Radiology Chapter 3: Radiation Biology

Radiation Biology

The study of the effects of ionizing radiation of living tissue

All ionizing radiation is...

Harmful and produces biological changes in the tissue

Two mechanisms of radiation injury are possible

- Ionizing

- Free radical formation

Some x-rays do not reach the dental x-ray film and are absorbed by the patients tissue

Chemical changes occur that result in biologic damage

Ionization

- Results when x-rays strike patient tissue

- This electron will interact with other atoms within the absorbing tissues causing chemical changes within the cell that results in biologic damage - Results in formation of a positive atom and dislodged negative electron

Freeradical formation

Cell damage occurs primarily through formation of free radicals when an x-ray photon ionizes water

Freeradical

- An atom that exists with a single, unpaired electron in its outermost shell

- Highly reactive and unstable

- Can combine with other molecules and not produce effects

- Can combine with other free radicals and cause changes in cells

- May combine with other molecules and produce a toxin

Freeradical formation steps

- X-ray photons interact with water in cells

- Ionization occurs

- Resulting in free radical formation

After free radical formation

- Free radicals combine to form toxins such as H2O2 (Hydrogen peroxide)

Radiation injury

Damage to living tissue caused by exposure

to ionizing radiation

Theories of radiation history

- Direct theory

- Indirect theory

Direct theory

A direct hit and absorption of an x-ray photon within a cell causing damage to critical areas such as DNA

Indirect theory

Absorption of an x-ray photon by water within a cell accompanied by free radical formation and formation of toxins causing biological damage

Biological effects of radiation can be classified into two effects

- Stochastic effects

- Nonstochastic effects

Stochastic effects

- A direct function of the dose

- No dose threshold (damage occurs at any dose)

- Examples (cancers or genetic mutations)

Nonstochastic effects

- Somatic effects that have a threshold

- Biological damage increases in severity with increasing absorbed dose

- Examples (hair loss or redness (erythema))

Sequence of radiation injury

- Latent period

- Period of injury

- Recovery period

Latent period

Time that elapses between exposure to ionizing radiation and the appearance of observable clinical signs

- The more radiation and faster the dose; the shorter the latent period

Period of injury

Cellular damage may occur

Recovery period

Cellular damage may be repaired

Tissue and radiation effects due to repeated exposure to radiation

- Bone marrow

- Reproductive organs

- Thyroid, skin, and salivary glands

- Eyes

Bone marrow

Leukemia

Reproductive organs

Genetic abnormalities/mutations

Thyroid, skin, and salivary glands

Carcinoma

Eyes

Cataracts

Determining factors for radiation history

- Total dose

- Dose rate

- Amount of tissue irradiated

- Cell sensitivity

- Age

Total dose

Quantity of radiation received

Dose rate

Rate at which exposure to radiation occurs and absorption takes place

Amount of tissue irradiated

Areas od the body exposed to radiation

Cell sensitivity

More damage occurs in cells that are most sensitive to radiation

Age

Children are more at risk (Cells are rapidly producing)

Short term effects

- Associated with large doses of radiation in a short amount of time

- Acute radiation syndrome (ARS)

- Examples: Nausea, vomiting, diarrhea, hair loss, and hemorrhage

Long term effects

- Small doses absorbed repeatedly over a long period of time

- Effects seen after years, decades, or generations

- Examples: Cancer, birth abnormalities, and genetic defects

Somatic effects

- Seen in the person irradiated

- Not seen in future generations

- All cells in the body except the reproductive

Genetic effects

- Not seen in the person irradiated

- Passed on to future generations

- The reproductive cells

Mutation

- When the DNA is damaged, cell function may be altered or

reproductive capacity may be accelerated. Cancer is the most harmful result of cellular mutation

A cell that is sensitive to radiation is called

Radiosensitive

A cell that is resistant to radiation is called

Radioresistant

Weather the cell is radioresistant or radiosensitive is determined by

- Mitotic activity

- Cell differentiation

- Cell metabolism

Mitotic activity

Cells that divide frequently or undergo many divisions over time are more sensitive to radiation

Cell differentiation

Cells that are immature or are not highly specialized are more sensitive to radiation

Cell metabolism

Cells that have a higher metabolism are more sensitive to radiation

Radiosensitive organs

- Lymphoid tissue (white blood cells)

- Bone marrow

- Reproductive Cells

- Immature cells

Radioresistant tissues

- Mature bone

- Muscle

- Nerve

Critical organ

An organ that, if damaged, diminishes the quality of a person's life

Critical organs exposed during dental radiographic procedures include

- Skin

- Thyroid gland

- Lens of the eye

- Bone marrow

How is radiation measured?

- Exposure

- Dose

- Dose equivalent

Traditional (older) units of radiation measurement

- Roentgen (R) - Exposure

- Radiation absorbed dose (rad) - Dose

- Roentgen equivalent (in) man (rem) - Dose equivalent

SI (newer) units of radiation measurement

- Coulombs/kilogram (C/kg)

- Gray (Gy)

- Sievert (Sv)

Measurement of ionizing radiation through the production of x-rays Roentgen

- Measures radiation exposure by determining the amount of ionization that occurs

- Exposure is stated in Coulombs per kilogram in the SI - Internationale or metric system

Dose measurement

The amount of energy absorbed by tissue when x-rays are produced

Rad

Traditional Unit (radiation absorbed dose)

Gray

SI equivalent

Dose equivalent measurement

Used to compare biologic effects of different kinds of radiation on different kinds of tissues

Dose Equivalent measurements (Examples)

Rem - traditional unit of measurement (roetgen equivalent man)

Sievert - SI equivalent

Sources of radation

- Cosmic radiation - Stars and sun

- Terrestrial radiation - Radioactive materials in the earth and air

- Artificial radiation - man-made sources

- Medical radiation - the largest contributor to artificial radiation

Each year, people are exposed to various types of ionizing

radiation and receive an average dose of...

People are exposed to 3.6 mSv per year

Risks and estimates

- "The potential risk of dental radiography inducing a fatal cancer in an individual has been estimated to be 3 in 1 million"

- "The risk of a person developing a cancer spontaneously is much higher, or 3300 in 1 million"

Risks and exposures (examples)

- 1 in one million risk 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

- Therapeutic x-rays contribute a small portion. Dental x-rays account for only 0.1% of the total annual exposure

Dental radiation and exposure risks

Risk estimates:

- Thyroid gland

- Bone marrow

- Skin

- Eyes

Patient exposure and dose

- Film speed - use fastest film speed

- Collimation - rectangular vs. round

- Technique - longer PID

- Exposure factors - higher kVp reduces dose to skin

Risk versus benefit of dental radiographs

- "Dental radiographs should be prescribed for a patient

only when the benefit of disease detection outweighs

the risk of biologic damage"

- When dental radiographs are properly prescribed and exposed, the benefit of disease detection far outweighs the risk of damage

Annual radiation exposure

- Each year, people are exposed to various types of ionizing

radiation and receive an average dose of 3.6 mSv (360

mrem ) per year.

- Radon 2 mSv(55%)

- Cosmic 0.27 mSv (8%)

- Rocks/soil 0.28 mSv (8%)

- Food/water 0.4 mSv (11%)

- Medical x-rays 0.39 mSv (11%)

- Nuclear medicine 0.14 mSv (4%)

- Consumer products 0.1 mSv (3%)

- Other sources <0.01 mSv (<1%)