Radiation Biology and Its Effects on Cells

Radiation Biology

• X-rays interact at the atomic level, ionizing or exciting orbital electrons, depositing energy in tissue.
• Ionization changes chemical properties of atoms in molecules.
• Broken or relocated atoms in large molecules can cause malfunction or death of cells.
• Ionized atoms can regain neutrality by attracting free electrons.
• Cells and tissues can regenerate.
• Early radiation effects occur within minutes or days.
• Late effects observed months or years later.

Radiobiology

• Studies effects of ionizing radiation on biological tissue.
• Aims to accurately describe effects of radiation on humans for safe diagnosis and effective treatment.

Composition of the Body

• Radiation interacts with body at atomic level.
• Over 85% of the body is hydrogen and oxygen (H - 60%, O - 25.7%, C - 10.7%, N - 2.4%).

Cell Theory

• Atomic level interactions lead to molecular changes, potentially causing cells to be deficient in normal growth and metabolism.
• Plants and animals are composed of cells; basic functional units.

Molecular Composition

• Five principal types of molecules in the body:
  • Water
  • Proteins
  • Lipids
  • Carbohydrates
  • Nucleic acids

Nucleic Acids

• DNA is the rarest and most radiosensitive, within the nucleus.
• DNA controls cell function and holds hereditary information.
• DNA within germ cells (sperm and egg) contains information for the entire individual.
• The DNA is the most sensitive part of a cell to ionizing radiation.
• Also, RNA, found in nucleus and cytoplasm, is involved in cell growth and development and controls protein synthesis.
• 3 general factors affecting cell or organ radiosensitivity:
  • Organ function
  • Cell maturation rate
  • Inherent cell sensitivity.

Response to Radiation based on cell type

• Lymphocytes, spermatagonia, erythroblasts, and intestinal crypt cells are highly radiosensitive.
• Endothelial cells, osteoblasts, spermatids, fibroblasts, liver cells, muscle cells, nerve cells, and chondrocytes have varying degrees of radiosensitivity.

Effects of Radiation on Cells

• Cells and tissues more immature or highly mitotic, more radiosensitive.
• Stem cells are highly sensitive.

Law of Bergonie and Tribondeau

• Immature, highly mitotic, and young cells/tissues are more radiosensitive to radiation.
• More differentiated cells are more resistant.
• Metabolically active cells are more sensitive.
• Higher proliferation rate = higher radiosensitivity.

Physical factors Affecting Radiosensitivity

• Linear Energy Transfer (LET): Measures the rate at which energy is transferred from radiation to soft tissue (keV/µm).
  • Higher LET implies more tissue damage.
  • Lower LET results in less damage (penetrating radiation)

Relative Biological Effectiveness (RBE)

• Quantifies the ability of radiation to cause biological damage
• Higher LET radiation, greater biological damage.

Fractionation and Protraction

• Large, single dose radiation has greater effects than fractionated doses.
• Lengthening the time/protraction of dose reduces effects.
• Fractionated doses allow time for tissue repair.

Oxygen Effect

• Oxygenated tissue is more sensitive to radiation than anoxic (hypoxic) tissue.
• Anoxic tissue is more radioresistant.
• Hyperbaric oxygen therapy increases tissue sensitivity for treatment.

Radiation Hormesis

• Suggests that low levels of radiation may stimulate repair response.

Mathematical dose-response relationships

• Some effects are purely deterministic; others are purely stochastic.
• Linear/nonthreshold relationships: Dose increase has direct effect.
• Non-linear/nonthreshold relationships: Dose increase has no specific relationship
• Threshold relationships: A specific dose is required before effects appear.

Diagnostic imaging and effects of radiation exposure

• Diagnostic image techniques use mathematical relationships.
• The goal of diagnostic imaging is to achieve optimal image with lowest radiation exposure.