L6. Radiation Protection

Absorbed dose

Amount of energy deposited by radiation in a mass; total energy from ionizing radiation absorbed per unit mass

What can the "mass" be in absorbed dose?

Anything: water, rock, air, people, etc.

Absorbed dose units

Expressed in milligrays (mGy); SI unit = Gray (Gy); traditional unit = rad; 1 Gy = 100 rad

Equivalent dose

Calculated for individual organs; based on the absorbed dose to an organ, adjusted to account for the effectiveness of the type of radiation

Equivalent dose units

Expressed in millisieverts (mSv) to an organ

Effective dose

Calculated for the whole body; the addition of equivalent doses to all organs, each adjusted for the sensitivity of the organ to radiation

Effective dose units

Expressed in millisieverts (mSv); SI unit = Sievert (Sv)

Effective dose: what it is used for and what it considers

Sum of equivalent doses weighted by radiosensitivity of exposed tissue/organ; used to estimate risk in humans; considers relative biologic effectiveness of different radiation types and radiosensitivity of different tissues in terms of risk for cancer formation or heritable effect

Total radiation exposure (US population, NCRP Report 160)

Grand total 6.2 mSv (natural background 3.1 + medical 3.0 + consumer products and other 0.1)

Natural background radiation total

3.1 mSv (radon 2.3, space 0.3, internal radionuclides 0.3, terrestrial 0.2)

Artificial exposure total

3.1 mSv (medical imaging 3.0 mSv; consumer products and other 0.1 mSv)

Radon exposure dose

2.3 mSv (largest natural source)

Space (cosmic) exposure dose

0.3 mSv

Internal radionuclides exposure dose

0.3 mSv

Terrestrial sources exposure dose

0.2 mSv

Medical exposure breakdown (mSv)

CT 1.5; nuclear medicine 0.8; interventional fluoroscopy 0.4; conventional radiography and fluoroscopy 0.3; dental 0.007; subtotal medical 3.0

Consumer products and other exposure dose

0.1 mSv

Source of the exposure data tables

National Council on Radiation Protection and Measurements (NCRP) Report 160, Ionizing Radiation Exposure of the Population of the United States, Bethesda, MD, 2009

Radon source of radiation

Radon-222 decays (polonium-218); emits alpha particles

Space (cosmic) radiation depends on what?

Altitude; sea level = 0.33 mSv/year; at 1600 m = 0.50 mSv/year

Internal radionuclides source

Foods containing uranium and thorium

Terrestrial radiation source

Potassium-40, uranium and others; found in the top 20 cm of soil

Radon Zone 1 (red)

Highest potential; predicted average indoor radon screening level greater than 4 pCi/L

Radon Zone 2 (orange)

Moderate potential; predicted average indoor radon screening level between 2 and 4 pCi/L

Radon Zone 3 (yellow)

Low potential; predicted average indoor radon screening level less than 2 pCi/L

Pennsylvania radon zones (Zone 1 and Zone 3)

Most PA counties are Zone 1 (red, highest potential); Philadelphia County is Zone 3 (yellow, low potential)

Pennsylvania Zone 2 (orange) counties

Erie, Crawford, Warren, McKean, Potter, Forest, Elk, Jefferson, Mercer, Lawrence, Cambria, Somerset, Fayette, Greene, Washington, Wayne, Pike

Natural background radiation pie chart (3.1 mSv/yr)

Radon 73%; space 11%; internal radionuclides 9%; terrestrial 7%

Artificial sources pie chart (3.1 mSv/yr)

CT 47%; nuclear medicine 25%; interventional radiography and fluoroscopy 14%; conventional radiography and fluoroscopy 10%; consumer products and other 4%; dental 0.26%

Medical x-ray exams in the US

400 million x-ray exams; 25% are dental

Largest medical contributors to artificial exposure

CT and cardiac nuclear medicine

Dental share of total medical exposure

0.26%

Consumer product sources of radiation

Cigarette smoking, building materials, air travel, mining agriculture, fossil fuels

Other artificial sources of radiation

Caregivers of nuclear medicine treatment patients, radiation workers

ALARA

As Low As Reasonably Achievable; guiding principle for exposure and dose in radiography

Deterministic (nonstochastic) effects: goal

Prevent their occurrence

Stochastic effects: goal

Reduce the likelihood of occurrence

Two ways to estimate risk from exposure

Using equivalent natural (background) exposure, or the probability of stochastic effect

Intraoral dose with rectangular collimation

Posterior bitewings (PSP or F-speed): 5 µSv = 0.6 day; full-mouth PSP or F-speed: 40 µSv = 5 days; full-mouth CCD sensor (estimated): 20 µSv = 2.5 days

Intraoral dose with round collimation

Full-mouth D-speed film: 400 µSv = 48 days; PSP or F-speed: 200 µSv = 24 days; CCD sensor (estimated): 100 µSv = 12 days

Extraoral exam doses

Panoramic 20 µSv = 2.5 days; cephalometric 5 µSv = 0.6 day; chest 100 µSv = 12 days

Cone beam CT doses

Small FOV (<6 cm) 50 µSv = 6 days; medium FOV (dentoalveolar, full arch) 100 µSv = 12 days; large FOV (craniofacial) 120 µSv = 15 days

Multidetector CT doses

Maxillofacial 650 µSv = 2 months; head 2 mSv = 8 months; chest 7 mSv = 2 years; abdomen and pelvis with and without contrast 20 mSv = 7 years

Organizations that recommend annual effective dose limits

NCRP (National Council on Radiation Protection and Measurements) and ICRP (International Commission on Radiation Protection)

Purpose of annual effective dose limits (stochastic effects)

Ensure the probability of stochastic effects is as low as reasonably and economically feasible

Purpose of annual effective dose limits (occupational risk)

Ensure the risk of radiation workers being afflicted with fatal cancer is ≤ the risk of fatal accidents in non-radiation occupations

Occupational dose limits

50 mSv per year (NCRP); 20 mSv per year averaged over 5 years (ICRP)

Non-occupational (public) dose limits

10% of occupational limits; 5 mSv per year for infrequent exposure; 1 mSv per year for continuous exposure

Operator protection: barrier materials

Lead; gypsum drywall, brick, or concrete (lead lining rarely needed); 3/4 inch glass

Operator protection: distance and position

At least 2 m (about 6.5 ft) from the x-ray source, at 90° to 135° to the x-ray beam

Who may assist in an exposure?

Patient's relative, with protection; never use a staff member

Three properties of x-rays relevant to operator protection

Travel in straight lines from the source; intensity diminishes rapidly with distance; can be scattered or deflected

Guiding principle for diagnostic radiology in dentistry (JADA 2001; reaffirmed ADA 2024)

Enhance the diagnostic benefits of dental radiographs and minimize the associated radiation risks to patients and staff

Patient protection: significance in diagnosis

Consider signs, symptoms, radiographic history, and positive findings

Intraoral receptor selection: film speed

Use the fastest speed film; F-speed requires only 40% of the exposure of D-speed film

Intraoral receptor selection: digital

Digital sensors (CCD/CMOS) give about 50% patient dose reduction vs F- or E-speed film; PSP plates are similar to F-speed film

Extraoral receptor selection: screens

Use intensifying screens; rare earth screens decrease patient exposure by 55% compared with older calcium tungstate screens

Digital receptors in panoramic and cephalometric radiography

No significant dose reduction

Collimation: exposure area size

Exposure area less than 7 cm (2.75 inches) in diameter

Collimation: PID type

Open end PID (position-indicating device)

Rectangular collimation benefits vs round

~60% less skin area exposed; ~50% lower thyroid dose; up to 5× lower effective dose

Focal-film distance: federal regulation

X-ray source to skin distance must not be less than 7 inches

Focal-film distance: recommendation

Use longer FFD (8, 12, or 16 inch PIDs)

Filtration requirement

2.5-mm equivalents of aluminum

Source-to-skin distance and collimator diagram (A, B, C)

A: short source-to-skin distance; B: longer source-to-skin distance; C: rectangular collimator, giving a smaller, less divergent beam and a smaller volume of tissue irradiated than A and B

Kilovoltage for intraoral radiographs

60-100 kVp; 70 kVp is recommended

Thyroid collars and lead aprons: national guidance (AAOMR 2023; ADA 2024)

No longer routinely recommended; no routine thyroid or gonadal shielding for any dental imaging

Rectangular collimation vs thyroid collar

Rectangular collimation lowers thyroid dose more than a collar does; collars can block the beam and cause retakes

Our clinic's thyroid collar policy

Still use a thyroid collar for intraoral imaging, because we use round collimation and learners are still building technique

Thyroid collar placement rule

Keep the collar out of the primary beam; if it shows on the image, it caused the retake

Receptor holders: benefit

Reduce retakes and radiation exposure

Patient protection: exposure and processing

Proper exposure and process time; darkroom practices (light tight, ventilation); infection control practice; no skin contact with chemicals

Patient protection: quality assurance, image viewing, education

Monitor x-ray generators, processors, and chemicals at regular intervals; ideal image viewing conditions; continuing education

Communicating radiation risk: first steps

Allow patients to fully express concerns (don't interrupt or dismiss); explain the specific diagnostic need for radiographs for their condition; describe protective measures (selection criteria, fast digital receptors, collimation)

Communicating radiation risk: background comparisons

Bitewings: less than 1 day with rectangular collimation, a few days with round; full-mouth series: about 12-24 days with round (2.5-5 days rectangular); panoramic: 2-3 days of natural background; 5-hour flight = 25 µSv

What to emphasize when talking with patients about dose

"Dental radiographs deliver negligible to minimal doses relative to other medical imaging"

5 steps for talking with patients about radiographs

1 Listen (let them voice concern fully); 2 Justify (why this image, for this patient, now); 3 Protect (name what we do to keep dose low); 4 Compare (days of background, or a flight); 5 Respect (if they decline, explain what we could miss, and document it)

Patient communication: what to say vs avoid

Say: "This x-ray shows me decay between your teeth that I can't see by looking." and "It's about the same as a few days of the natural radiation we all get." Avoid: "There's zero radiation. It's totally safe." (not accurate) and "Everyone gets these every year." (images are prescribed by need, not by the calendar)

ACS lifetime cancer probability (2008-2010)

Developing any cancer: males 43.92% (1 in 2), females 38.00% (1 in 3); dying from cancer: males 22.94% (1 in 4), females 19.34% (1 in 5); prostate 15.33% (1 in 7) in males

Risk perspective: baseline, dental share, occupational dose

~40% of people develop cancer in their lifetime; dental radiography is 0.26% of total medical radiation exposure; average occupational dose in dentistry is about 0.06 mSv per year (ADA 2024), far below the 20-50 mSv annual limits

Risk perspective: pregnancy, everyday risk, benefit vs risk

Fetal doses from dental radiography are 42,000-fold lower than the threshold for deterministic effects; dental x-rays carry less risk than many everyday activities; diagnostic benefits far outweigh minimal radiation risks