Wk 2: Radiation Biology

What are the two possible mechanisms of radiation injury?

Ionization; Free radical formation.

What happens during ionization when x-rays strike patient tissue?

Produced through the photoelectric effect or Compton scatter; results in formation of a positive atom and dislodged negative electron; this electron will interact with other atoms within the absorbing tissues, causing chemical changes within the cell that results in biologic damage.

How does cell damage occur primarily through free radical formation?

Free radicals are formed when an x-ray photon ionizes water.

What is a free radical?

An uncharged atom or molecule that exists with a single, unpaired electron in its outermost shell; highly reactive and unstable.

How can free radicals achieve stability?

Combine without causing changes in a molecule; combine with other free radicals and cause changes; combine with ordinary molecules to form toxins.

What can free radicals combine with each other to form?

Toxins such as hydrogen peroxide.

What are the two theories used to describe how radiation damages biologic tissues?

Direct theory; Indirect theory.

What is the direct theory of radiation injury?

Cell damage results when ionizing radiation directly hits critical areas within the cell; this occurs infrequently. X-ray photon interacts with important cell chemicals and breaks them apart, causing critical damage to molecule.

What may happen to minor damage caused according to the direct theory?

Minor damage can be repaired within the cell; most photons pass through cells with no observable effects.

What is the indirect theory of radiation injury?

X-ray photons are absorbed within the cell and cause the formation of toxins, which in turn damage the cell.

What happens when x-ray photons are absorbed by water within a cell according to the indirect theory?

Free radical formation results; the free radicals combine to form toxins that damage cells.

What occurs during radiolysis of water?

Ionization causes water molecules to break into hydrogen and hydroxyl radicals and reform into hydrogen peroxide, which is poisonous to cells, causing dysfunction of affected cells.

What percentage of the human body is water, and what molecule is most likely damaged?

Human body 80 percent water; water molecule most likely damaged.

What tends to happen to water ions after ionization?

Water ions tend to recombine back into a water molecule.

When will entire body tissues be destroyed by radiation, and what potential always exists?

Only in extreme instances concerning mass amounts of radiation will entire body tissues be destroyed; potential always exists for a cell to become malignant.

How do the direct theory and indirect theory differ?

In the direct theory x-ray photons collide with large molecules and break them apart by ionization. The indirect theory based on the assumption that radiation can cause chemical damage to the cell by ionizing the water within it.

What factors affect the potential harm from radiation?

Quantity of radiation; Duration of exposure.

What is the purpose of the dose-response curve?

Curve is used to correlate the damage of tissue with the dose of radiation received.

What relationship is seen in the dose-response curve?

A linear, nonthreshold relationship is seen.

What does the linear relationship of the dose-response curve indicate?

The response of the tissues is directly proportional to the dose.

What does the nonthreshold dose-response curve suggest?

No matter how small the amount of radiation received, some biologic damage occurs.

What are stochastic effects?

A direct function of the dose; no dose threshold; effects do not depend on the magnitude of the absorbed dose. Examples: Cancer and genetic mutations.

What are nonstochastic (deterministic) effects?

Somatic effects that have a threshold; effects increase in severity with increasing absorbed dose. Examples: Erythema, loss of hair, cataracts, and decreased fertility.

What is the sequence of events following radiation exposure when the dose received is non-lethal?

Latent period; Period of injury; Recovery period.

What is the latent period?

The time that elapses between exposure to ionizing radiation and the appearance of observable clinical signs.

What does the latent period depend on?

The total dose of radiation received and the amount of time it took to receive the dose.

How does dose affect the length of the latent period?

Short if large dose over short period of time—minutes, days, weeks. Long if smaller dose over longer period of time—years, decades, generations.

What may occur during the period of injury?

A variety of cellular injuries may result.

What effects are observed during the period of injury?

Stoppage of mitosis or abnormal mitosis; breaking or clumping of chromosomes; formation of giant cells—associated with cancer.

What occurs during the recovery period?

Depending on a number of factors, cells can repair the damage caused by radiation; there may be irreparable damage.

What are cumulative effects of radiation?

Effects of radiation exposure are additive; unrepaired damage accumulates in tissues.

At what radiation levels can radiobiologists not determine radiation effects?

100mSv or less.

What are the determining factors for radiation injury?

Total dose; Dose rate; Amount of tissue irradiated; Cell sensitivity; Age.

What is total dose?

Quantity of radiation received, or total amount of radiation energy absorbed.

What is dose rate?

Rate at which exposure to radiation occurs and absorption takes place.

How does the amount of tissue irradiated affect radiation injury?

Amount of tissue irradiated – areas of the body exposed to radiation.

How does cell sensitivity affect radiation injury?

More damage occurs in cells that are more sensitive to radiation.

How does age affect radiation injury?

Children are more susceptible to radiation damage than adults.

What is the helpful hint for remembering determining factors for radiation injury?

Total Dose – MORE damage with MORE dose; Dose Rate – MORE damage with FAST rate; Amount of Tissue – MORE damage with MORE tissue exposure; Age- MORE damage in children.

What are short-term effects of radiation?

Short-term effects of radiation are those seen minutes, days, or months after exposure; results when a very large dose delivered in a VERY short period of time.

What are the symptoms of Acute Radiation Syndrome (ARS)?

Erythema, nausea, vomiting, diarrhea, hemorrhage, and hair loss.

What dose is associated with Acute Radiation Syndrome (ARS), and is it a concern in dentistry?

VERY large (1.0Gy whole body) dose in short period; not a concern in dentistry.

What are long-term effects of radiation?

Long-term effects of radiation are those that are seen years after the original exposure; previous high exposure OR chronic low-level exposures.

What may result from the long-term effects of radiation?

An increased incidence of cancer, embryological defects, low birth weight, cataracts, and genetic mutations.

What are somatic cells and genetic cells?

Somatic cells: All cells in the body except the reproductive cells. Genetic cells: The reproductive cells.

What are somatic effects of radiation?

Seen in the person irradiated; not seen in future generations.

What are genetic effects of radiation?

Not seen in the person irradiated; passed on to future generations.

What is a cell that is sensitive to radiation termed, and what is a resistant cell termed?

A cell that is sensitive to radiation is termed radiosensitive; one that is resistant is termed radioresistant.

What factors determine the response of cells to radiation?

Mitotic activity - cells that divide frequently; Cell differentiation - cells that are immature or not highly specialized; Cell metabolism – cells that have a higher metabolism.

What is the order of cell sensitivity to radiation exposure from high sensitivity to low sensitivity?

White blood cells (lymphocytes); Red blood cells (erythrocytes); Immature reproductive cells; Epithelial cells; Endothelial cells; Connective tissue cells; Bone cells; Nerve cells; Brain cells; Muscle cells.

Which organs are radiosensitive?

Lymphoid tissue; Bone marrow; Testes; Intestines.

Which tissues are radioresistant?

Salivary glands; Kidney; Liver.

What is a critical organ?

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

Which critical organs are exposed during dental radiographic procedures?

Skin; Thyroid gland; Lens of the eye; Bone marrow.

What are the five possible outcomes following radiation exposure to a cell?

Nothing, the cell is unaffected by the exposure; cell is injured or damaged but repairs itself and functions at pre-exposure levels; cell dies, but is replaced through normal biological processes; cell is injured or damaged, repairs itself, but now functions at a reduced level; cell is injured or damaged, and repairs itself incorrectly or abnormally, resulting in a biophysical change (tumor or malignancy).

What are the traditional (older) units of radiation measurement?

Roentgen (R); Radiation absorbed dose (rad); Roentgen equivalent (in) man (rem).

What are the System International d’Unites (newer) units of radiation measurement?

Coulombs/kilogram (C/kg); Gray (Gy); Sievert (Sv).

What does the roentgen measure?

Roentgen measures radiation by determining the amount of ionization that occurs in air; it does not describe the amount of radiation absorbed.

How is exposure stated?

Exposure is stated in coulombs per kilogram.

What is dose measurement?

The amount of energy absorbed by tissue.

What are the traditional and SI units for dose measurement?

Traditional unit is the rad (radiation absorbed dose); SI equivalent is the gray; 1 Gy = 100 rads.

What is absorbed dose?

Amount of energy deposited in any form of matter, such as the tissues of the head and neck of a patient, by any type of radiation (alpha or beta particles, x- or gamma rays).

What units are used for measuring absorbed dose?

The gray (Gy) or (rad). One gray equals 100 rads.

What is dose equivalent measurement used for?

Dose equivalent measurement is used to compare biologic effects of different kinds of radiation.

What are the traditional and SI units for dose equivalent measurement?

Traditional unit is the rem (roentgen equivalent man); SI equivalent is the sievert; 1 Sv = 100 rems.

How is dose equivalent determined?

Product of the absorbed dose times a biological-effect qualifying or weighting factor determined by quantum physicists.

What is effective dose equivalent used for?

Compare the risk of radiation exposure producing a biological response; compensates for differences in area exposed and tissues that may be in path of the beam; allows for comparison of different types of exposure.

What does milli mean, and why is it used in dental imaging?

Milli means 1/1000; used to express the small doses used in dental imaging.

What is natural background radiation?

A form of ionizing radiation that is ubiquitous in the environment.

What are the sources of natural background radiation?

Cosmic radiation—stars and sun; terrestrial radiation—radioactive materials in the earth and air.

What is the average dose of background radiation received by an individual in the United States?

150 to 300 mrads per year.

What is artificial or humanmade radiation?

Resulting from modern technology; includes consumer products, fallout from atomic weapons, weapons production, and the nuclear fuel cycle.

What forms of medical radiation are listed as sources of artificial radiation exposure?

Medical radiographic procedures, dental imaging, fluoroscopy, nuclear medicine, and radiation therapy.

What is the estimated potential risk of dental imaging inducing a fatal cancer?

3 in 1 million.

What is the risk of a person developing a cancer spontaneously?

3300 in 1 million.

What is the estimated risk of an FMS/FMX exam inducing cancer?

3 per 1,000,000 exams.

What radiation exposure is listed for an FMX using F-speed film and round PID?

FMX using F-speed film and round PID = 23.4mSv; one day of background radiation = 8mSv; FMX = 2.9 days of any time of background radiation exposure.

What factors cause dental radiation risk estimates to vary?

Speed of film; collimation; the technique used; exposure factors.

When should dental images be prescribed for a patient?

Only when the benefit of disease detection outweighs the risk of biologic damage.

When dental images are properly prescribed and exposed, how does the benefit compare with the risk?

The benefit of disease detection far outweighs the risk of damage.

What is the goal of patient protection from x-radiation?

Minimize the amount of radiation received by the patient and maximize the benefits.

When may techniques be used to protect the patient?

Before, during, and after the procedure.

What patient protection measures are used before exposure?

Prescribing dental images; Proper equipment.

How are the number, type, and frequency of dental images determined?

Professional judgment is used to determine the number, type, and frequency of dental image.

What proper equipment features minimize radiation a patient receives?

Filtration—Inherent filtration, Added filtration, Total filtration; Collimation; Position-indicating device.

What is inherent filtration?

Inherent filtration takes place when the primary beam passes through the glass window of the x-ray tube, the insulating oil, and the tubehead seal.

How much aluminum is inherent filtration equivalent to, and is it sufficient by itself?

Approximately 0.5 to 1.0 mm of aluminum; it does not meet the standards regulated by state and federal law, so additional filtration is required.

What is the purpose of filtration?

Absorption of the longer wavelength (less penetrating) x-rays to reduce patient dose.

What material is usually used for filtration?

Usually an aluminum filter.

What is the half-value layer?

Thickness (mm) of aluminum that will reduce the density of the beam by half (more accurate than kVp because machines can vary); measures intensity of the beam.

What is added filtration?

An aluminum disk is placed between the collimator and the tubehead seal to filter out longer wavelength, lower energy x-rays from the x-ray beam.

What effect does filtration have on the useful beam?

Filtration results in a higher-energy and more penetrating useful beam.

What is total filtration?

The sum of inherent and added filtration; regulated by state and federal law.

What total filtration is required for machines operating at or below 70 kVp?

Require a minimum total of 1.5 mm. aluminum filtration.

What total filtration is required for machines operating above 70 kVp?

Require a minimum total of 2.5 mm aluminum filtration.

What is the purpose of collimation?

Controls size and shape of useful beam, thus reducing volume of tissue irradiated.

How is collimation accomplished?

Using lead diaphragm or collimator; placed in the path of the primary beam as it exits the tube housing at the port; may be round or rectangular.

What do federal guidelines require for round opening collimators?

Restrict the x-ray beam to 2.75 inches (7 cm) at the patient’s face.

How do rectangular collimators affect patient exposure compared with round collimators?

Rectangular collimators further restrict the beam to about the size of the image receptor—can reduce exposure by up to 70 percent over round collimators.

What effect does collimation have on scatter radiation and image quality?

Collimation reduces scatter radiation that can fog the image and cause poor contrast.