau: Radiosensitivity varies with the metabolic state of the irradiated tissue; sensitivity is higher in undifferentiated, rapidly proliferating cells.

  • Physical Factors:

    • Example: Radiation dose in Gyt (rad) determines the response.

    • Oxygen Effect: Tissue is more sensitive when oxygenated.

  • Linear Energy Transfer (LET): Rate at which energy is deposited per unit length by ionizing radiation.

    • Measured in keV/μm.

  • Relative Biological Effectiveness (RBE): Measures varying biologic effects caused by different types of radiation.

    DETERRING RADIATION DAMAGE

  • Radiosensitizers: Chemicals that enhance radiation effects when present during exposure.

  • Radioprotectors: Substances that provide protection against radiation damage, often requiring administration at toxic levels.

    HORMESIS

  • Defense response activated at low doses of radiation may yield beneficial effects that involve stimulation of immune and repair mechanisms.

    RADIATION DOSE-RESPONSE RELATIONSHIPS

  • The relationship correlates the magnitude of radiation doses to the biological response observed across various exposure levels.

  • Categories include deterministic and stochastic effects based on thresholds and severity of responses.

    MOLECULAR RADIOBIOLOGY

  • Examines the interaction of radiation with biological macromolecules, focusing on direct DNA damage versus indirect mechanisms via free radicals.

    CYTOGENETIC EFFECTS

  • Chromosomal damage after exposure can be quantitatively assessed for genetic effects.

    CLINICAL IMPLICATIONS OF RADIATION EXPOSURE

  • Understanding radiation effects is critical for providing safer diagnostic imaging and effective cancer therapies, emphasizing adherence to the ALARA principle not only in occupational settings but also for patient care.

    Thank You!