Radiation Exposure: Background & Sources

Sources of Radiation Exposure

• The general population is exposed to radiation primarily from natural background and medical sources
• Understanding these exposure sources provides a useful framework for considering dental exposure

Background Radiation

• All life on earth has evolved in response to continuous exposure to natural background radiation
• Background radiation from space and terrestrial sources yields an average annual effective dose of about 3.1 mSv in the United States

Radon and Its Progeny

• Radon 222, a gas released from the ground, enters homes and buildings
• Radon decays, releasing α particles, to polonium 218 and lead 214
• These isotopes decay further, emitting more α particles
• Radon and its decay products attach to dust particles and can be inhaled and deposited on the bronchial epithelium
• Radon is responsible for approximately 73% of the world's population background exposure
• Exposure to this quantity of radiation may cause 10,000 to 20,000 lung cancer deaths per year in the United States, mostly in smokers

Space Radiation

• Space radiation comes from the sun or cosmic rays
• It's composed primarily of protons, helium nuclei, and nuclei of heavier elements as well as other particles generated by interactions of primary space radiation with the earth’s atmosphere
• Exposure from space radiation almost doubles for each 2000-m increase in elevation
• At sea level, the exposure from space radiation is about 0.33 mSv/y
• At 1600 m, the approximate elevation of Denver, Colorado, it is about 0.50 mSv/y
• Space radiation contributes about 11% of background exposure

Internal Radionuclides

• Another source of background radiation is radionuclides that are ingested
• Greatest internal exposure comes from foods containing uranium and thorium and their decay products
• Potassium 40, rubidium 87, carbon 14, and tritium contribute

Terrestrial Radiation

• The final source of background radiation comes from exposure from radioactive nuclides in the soil
• Primarily potassium 40 and the radioactive decay products of uranium 238 and thorium 232
• Most of the γ radiation from these sources comes from the top 20 cm of soil
• Indoor exposure from radionuclides is close to the exposure occurring outdoors given that structural materials provide shielding materials
• Terrestrial exposure contributes approximately 7% of background exposure

Medical Exposure

• Medical imaging adds additional sources of radiation to the environment
• Approximately 400 million x-ray examinations are performed annually in the United States; about one quarter of these are dental
• Medical exposure medical exposure in developed countries has grown rapidly in recent decades

Consumer Products

• Consumer products includes cigarette smoking, building materials, air travel, mining and agriculture, and combustion of fossil fuels
• Cosmic radiation becomes a more significant contributor to exposure above earth's atmosphere
• An airline flight of 5 hours may result in an exposure of about 25 μSv
• Minor sources of exposure includes dental porcelain, television receivers, and smoke alarms
• Consumer products contribute only about 1.6% of the total average annual exposure

Other Sources

• Other sources of exposure affect caregivers or others in contact with patients receiving nuclear medicine treatments
• People who work in nuclear power generation or in medical x ray facilities, affect caregivers
• Industrial, medical, educational, or research activities and commercial pilots are also factors
• All these sources of radiation combined contribute only about 0.1% of the total average annual exposure

Dose Limits Exposures and Risk

• This section considers governmental dose limits for individuals who are occupationally exposed to radiation and for members of the general population
• It also covers amounts of radiation received by patients in dental and medical radiography and the estimated risks from these exposures

Dose Limits

• Recognition of the harmful effects of radiation and the risks involved with its use led the International Commission on Radiological Protection (ICRP) to establish guidelines for limitations
• ICRP set the amount of radiation received by both occupationally exposed individuals and the public
• These limits pertain to planned exposure situations, not to background radiation and not to emergency situations
• Occupationally exposed individuals include dentists and their staffs
• Individuals in the reception area are members of the public
• Dose limits for public are generally set at 10% of occupationally exposed individuals
• Occupational exposure limits ensure that no individuals have deterministic effects and that the probability for stochastic effects is as low as reasonably and economically feasible
• The average dose for individuals occupationally exposed in the operation of dental x-ray equipment is 0.2 mSv, only 1% of the allowable exposure
• Increasing concern for patient exposure has led multiple institutions to issue diagnostic reference levels (DRL) for medical and dental diagnostic imaging

Patient Exposure

• Patient dose from dental radiography is usually reported as effective dose
• Effective dose is a measure of the amount of radiation received by various radiosensitive organs during the radiographic examination
• Dental exposures are a small fraction of the annual average background exposure

Estimating Risk

• The primary risk from dental radiography is the small risk of Cancer
• Cancer affects about 40% of people and accounting for about 20% of all deaths
• Literature links exposures of radiation to cancer formation
• Studies have led to the development of the linear nonthreshold (LNT) hypothesis
• LNT holds that there is a linear relationship between dose and the risk of inducing a new cancer, even at very low doses, and that there is no safe dose to add no risk

LNT

• It is a hypothesis that has been accepted widely for setting policy in radiation safety and protection and is not a demonstrated scientific fact
• There is solid work demonstrating increased risk of tumors in individuals exposed to more than about 100 mGy
• Insufficient studies show risk so the validity of model is uncertain
• The policy sets exposure limits for individuals exposed in the low-dose range
• DNA damage may occur with even one x-ray photon
• Most radiation protection organizations believe that it is prudent to assume that risk is proportional to dose, even for diagnostic exposures, and that there is no safe threshold Estimates are often based on the LNT and are highly speculative

Reducing Dental Exposure

• There are three guiding principles in radiation protection:
  • Justification
  • Optimization
  • Dose limitation
• The principle of justification means the dentist should identify situations where the benefit to a patient from the diagnostic exposure likely exceeds the risk of harm

Patient Selection Criteria

• Radiographic screening for detecting disease before clinical examination should not be performed
• A clinical examination, consideration of the patient history, review of any prior radiographs should precede radiographic examination
• Radiographs should be made only when there is a specific indication for a specific patient
• Clinical or historical findings identify patients for whom a high probability exists that a radiographic examination would provide information affecting their treatment or prognosis
• Dentists should the lowest dose image that would satisfy the task

Conducting the Examination

• Conducting the examination and the principle of optimization greatly influences patient exposure to radiation
• Conduct of the examination affects what equipment is used, the technique, operation of equipment, and how the radiographic image is processed and interpreted

Film and Digital Imaging

• Use the fastest image receptor compatible with the diagnostic task
• Film of speed group E/F film is about twice as fast (sensitive) as film of group D and thus requires only half the exposure
• Fast films are desirable from standpoint of exposure reduction
• E/F-speed film is preferred for most situations as it has the same useful density range, latitude, contrast, and image quality as D-speed films without sacrificing diagnostic information
• Digital sensors offer equal or greater dose savings than E/F-speed film and comparable diagnostic utility

Intensifying Screens and Film

• Contemporary intensifying screens used in extraoral radiography use the rare earth elements gadolinium and lanthanum
• Rare earth screens decrease patient exposure by 55% in panoramic and cephalometric radiography
• No dose reduction is gained by replacing fast extraoral screen-film systems with digital imaging
• Image resolution with digital systems is roughly the same as with rare earth screens

Source-to-Skin Distance

• Longer source of 40 cm decreases exposure by 10 to 25 percent
• Distances for intraoral machines, use 20 cm (8 inches) and 41 cm (16 inches)
• Exposed tissue volume is reduced because the x-ray beam is less divergent
• Use of a longer source-to-object distance results in a smaller apparent focal spot size increasing the resolution

Rectangular Collimation

• A rectangular collimator decreases the radiation dose by up to fivefold compared to a circular one
• Most state regulations require that the x-ray beam be collimated so that the field of radiation at the patient skin's surface is no more than 7 centimeters
• The field size is almost three times larger than necessary to expose the image so reduce to image receptor decreases patient exposure
• Decreased tissue volume reduces the amount of scattered radiation decreases image fogging and resultant image has improved diagnostic quality
• Beam size can be reduced by attaching a rectangular position-indicating device (PID)to the radiographic tube housing reducing skin surface by 60 percent
• A film-holding instrument centering the beam over the film or sensor is recommended to better film position

Filtration

• The x-ray beam emitted from the radiographic tube is a spectrum of x-ray photons, particularly lower energy photons which are almost completely absorbed by the patient
• Filtration removes low-energy photons preferentially from the x-ray beam
• Filtration results in decreased patient exposure with no loss of radiographic information
• Federal government says for dental x-ray machines have 1.5 mm of aluminum total filtration when operating from 50 to 70 kVp and 2.5 mm when operating above 70 kVp

Leaded Aprons and Thyroid Collars

• The thyroid gland is more susceptible to radiation exposure during dental radiographic exams especially in children
• Reduce radiation exposure to secondary organs with use of collimation
• The NCRP and ADA concluded that leaded aprons are unnecessary
• Reducing exposure of primary beam is paramount and more recent research shows heritable effects are insignificant
• Most states currently require leaded aprons
• In recent years, lead-free aprons that are are lighter and avoid the use of lead have been offered for sale; they attenuate like conventional aprons but weigh less
• Concerns about radiation exposure to the thyroid mean it is appropriate to protect the thyroid glands of children
• Best ways to accomplish this are to use fast receptors, rectangular collimation, and thyroid collars

Film and Sensor Holders

• Film holders align the film and reduce retakes
• Film or digital sensor holders should be used when intraoral radiographs are made because they improve the alignment of the film
• Use results in significant reduction in unacceptable images and thus avoidable retakes
• Most such devices have an external guide that shows the operator where to align the aiming cylinder (PID)

Kilovoltage

• The optimal kilovoltage is between 60 and 70
• Patient dose is reduced with higher kVp exposures constant-potential units enable can produce radiographs with lower kilovoltage and at reduced levels of radiation

Milliampere-Seconds

• Image quality should be prioritized
• Exposure time is the most crucial factor in influencing diagnostic quality
• Optimal image quality results when radiograph is of diagnostic density, neither overexposed nor underexposed
• Image density is controlled by the quantity of x rays produced
• If your x-ray machine has a variable milliampere control, it should be set at the highest choice
• Proper exposure times should be determined empirically or by using manufacturer's suggestions

Film Processing

• All film should be processed by with the recommended guidelines
• Major cause of unnecessary patient exposure is overexpose
• Time-temperature processing is the best way to ensure optimal film quality

Interpreting the Images

• Radiographs should be reviewed under appropriate conditions
• Best to study in a semidarkened room
• Also it is useful is to use a magnifying glass to look at the photo's

Protecting Personnel

• Methods of dose reduction primarily help patient exposure which reduces the risk to operator as well
• Operators of radiographic equipment should use barrier protection with leaded glass window when possible
• Shielding means operator should stand at least two meters or six feet away from tube head and out of the primary beam
• Dental operatories should be designed and constructed to meet State regulations
• Barriers should be used if possible
• Strict position-and-distance rule means you have to be a certain distance in a spot that is safe

Personnel Monitoring

• Personnel-monitoring devices measures the accumulation of occupational risks
• Devices are recommended if someone is thought of to accumulate a dose greater of the limit

Continuing Education

• Practitioners should be up to date in all means with safety
• Exposure and dose reduction