Dental Radiography: Radiation Biology

Radiation Biology

Radiation Injury

  • Mechanisms of injury

    • Some x-rays are absorbed by the patient's tissue.

    • Chemical changes occur, leading to biologic damage. Theory of damage

  • Severity increases with absorbed dose.

    • Examples: erythema, hair loss, cataracts, decreased fertility.

  • Sequence of radiation injury

    • Latent period

      • Time between exposure and observable signs.

      • Depends on total dose and time to receive dose.

    • Period of injury

      • Cellular injuries occur.

    • Recovery period

      • Cells repair damage.

    • Cumulative effects

      • Effects are additive.

      • Unrepaired damage accumulates.

  • Determining factors for radiation injury

    • Total dose

    • Dose rate

    • Amount of tissue irradiated

    • Cell sensitivity

    • Age

Mechanisms of Injury

  • X-rays absorbed by patient's tissue lead to chemical changes and biologic damage.

  • Two mechanisms:

    • Ionization

    • Free radical formation

Ionization

  • X-rays strike patient tissue.

  • Occurs through photoelectric effect or Compton scatter.

  • Formation of a positive atom and dislodged negative electron.

  • The electron interacts with other atoms, causing chemical changes within the cell.

Free Radical Formation

  • Cell damage primarily through free radicals.

  • Free radicals form when x-ray photon ionizes water.

  • Free radical – Uncharged atom or molecule with a single, unpaired electron in the outermost shell

    • Highly reactive and unstable.

Theories of Radiation Injury

  • Damage caused by ionizing radiation may result from:

    • Direct hit and absorption of an x-ray photon.

    • Absorption by water, resulting in free radical formation.

  • Two theories:

    • Direct theory

    • Indirect theory

Direct Theory

  • Ionizing radiation directly hits critical areas within the cell.

  • Occurs infrequently.

Indirect Theory

  • X-ray photons absorbed within the cell cau sister what down

Sequence of Radiation Injury

  • Latent period

    • Time between exposure and observable signs.

    • Depends on total dose and time to receive the dose.

  • Period of injury

    • Variety of cellular injuries may result.

  • Recovery period

    • Cells can repair damage.

  • Cumulative effects

    • Radiation exposure effects are additive.

    • Unrepaired damage accumulates.

Determining Factors for Radiation Injury

  • Total dose

  • Dose rate

  • Amount of tissue irradiated

  • Cell sensitivity

  • Age

Shorter

Somatic and Genetic Effects

  • Somatic cells: All cells except reproductive cells.

  • Genetic cells: Reproductive cells.

  • Biologic effects classified as:

    • Somatic: Seen in the person irradiated; not in future generations.

    • Genetic: Not seen in the person irradiated; passed to future generations.

Radiation Effects on Cells

  • Radiosensitive: Sensitive to radiation.

  • Radioresistant: Resistant to radiation.

  • Response determined by:

    • Mitotic activity

    • Cell differentiation

    • Cell metabolism

Radiation Effects on Tissues and Organs

  • Radiosensitive organs

    • Lymphoid tissue

    • Bone marrow

    • Testes

    • Intestines

  • Radioresistant tissues

    • Salivary glands

    • Kidney

    • Liver

  • Critical organ

    • Organ that, if damaged, diminishes the quality of life.

    • Critical organs exposed during dental radiography: hello

  • Patient exposure and dose

  • Risk versus benefit of dental images

Sources of Radiation Exposure

  • Natural background radiation

    • Ionizing radiation in the environment.

      • Cosmic radiation: stars and sun.

      • Terrestrial radiation: radioactive materials in earth and air.

      • U.S. average: 150 to 300 mrads per year.

  • Artificial or human-made radiation

    • From modern technology.

      • Consumer products, fallout from atomic weapons, weapons production, nuclear fuel cycle.

      • Medical radiation: radiography, dental imaging, fluoroscopy, nuclear medicine, radiation therapy.

Risk and Risk Estimates

  • Risk of fatal cancer from dental imaging: 3 in 1 million.

  • Risk of spontaneous cancer development: 3300 in 1 million.

  • 1 in a million risks of a fatal outcome:

    • 10 miles on a bicycle

    • 300 miles in an auto

    • 1000 miles in an airplane

    • Smoking 1.4 cigarettes a day

Dental Radiation and Exposure Risks

  • Risk estimates for:

    • Thyroid gland

    • Bone marrow

    • Skin

    • Eyes

Patient Exposure and Dose

  • Film speed

  • Collimation

  • Technique

  • Exposure factors

Risk Versus Benefit of Dental Images

  • Prescribe dental images only when the benefit of disease detection outweighs the risk of biologic damage.

  • When properly prescribed and exposed, the benefit outweighs the risk.