Radiation Biology Chapter 3 - Radiation Injury Mechanisms

Radiation Biology Chapter 3

Mechanisms of Injury
- Theories of radiation injury
- Dose-response curve
- Stochastic and nonstochastic radiation effects
- Sequence of radiation injury
- Determining factors for radiation injury

Radiation causes chemical changes that result in biological damage.
A portion of x-rays do not reach dental x-ray film; instead, they are absorbed by the patient’s tissue leading to:
- Ionization
- Free radical formation

Results when x-rays strike patient tissue.
Produced through the:
- Photoelectric effect
- Compton scatter
Ionization Process:
- Forms a positive atom and a dislodged negative electron.
- The dislodged electron interacts with other atoms in the absorbing tissues, causing chemical changes within cells that result in biological damage.

Cell damage primarily occurs through the formation of free radicals.
Free Radical:
- An uncharged atom or molecule with a single, unpaired electron in its outermost shell.
- Highly reactive and unstable.
Free radicals form when an x-ray photon ionizes water.

Damage to tissue from ionizing radiation can occur through:
- A direct hit and absorption of an x-ray photon within a cell.
- Absorption of an x-ray photon by water within a cell leading to free radical formation.
There are two primary theories of radiation damage:
- Direct theory:
- Cell damage occurs when ionizing radiation directly hits critical areas within a cell.
- This is a rare event.
- Indirect theory:
- X-ray photons absorbed by water cause the formation of toxins that subsequently damage the cell.
- Free radicals combine to form toxins that harm cells.

Represents the relationship between radiation doses and biological responses in tissues.
Features of the curve:
- Linear Nonthreshold Relationship:
- Indicates a direct proportionality between tissue response and radiation dose.
- Suggests biologic damage occurs at any radiation level, no matter how small.
Used to correlate tissue damage with the radiation dose absorbed.

Stochastic Effects:
- No dose threshold; effects are proportional to the dose with examples including:
- Cancer
- Genetic mutations
Nonstochastic (Deterministic) Effects:
- Somatic effects that have a threshold; severity increases with absorbed dose.
- Examples include:
- Erythema
- Loss of hair
- Cataracts
- Decreased fertility

Latent Period:
- Time between exposure and visible clinical signs of injury.
- Depends on total radiation dose and exposure duration.
Period of Injury:
- Various cellular injuries can result from radiation exposure.
Recovery Period:
- Cells have the ability to repair radiation-induced damage, contingent on several factors.
Cumulative Effects:
- Radiation exposure effects are additive; unrepaired damage accumulates in tissues.
Determining Factors for Radiation Injury:
- Total dose
- Dose rate
- Amount of tissue irradiated
- Cell sensitivity
- Age

Short- and Long-Term Effects:
- Short-term Effects:
- Associated with high radiation doses in a short time (e.g., Acute Radiation Syndrome (ARS)).
- Symptoms include nausea, vomiting, diarrhea, hair loss, and hemorrhage.
- Long-term Effects:
- Result from small doses absorbed over long periods (noted years later).
- Consequences can include cancer, birth defects, and genetic anomalies.
Somatic Cells:
- All body cells except reproductive cells.
Genetic Cells:
- Reproductive cells.
Biologic effects categorization:
- Somatic effects: Occur in the irradiated individual
- Genetic effects: Not seen in the irradiated person but may affect future generations.

The response to radiation is determined by:
- Mitotic Activity:
- Cell Differentiation:
- Cell Metabolism:
Radiosensitive:
- A cell sensitive to radiation.
Radioresistant:
- A cell resistant to radiation.

Radiosensitive organs:
- Includes lymphoid tissue, bone marrow, testes, and intestines.
Radioresistant Tissues:
- Includes salivary glands, kidneys, and liver.
Critical Organ:
- An organ that diminishes the quality of life if damaged.
- Key critical organs affected during dental radiographic procedures:
- Skin, thyroid gland, lens of the eye, bone marrow.

Radiation Exposure Measurements:
- Traditional Units:
- Roentgen (R): Measures radiation based on ionization in air; does not indicate absorption.
- Radiation Absorbed Dose (rad): Amount of energy absorbed.
- Roentgen Equivalent (in) Man (rem): Compares biologic effects of differing irradiations.
- SI Units (newer):
- Coulombs/kilogram (C/kg)
- Gray (Gy): SI equivalent of rad; 1 Gy = 100 rads.
- Sievert (Sv): SI equivalent of rem; 1 Sv = 100 rems.

Sources of Radiation Exposure:
- Natural Background Radiation:
- Cosmic radiation (from stars and sun).
- Terrestrial radiation (from radioactive materials in the earth/air).
- U.S. average background radiation dose: 150 to 300 mrads per year.
Artificial Radiation:
- Resulting from technology, medical procedures, atomic weapons fallout, and nuclear fuel cycles.
Risk Estimates:
- Estimated risk of fatal outcome due to dental imaging is 3 in 1 million.
- Comparison against other activities:
- 1 in 10 million risks for 10 miles of biking
- 1 in 300 million risks for 1000 miles of flying
- Risk of developing cancer spontaneously: 3300 in 1 million.

Dental Radiation and Exposure Risks:
- Important to understand risk estimates related to critical organs: thyroid gland, bone marrow, skin, and eyes.
Risk Versus Benefit of Dental Images:
- Dental images should only be prescribed when the benefits for disease detection outweigh potential risks for biological damage.
- When properly prescribed, the benefits of dental images significantly outweigh potential risks of damage.