Rethinking Patient Shielding in Dental Radiography

The foundation to justify radiographic imaging is that the diagnostic benefit far outweighs the radiation-associated risks. With this in mind, dental professionals have used various approaches to minimize patient radiation exposure such as the use of selection criteria, beam collimation, fast receptor systems, and patient shielding.¹ Thyroid and gonad shielding during dental radiography are long-standing practices introduced more than 70 years ago.²

With technological advances, radiation doses from dental radiographs have significantly decreased.³ Scientific investigation has provided further clarity on the magnitude of risk for radiation-induced effects.^4-6 Considering these changes, several expert organizations have re-evaluated the protection offered by shielding and challenged its use.^3,7,8

X-rays fall under the spectrum of ionizing, electromagnetic radiation, which can ionize an atom.⁹ A diagnostic X-ray beam consists of photons of various energies. The energy of those photons determines how they interact with hard and soft tissues. As the beam passes through a patient to strike an image receptor, it is either attenuated (absorbed), scattered, or passes without interaction.^9,10

Sources of Ionizing Radiation

Radiation exposure to the general United States population comes from natural background radiation, medical imaging and procedures, consumer products and activities, research/​educational activities, and occupational exposure. The average effective dose from background radiation is 3.1 mSv per year.¹¹ Background radiation comes from natural sources, including: radon (73%), cosmic rays (11%), internal radionuclides (9%), and terrestrial (7%).¹¹

The average effective dose from medical sources including computed tomography, nuclear medicine, interventional radiography, conventional radiography, and dental imaging is 2.2 mSv per year. Radiation dose from dental sources, including cone-beam computed tomography (CBCT), amounts to approximately 0.04 mSv per year. Radiation exposure from dental imaging contributes only 1.8% of the total dose from medical sources.¹²

Biological effects from exposure to ionizing radiation are of two types: stochastic effects and tissue reactions (formerly known as deterministic effects). Stochastic effects occur without a threshold, either occurring or not occurring, with no varying levels of the severity of the effect; one either gets the effect or does not. The primary stochastic effect in radiation safety is cancer induction.

Tissue effects only occur when the radiation dose exceeds a threshold, and then the tissue effect is proportional to the dose absorbed above the threshold, becoming more severe as the absorbed radiation dose increases. Examples of tissue reactions include radiation burns, xerostomia following radiation therapy to the head and neck, and radiation cataractogenesis. These are all effects on somatic cells; to date, there is no evidence of radiation-induced heritable effects in humans.^4,5

Factors Influencing Radiation Risk

Radiation-induced cancer is the only risk from diagnostic dental radiography.^4,5 Several factors influence this risk, including X-ray beam factors (dose, dose rate, field size) and subject factors (tissue type irradiated and age at exposure). Each of these has some relevance to risks from diagnostic radiation.

X-ray Beam Factors. Radiation-induced cancer risk grows with increasing dose.⁴ The dose should be the lowest level needed to make diagnostic images. Optimum settings are based on receptor type, anatomic location, and patient size.¹³ Cancer risk is cumulative⁴ and thus frequency of radiographic examinations is based on diagnostic need.¹ Using guidelines to select patients for radiographic examination is a core safety practice. To also minimize risk, the radiation field is limited to the area of interest and should be no more than the size of the receptor.¹⁴

Subject Factors. In the maxillofacial area, the thyroid gland, red bone marrow (in growing children), and salivary glands are at higher risk for radiation-induced cancer.⁴ Thus, to the extent possible, radiation protection practices try to limit dose to these tissues during imaging. Age at exposure is the strongest modifier of radiation-induced cancer risk. For thyroid cancer, the risks are present when exposed at age 19 and younger with no perceptible risk when exposed as adults.¹⁵ Thus, children and adolescents are a risk group that would benefit the most from radiation protection.

Reducing Patient Dose

Several measures can be implemented to reduce a patient’s radiation dose, such as selection criteria and equipment. Dental professionals should optimize these radiation exposure protocols to ensure diagnostic quality with the least amount of radiation.

Patient selection recommendations and guidelines for dental and maxillofacial imaging are continuously revised by the American Dental Association, US Food and Drug Administration, American Academy of Oral and Maxillofacial Radiology (AAOMR), and National Council on Radiation Protection and Measurements (NCRP).^1,14,16-20 These evidence-based guidelines support oral health professionals in making informed, patient-centered decisions about the type and frequency of radiographic examinations based on the clinical presentation and individual needs. Using professional judgment when applying available guidelines will minimize the radiation exposure and associated risks.

Rectangular collimation is an effective way to limit the beam to the size of the image receptor, reducing the total tissue volume exposed and the patient dose by 60%.⁴ In contrast, a round collimator results in a circular beam with an area up to three times the area of a Size 2 receptor, and four to five times the area of a Size 0 receptor. Rectangular collimation also reduces the dose to radiation-sensitive tissues, such as the parotid and thyroid glands, and improves image quality by reducing scattering to the receptor.^6,7,21,22

The use of digital receptors for intraoral, panoramic, and cephalometric radiography reduces radiation exposure by more than 50% compared to film-based imaging.¹⁴ In most US dental offices, intraoral imaging is performed using direct digital sensors (68%), which offer the highest dose reduction.²³

Shielding of pelvic and abdominal structures was instituted in the early 1950s following recommendations from the International Commission on Radiological Protection.² Since then marked improvements in radiographic equipment have been made with abdominal and pelvic doses of today being negligible. There has been a 95% dose reduction to pelvic organs from these procedures, and the doses from oral and maxillofacial (OMF) imaging are orders of magnitude lower.³

Shielding Recommendations

Given that heritable effects from radiation exposures have been shown to be essentially nonexistent in humans^4,5 and that the abdominal and pelvic doses from OMF imaging come almost entirely from internal scatter off the spine,^6,7 the AAOMR provides the following updated guidelines:

Patient gonadal, fetal, and thyroid “shielding during diagnostic intraoral, panoramic, cephalometric, and CBCT imaging should be discontinued as routine practice.”⁸ Patient gonadal and fetal shielding during diagnostic OMF imaging should be discontinued as routine practice⁸ based on the well-established lack of radiation-induced heritable effects in humans^4,5 and the negligible decrease in gonad radiation dose from shielding.⁶

Radiation exposure to the embryo and fetus has long been a concern of pregnant patients. Cancer induction, as well as growth retardation, microcephaly, intellectual disability, and prenatal death, in irradiated embryo/​fetus has been shown.²⁴ However, the doses that result in such adverse events are several thousand-fold higher than the pelvic doses delivered during OMF imaging.⁶ Therefore, OMF imaging poses no risk of these effects to the embryo or fetus during examinations on pregnant patients.⁸

In summary, lead aprons do not protect against internal scatter, which is the main source of radiation to the gonads and fetus during OMF imaging.^3,6,7 Furthermore, there is no scientific evidence of radiation-induced heritable effects from OMF imaging,^4,5 thus gonadal shielding during OMF imaging is deemed unnecessary.⁸

Thyroid shielding during diagnostic OMF imaging should be discontinued as routine practice.⁸ Previous recommendations for thyroid shielding are based on risks of radiation-induced thyroid cancer at doses of approximately 50 mGy and higher and on the linear no-threshold model, which is the accepted approach to model radiation risks from low doses.^25-27 However, the thyroid dose from a full-mouth intraoral radiographic examination is at least 50-fold lower than the lowest doses associated with thyroid cancer risk.^4,8,26

Although there is evidence to support a higher sensitivity to thyroid cancer induction in children and adolescents relative to adults, the risk when exposed after age 30 is small to none.⁴ Furthermore, the extent of the intraoral radiographic examination strongly influences thyroid dose. Bitewing and periapical radiographs are below detection levels and bitewing imaging is the most frequently used pediatric imaging modality.²⁸

Interestingly, dental radiography accounts for more than 25% of radiographic examinations performed in the US; however, it contributes only 0.04% to the average annual effective dose.²⁹ Thus, the overall population radiation exposure with intraoral radiography has negligible effects on thyroid carcinogenesis.⁸ More important, rectangular collimation decreases thyroid dose by approximately 50%, and is more effective at reducing thyroid dose than thyroid shielding during intraoral imaging.³⁰

For extraoral imaging, thyroid shields could cause artifacts that degrade image quality and negatively affect diagnostic evaluation.⁸ Thyroid gland absorbed doses from imaging with contemporary digital panoramic units are less than 0.1 mGy.^28,31 Radiation doses from CBCT imaging vary; however, thyroid doses from CBCT imaging are within the range of those from intraoral imaging and are markedly lower than doses from head and neck multidetector CT examinations.^14,28,31

The most recent Nationwide Evaluation of X-Ray Trends (NEXT) study indicated that approximately 20% of 380 million intraoral examinations performed in the US were on pediatric patients.²³ Substantial movement from film to digital receptors has resulted in a 40% dose reduction since the prior NEXT survey.²³ Thus, the risk of radiation-induced thyroid cancer from OMF imaging in children, adolescents, and young adults is negligible, assuming digital receptors, rectangular collimation, appropriate acquisition parameters, and selection criteria are used.

Federal, state, and local dental regulations should be revised to remove any actual or implied requirement for routine patient shielding during OMF imaging to align with a scientifically valid change in radiation protection practices.^3,7,8,32 In 2019, the American Association of Physicists in Medicine (AAPM) released a position statement stating that patient and fetal shielding during X-ray based diagnostic imaging should be discontinued as routine practice. It stated that patient shielding may jeopardize the benefits of undergoing imaging and compromise the diagnostic information of the exam due to interference with automatic exposure control and obscuring of anatomic structures by the shield.⁷

The NCRP also recommended ending routine gonadal shielding during abdominal and pelvic radiography.³ Various factors that led to this recommendation include much lower than previously estimated risks of heritable effects due to diagnostic radiography, improvements in imaging technology, interference of gonadal shielding with automatic exposure control, anatomic variations resulting in lack of complete shielding of region, and dose to ovaries predominantly from internal scatter radiation is not prevented by shielding.³

The current recommendations support discontinuing patient shielding during diagnostic intra- and extraoral imaging, including pregnant, apprehensive, and pediatric patients.⁸ Since this contradicts long-standing practices, the importance of training and education of the oral healthcare team in effective communication is heightened. The ultimate goal is to allay patient or parent concerns, as well as increase the oral healthcare team’s confidence in evidence-based best practices. This communication needs to explain the following: patient shielding does not protect the patient from internal scatter,^3,6,7 heritable effects are absent in humans,^4,5 and intraoral radiographs have a negligible effect on thyroid carcinogenesis.⁸

Conclusion

Based on a review of current literature, the AAOMR recommends that patient gonadal, fetal, and thyroid shielding for OMF imaging be discontinued as a routine practice. The guiding principles of dental radiographic prescription are that imaging will likely provide answers to diagnostic questions with minimized radiation dose and that benefits from imaging should vastly outweigh the estimated radiation-associated risks. The most effective approaches to reducing unnecessary radiation exposure are appropriate radiographic prescription through selection criteria, optimal exposure settings, beam collimation, and use of digital receptors.

References

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From Dimensions of Dental Hygiene. June/July 2024; 22(4):36;39-41