Effective and Safe Use of X-Rays: Understanding the Risks for Decision-Making

Sanjay M. Mallya, BDS, MDS, PhD, is an associate professor and the chair of oral and maxillofacial radiology at the University of California, Los Angeles, School of Dentistry. He has authored numerous scientific manuscripts and book chapters on oral radiology, periodontology, endodontology and implantology. He is the editor of “White and Pharoah’s Oral Radiology” and is a past president of the American Academy of Oral and Maxillofacial Radiology. Dr. Mallya is a diplomate of the American Board of Oral and Maxillofacial Radiology and serves on the board for the Intersocietal Accreditation Commission. Conflict of Interest Disclosure: None reported.

ABSTRACT:

Background: X-radiation is part of the dentist’s diagnostic armamentarium and is used to make radiologic images for diagnosis and treatment planning. However, the hazards of X-radiation — in particular, its potential to induce cancer — have concerned health care providers and patients. Application of diagnostic imaging is based on the premise that the benefits will vastly outweigh the risks.

Results: The first part of this article discusses increasing imaging effectiveness with proper patient selection — a critical and frequently overlooked step toward maximizing diagnostic benefit. The second part provides a basic understanding of the risks from diagnostic X-radiation, including typical doses from maxillofacial radiologic procedures and the magnitude of associated radiation-related risks.

Practical applications: This article provides approaches for the dentist to communicate radiation risks to their patients in lay terms. It also highlights recent developments of importance to dental practice — developing changes to practices of gonadal and fetal shielding during diagnostic imaging.

Key words: Evidence-based imaging, radiation risk, radiation safety

X-rays have sufficient energy to ionize biologic molecules and possibly cause damage that is manifested as disease. Nevertheless, despite its potentially hazardous effects, X-ray-based imaging continues to be an essential tool for diagnosis and treatment planning in health care, including dentistry. For example, asymptomatic patients are screened to detect incipient caries lesions using bitewing radiography. Alternatively, a patient with a periapical abscess may be imaged using intraoral periapical imaging. In another clinical scenario, cone beam computed tomography (CBCT) may be prescribed to evaluate the relationship between an impacted mandibular third molar and the inferior alveolar canal. Although the specific diagnostic objective in each scenario is unique, the principles that guide the decision to make radiologic images are the same. The first principle addresses the effectiveness of imaging and is based on the tenet that radiologic examination will likely provide information relevant to diagnosis and treatment planning.

Guided by this principle, the dentist must design an appropriate radiologic examination to yield high diagnostic benefit. The second principle addresses the safety of the radiologic examination and ensures that radiation-associated risks will be negligible relative to the diagnostic benefit. To appropriately apply these principles in practice, dentists must understand the advantages and limitations of the imaging modalities used and the nature and magnitude of radiationassociated risks. When appropriately applied, dentists create a positive balance where the anticipated benefits vastly outweigh the estimated risks and thereby frame the logical basis for effective and safe use of X-rays in dentistry.

Radiologic imaging is one component of the comprehensive process of diagnosis and treatment planning. Prior to prescribing radiologic imaging, the clinician must first establish a provisional diagnosis based on the medical and dental history, clinical presentations, findings from the visual and physical examinations and other diagnostic tests. These findings should guide the clinician to develop distinct diagnostic objectives that will be addressed by imaging and to design the appropriate radiological examination to achieve these objectives.

Effectiveness of imaging refers to the likelihood that radiologic examination will accomplish the objectives of imaging. In practice, the clinical team has an important role to maximize imaging efficacy in their individual clinic environments. This is accomplished by customizing radiologic examinations to maximize diagnostic yield in individual patients and optimizing the clinic’s imaging and interpretation procedures.

The first facet of imaging effectiveness is to custom design radiologic examinations to meet the diagnostic needs of individual patients. This requires familiarity with the performance efficacy of different imaging procedures. To facilitate application of data-driven imaging in practice, professional societies have developed guidelines and position statements that address selection of patients for intraoral, panoramic, cephalometric [1] and CBCT imaging [2–7] (TABLE 1). These guidelines provide evidence-based recommendations for distinct diagnostic objectives. Application of patient-selection guidelines in practice will allow dentists to maximize the diagnostic yield, thereby enhancing imaging effectiveness.

Imaging effectiveness, also termed efficacy, is expressed using the measures sensitivity and specificity. [8] Sensitivity refers to the ability of a diagnostic test to correctly identify presence of disease. Specificity refers to a diagnostic test’s ability to identify absence of disease. It is important to recognize that sensitivity and specificity are measures of a distinct diagnostic task. For example, although both tasks address caries detection with bitewing imaging, the sensitivity and specificity for detection of noncavitated caries lesions on proximal surfaces of posterior teeth differs from the sensitivity and specificity of detection of cavitated caries lesions on proximal surfaces of posterior teeth.

Note that sensitivity is a measure of how well your test can detect disease when it is present. This influences your selection of the imaging method you apply for disease detection — for example, in the case of proximal caries detection, we rely on bitewing radiographs and not panoramic radiographs. In practice, the chairside performance of the bitewing radiograph is reflected in its predictive value — the reliability of a positive (or negative) test result. The positive predictive value (PPV) is the fraction of positive tests that are truly positive. The negative predictive value (NPV) is the fraction of negative tests that are truly negative. Although sensitivity and specificity measure a diagnostic test’s performance, they are not of immediate relevance to clinical decision-making as described in BOX 1.

A practical example of appropriate application of imaging is illustrated with an example decision of how frequently to image an asymptomatic recall patient (BOX 1). The diagnostic objective of this screening radiograph is to detect caries before it becomes clinically apparent. In practice, how frequently should dentists apply this screening to their patients? To address this issue, the application of bitewing imaging is simulated for two scenarios that vary in caries prevalence — 50% versus 5%. Both scenarios use bitewing radiography, so radiation-associated risks to patients are equivalent. The major difference between the two scenarios is reflected in their positive predictive value — in scenario one, a dentist scoring presence of caries would be correct 86% of the time. In stark contrast, this value drops sharply to 25% in scenario two and would result in increased misdiagnosis and potential unnecessary treatment. It may seem counterintuitive that using the same technology with the same interpretation criteria will yield different results when applied to different populations. However, this principle is the key element to maximizing diagnostic yield — application of the same diagnostic test in different patient populations yields different benefits. Thus, the benefit-torisk ratio in scenario one is substantially higher than in scenario two. This difference in the benefit-to-risk ratio is reflected in the recommended screening radiography schedules; 1 in patients with high caries risk, the schedules are more frequent, every six to 18 months depending on risk. In contrast, for patients with no caries risk, where the benefits are lower, the recommended schedule is every 24 to 36 months. [1] Note that by applying appropriate patient selection guidelines, dentists can make major changes in the effectiveness of their imaging results. A recent study in the Journal of the American Dental Association showed that less than 50% of dentists apply evidence-based imaging guidelines in their practices with resultant overprescription of imaging. [9] The reasons for such practices are likely multiple and may derive from financial benefits, lack of guidelines knowledge, easy availability of in-office CBCT imaging and misperceptions of medicolegal risk and underscore the need to better promote evidence-based imaging. The most recent report from the Nationwide Evaluation of X-ray Trends (NEXT) estimated almost 400 million intraoral radiographs and approximately 5.2 million CBCT examinations are done every year in the U.S. [10] Efforts to decrease unnecessary imaging will not only reduce radiation dose, but also impact dental treatment costs.

The second facet of imaging effectiveness is making and interpreting diagnostically acceptable radiologic images. To this end, the dental team has four broad roles as summarized next. For current practical relevance, the discussion is limited to digital imaging technologies.

Know the technical capabilities of the imaging system:

In-office imaging modalities used in dentistry include intraoral, panoramic, cephalometric and CBCT imaging. Dentists must be familiar with the basic technical specifications of the imaging systems used in their offices. This includes both the hardware and software components. Importantly, dentists must understand the advantages and limitations of different imaging approaches so they can design appropriate imaging examinations to meet specific diagnostic needs. Most vendors of digital intraoral sensors provide software to manipulate the display of the radiologic image — for example, brightness and contrast adjustments, image sharpness, etc. Understanding the appropriate application of such tools will allow the dentist to better use information for these images.

Optimize exposure factors to make high-quality diagnostic images:

An essential and critical step in maximizing imaging effectiveness is to optimize imaging protocols to make diagnostically acceptable images with optimal density, contrast and resolution that are adequate for the diagnostic task. For example, when a practice buys a new digital sensor, they should make phantom images to select the lowest exposure time to make diagnostic images and establish a technique-setting chart that is followed by all office staff. Each imaging system must be optimized to define exposure settings, which is the focus of a separate manuscript in this issue.

Optimize the viewing environment:

The image viewing display and room lighting conditions are practical factors that influence the quality of the image presented to the dentist for interpretation. The diagnostic assessments may be made on a display monitor, a laptop computer or a tablet. Standard test patterns are available to aid the dentist in adjusting display parameters to confirm that the required latitude and contrast are perceptible. [11]

Systematically interpret the image:

The dentist’s ability to extract information from the image is central to achieving high effectiveness of imaging. This requires that dentists maintain and update their radiologic interpretive skills. This is especially important when adopting newer imaging technologies such as CBCT, where foundational knowledge in CT technology and 3D anatomy are necessary for interpretation. As needed, dentists should seek appropriate consultative specialty opinion to maximize the diagnostic information obtained from the radiologic examination. The use of teleradiology has facilitated easy access to this consultative service.

Broadly, the phrase “safe use of X-rays” refers to approaches taken by the dental team take to minimize radiation dose to their patients, themselves and the public. This manuscript focuses on radiation-associated risks from dentomaxillofacial diagnostic imaging and presents the dentist with data to better understand the magnitude of these risks.

Radiation effects can be categorized as stochastic effects and tissue reactions. Each category has different implications for radiation safety, and aspects relevant to dental practice are discussed below.

Stochastic Effects:

Stochastic effects occur due to mutations consequent to misrepair of radiation-induced DNA damage. One stochastic outcome of radiation exposure is cancer resulting from mutations occurring in somatic cells. Another stochastic outcome is hereditary effects resulting from germ cell mutations following gonadal radiation exposure and manifested as disease in the exposed individual’s offspring. As discussed below, hereditary effects are not a concern from dental diagnostic imaging, and cancer induction is the only radiation-associated risk from dentomaxillofacial diagnostic imaging and the magnitude of these risks are low.

Radiation-induced cancer has been scientifically established by animal studies as well as studies of several human populations that were exposed to radiation. The Life Span Study, which monitors radiation effects in atomic bomb survivors, has provided considerable information on radiation’s carcinogenic effects. Overall, these studies have shaped important concepts of relevance to radiation safety and protection. [12]

■ Cancer induction has been demonstrated at doses above 100 mSv, and risk modeling demonstrates a linear dose response. However, the carcinogen effects of radiation are unclear at doses below 100 mSv — the dose range of all diagnostic radiologic procedures. Although radiation doses for stochastic effects are considered cumulative, the relatively low magnitude of dental exposures would require thousands to tens of thousands of dental exposures to total 100 mSv. The current model for risk prediction at lower doses is based on linear extrapolation of higher-dose data. This model, the linear no-threshold (LNT) model, considers that there is no threshold dose for cancer induction and implies that even the smallest amount of X-radiation carries a risk for cancer induction. The no-threshold model guides current radiation protection practices. [13] In the absence of scientific data to support or refute the LNT model, radiation protection agencies worldwide have adopted a prudent approach to low-dose risk estimation, and current protection efforts are based on the premise that there is no threshold dose for this adverse outcome.

■ Opponents of the LNT model argue that the presence of repair mechanisms and adaptive responses contributes to lower risk at low doses than is predicted by the LNT model, with some groups arguing the presence of a threshold dose for cancer induction. [14] These groups argue that a conservative approach could compromise application of diagnostic imaging and that the increased protection efforts add unnecessary costs. Nevertheless, current federal and state radiation protection regulations are based on the LNT model’s risk prediction, and this issue continues to be intensely debated.

■ Certain organs and tissues demonstrate increased sensitivity to cancer induction by radiation. In the head and neck, these include the thyroid gland, salivary glands, red bone marrow and the brain. Minimizing dose to these sensitive organs during imaging will decrease radiation-associated risks. The acronym ALARA (as low as reasonably achievable) provides the guiding principle for dose reduction efforts. Approaches to dose reduction are listed in BOX 2.

■ Children are more sensitive to radiation-induced cancer, emphasizing the need to specifically enhance risk reduction efforts in children.

■ Cancer is manifested years to decades following radiation exposure. Thus, an exposed child has a higher likelihood of manifesting radiation-induced cancer during their lifetime in contrast with an elderly exposed individual. This emphasizes the need to specifically enhance riskreduction efforts in children.

Radiation-induced thyroid cancer is of particular relevance to dentomaxillofacial imaging. The thyroid gland is sensitive to cancer induction by radiation, especially before age 20. [15] Cancer risks decrease sharply when exposed at ages older than 20 years, and there have been no demonstrated risks in adults exposed at an age older than 40 years. The thyroid gland is exposed either directly by the primary beam or by scatter radiation from adjacent exposed areas. Thyroid shields can minimize the dose and should be used when they will not obscure necessary anatomic details. The NCRP Report No. 177 [16] recommends that “Thyroid shielding shall be provided for patients when it will not interfere with the examination.” This recommendation is also the position of the American Thyroid Association [17] and the American Dental Association. 1 BOX 3 summarizes the scientific data on radiation-induced thyroid cancer that supports use of thyroid shielding during dentomaxillofacial imaging.

Heritable effects result from germ cell mutations that may manifest as disease in the exposed individual’s offspring. Although demonstrated in animal studies, there is no evidence of radiationinduced heritable disease in humans. [18] Thus, heritable risks are practically nonexistent with diagnostic imaging. However, the U.S. federal regulations and several U.S. state radiation regulations require use of gonadal shielding when imaging patients. Current California state regulations require that “Each patient undergoing dental radiography shall be draped with a protective apron of not less than 0.25 mm lead equivalent to cover the gonadal area” (17 CCR § 30311).

This practice was recently questioned by the American Association of Physicists in Medicine (AAPM) position statement advocating for discontinuance of routine use of gonadal and fetal shielding [19] and has been endorsed by several professional organizations, including the American College of Radiology, [20] the Radiological Society of North America and the Image Gently and Image Wisely Radiation safety campaigns. This changing trend in radiation protection practice has considerable implications for dentistry. The majority of X-ray examinations are done in dental offices — and this positions dentistry as a key player in this trend. As these practices evolve in health care, dentists must become informed providers so that they understand the rationale for these changes and can communicate them to patients who may be confused by changing practices. BOX 4 provides relevant data and information to this end.

The unit effective dose is used to compare risks from different diagnostic imaging protocols. Effective dose is the sum of the doses to specific at-risk tissues, weighted for their relative stochastic risk. This unit considers the tissues and organs exposed and provides a summation of the estimated stochastic risks weighted based on exposed tissue sensitivities. Because cancer induction is the only risk from diagnostic radiation exposure, an effective dose provides an estimate of the cancer risks. However, there are limitations to a very rigid interpretation of cancer risks from an effective dose. First, published effective doses (TABLE 2) are broad estimates and individual patient doses will vary based on protocol and patient size. Second, effective dose does not consider age- and gender-associated variations in risks. Nevertheless, this unit provides a convenient measure to compare risks from radiation. TABLE 2 lists typical effective doses from common dentomaxillofacial imaging procedures and provides an approach for dentists to understand the risks and convey them to patients.

Deterministic effects, also termed tissue reactions, occur due to radiation-induced cell killing. Diagnostic X-rays cause negligible cell killing and do not produce clinically evident effects. At higher doses, radiation-induced cell death impairs tissue or organ function and manifests as a detectable effect. The threshold dose, the minimum dose to induce a demonstrable effect, varies with the tissue and effect. When the threshold dose is exceeded, the likelihood of adverse manifestation is high. Moreover, the severity of the effect increases with dose. Deterministic radiation effects are of practical relevance to dentistry — patients with head and neck tumors are managed by radiation therapy, where the high doses cause clinically evident effects. These effects include mucositis, atrophy of the skin and oral mucosa, fibrosis and xerostomia resulting from permanent damage to the salivary gland acini. Sunburn is a classic example of a deterministic effect — it manifests only after a certain amount of sunlight (specifically ultraviolet) exposure and the severity of the burn increases with increasing sunlight exposure. By limiting sunlight exposure, we can prevent the occurrence of sunburn. The same principle is applied in diagnostic imaging. Radiation doses from diagnostic dentomaxillofacial radiologic examinations are several thousandfold lower than the threshold doses for radiation-induced deterministic effects. Thus, the risk of deterministic effects from dentomaxillofacial radiologic imaging is zero.

Cataract induction is a deterministic effect of practical relevance to dental imaging. Cataract induction — lens opacification with visual impairment — is a scientifically established radiation effect. Recent revaluation of the cataract-inducing effects of radiation estimates the threshold dose at approximately 0.5 Gy to 1 Gy, lower than the previously estimated 2 Gy. Considering these new developments, it has been proposed that lead glasses may provide protection during dentomaxillofacial imaging. Depending on the imaging procedure and anatomic coverage, the lens dose from dentomaxillofacial imaging ranges from 0.02 mGy to 0.4 mGy, which is 1,200-fold lower than the threshold dose. Thus, the use of lead glasses is not practically necessary.

Deterministic effects, also termed tissue reactions, occur due to radiation-induced cell killing. Diagnostic X-rays cause negligible cell killing and do not produce clinically evident effects. At higher doses, radiation-induced cell death impairs tissue or organ function and manifests as a detectable effect. The threshold dose, the minimum dose to induce a demonstrable effect, varies with the tissue and effect. When the threshold dose is exceeded, the likelihood of adverse manifestation is high. Moreover, the severity of the effect increases with dose. Deterministic radiation effects are of practical relevance to dentistry — patients with head and neck tumors are managed by radiation therapy, where the high doses cause clinically evident effects. These effects include mucositis, atrophy of the skin and oral mucosa, fibrosis and xerostomia resulting from permanent damage to the salivary gland acini. Sunburn is a classic example of a deterministic effect — it manifests only after a certain amount of sunlight (specifically ultraviolet) exposure and the severity of the burn increases with increasing sunlight exposure. By limiting sunlight exposure, we can prevent the occurrence of sunburn. The same principle is applied in diagnostic imaging. Radiation doses from diagnostic dentomaxillofacial radiologic examinations are several thousandfold lower than the threshold doses for radiation-induced deterministic effects. Thus, the risk of deterministic effects from dentomaxillofacial radiologic imaging is zero. Cataract induction is a deterministic effect of practical relevance to dental imaging.

Cataract induction — lens opacification with visual impairment­ — is a scientifically established radiation effect. Recent revaluation of the cataract-inducing effects of radiation estimates the threshold dose at approximately 0.5 Gy to 1 Gy, lower than the previously estimated 2 Gy. Considering these new developments, it has been proposed that lead glasses may provide protection during dentomaxillofacial imaging. Depending on the imaging procedure and anatomic coverage, the lens dose from dentomaxillofacial imaging ranges from 0.02 mGy to 0.4 mGy, which is 1,200-fold lower than the threshold dose. Thus, the use of lead glasses is not practically necessary.

Deterministic effects on the embryo and fetus require additional consideration when imaging a pregnant patient. Animal studies have demonstrated that the preimplantation embryo is highly sensitive to killing by radiation. In humans, embryonic death is unlikely at doses less than 100 mGy.[21] Studies of atomic bomb survivors irradiated in utero have identified microcephaly and mental retardation as radiation-associated effects.[22–25] These effects occur when in utero radiation occurs before 15 weeks, and the threshold dose for causation is approximately 0.3 Gy. Notably, these doses are several thousandfold lower than estimated fetal doses from dentomaxillofacial imaging. BOX 5 provides data that allow the dentist to evaluate the risks to the embryo and fetus from radiologic imaging.

When prescribing radiologic imaging, dentists must ensure that the anticipated benefits of imaging will outweigh the radiation-associated risks of cancer induction. Doses delivered by dentomaxillofacial imaging are very low, and dentists who practice appropriate patient selection can very easily tip the scale toward this benefit.

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THE AUTHOR, Sanjay M. Mallya, BDS, MDS, PhD, can be reached at smallya@dentistry.ucla.edu.