What Are the Cancer Risks from Dental Computed Tomography?

Medical and dental radiography are common and modifiable risk factors for adverse health outcomes that include cancer, mortality, and adverse pregnancy outcomes. Currently, 50% of all the radiation exposures for US residents are due to medical X-rays (National Council on Radiation Protection and Measurements et al. 2009). This proportion continues to increase (Smith-Bindman et al. 2012) and maybe unnecessarily so. For instance, 3 professional organizations recommended against head computed tomography (CT) for children with headaches. Yet, 1 of 4 children between 2007 and 2008 had at least 1 CT when reporting headaches (Devries et al. 2013).

In this issue of the Journal of Dental Research, Wu et al. (2015) estimated that the CT cancer risks in an adult population of 30 y and older ranged from 1 in 20,000 to less than 1 in 100,000 depending on age and sex. These cancer risks were low because of the use of low doses, low image resolution, a narrow field of view, and adult patients (>30 y) but can be substantially higher. For instance, a head CT in girls younger than 5 y can lead to a 10- to 50-fold larger cancer risk (Miglioretti et al. 2013).

Such morbidity and mortality risks were often discussed in a “theoretical context.” From 1919 to 1956, the possibility that diagnostic X-ray could cause harm was largely unappreciated. “Safe from radiation” in advertisements from this era referred to keeping unexposed dental film safe from the radiation, not the patient. Then, as the epidemiological evidence grew that small radiation doses caused harm, the information became often dismissed. Up until 2013, the Health Physics Society published on their “Ask the Experts” website in an answer to a concerned patient that 3 full sets of X-rays in a week do not produce “any ill effects whatsoever” and that “these doses may be beneficial.” Such disregard for safety and for the precautionary principle remained in place even though the influential BEIR VII report (Hirschmann 1995; United Nations Scientific Committee 2000; Valentin 2005; Valentin and International Commission on Radiological Protection 2005; National Research Council 2006; National Council on Radiation Protection and Measurements et al. 2009) had joined the international community of scientists in reporting that small doses do increase cancer risk and that suggestions of beneficial effects were unwarranted.

Ninety years ignoring the harms of small doses of X-rays through advertising and education cannot be undone in a few decades. D-speed radiographic film, which requires higher doses (longer exposure times), continues to be highly prevalent in dental practices, despite the fact that guidelines have advised against its use for 3 decades now (Council on Dental Materials, Instruments, and Equipment 1989). Thyroid collars, which can be used to protect one of the most radiosensitive organs in the body, were reported to be used rarely (Hujoel et al. 2006). One of the “world’s foremost experts” on CT compared the dose received from a CT scan to an airport scan (Bogdanich and McGinty 2010), which is “very wrong—by a lot.” Some orthodontists are reported to use a CT instead of digital pictures for convenience and cost reasons (Bogdanich and McGinty 2010). These examples indicate that efforts on the part of organizations such as the American Dental Association, the American Academy of Oral and Maxillofacial Radiology, and the American Academy of Pediatric Dentistry to always use X-ray doses that are “as low as reasonably achievable” (ALARA) are insufficient to guide practice (United Nations Scientific Committee 2000; National Council on Radiation Protection and Measurements 2001; National Research Council 2006).

The strong epidemiological evidence of harm of small radiation doses contrasts starkly with the weak evidence of benefits promoted in commercial advertising. George Eastman—whose company manufactured dental X-ray film—was a strong believer in marketing, and in 1919, his company started advertising the preventive benefits of X-rays on a monthly basis in the Journal of the American Dental Association. Bitewings were advertised as “one of the greatest aids in preventive dentistry,” as “the only explorer for 60 of the dental surfaces,” and as the ideal tool to “banish the pulpless tooth” and to “avoid tooth-ache.” Such marketing slogans often flop when put to test in comparative studies. Preliminary evidence suggests the only benefit of bitewings may be an increased incidence of unnecessary fillings (Baelum et al. 2012). While the sophistication and aggressiveness of marketing have increased exponentially since 1919, the conduct of comparative studies on the hypothesized benefits of ionizing radiation has not.

Good evidence on potential benefits of CT is most sorely needed when infants, children, and young adults become involved (Aps 2013a, 2013b). It is ironic that the most vulnerable population in terms of cancer risk may have the highest prevalence of X-ray exposures and the weakest evidence in terms of demonstrated benefits. Everything seems to go against them, with even the exposure parameters often set incorrectly (Food and Drug Administration 2001; Paterson et al. 2001). A recent nonscientific survey suggested that 75% of the orthodontic training programs in the United States take a CT on some of the orthodontic patients and that 1 in 10 programs take a CT on all patients (Bentson Clark & Copple 2013). This is alarming, given that 2 million US children undergo orthodontic treatment each year (Bollen et al. 2007) and that we are still waiting for the first randomized study to suggest that CT delivers a nicer smile or any tangible patient outcome that would justify the use of ionizing radiation. Add the possible use of CT for wisdom teeth extraction, caries screening, a bump on the head, or a migraine headache, and it becomes understandable why cancer risks induced by ionizing radiation may have become unacceptably high for the young.

Reversing the trend of ever increasing radiation doses has proven to be challenging in medicine and may be equally so in dentistry. One could call for randomized trials to provide an evidence-based approach to diagnostics. One could support initiatives such as the “Image Gently Image Wisely” by the American College of Radiologists and suggest adherence to ALARA guidelines. One could call for informed consent on cancer risks. One could attempt to lobby professional organizations to broadcast more explicitly a proper standard of care. One could call for adherence to the Food and Drug Administration guideline that parents should keep a record of the radiation doses their child receives. Evidence that such efforts have had an impact, however, remains largely lacking. Legislation may be the only solution to keep medical and dental diagnostic radiation within acceptable limits.