Over the past 25 years, medical advances in the use of ionizing radiation have allowed faster and more accurate diagnosis and treatment of patients. During that time, there has been a dramatic increase in the use of computed tomography (CT) and nuclear medicine examinations in the United States. In 1980 there were approximately 3 million CT scans performed. In 2005 the number had grown to 63 million. (1) Every time a patient is exposed to radiation in a CT scan, an x-ray, or a nuclear medicine test or treatment, there is a cumulative health risk to that patient. (1) Medical staff conducting these tests must also monitor their own radiation exposure over time. "There is no dose below which radiation induced injury is absent." (2) Both a recent study by the Center for Radiological Research at Columbia University Medical Center and the 2007 American College of Radiology White Paper on Radiation Dose in Medicine have raised concerns that the increased use of medical tests employing radiation could foster a future public health issue. (1, 3)
Radiation risks
"Ionizing radiation, especially at high doses, has long been known to increase the risk for developing cancer. In fact, x-rays have recently been officially classified as a 'carcinogen' by the World Health Organization's International Agency for Research on Cancer, the Agency for Toxic Substances and Disease Registry of the Centers for Disease Control and Prevention, and the National Institute of Environmental Health Sciences." (1) While it is commonly accepted that the benefits to patients far outweigh the risks, there is current debate about whether the widespread use of x-rays, CT scans, and nuclear medicine could result in increased incidence of cancers in the population. Cancers resulting from radiation may appear decades after exposure.
Currently, there is no data detailing what dose levels of radiation may cause an increase in cancer risk. The most thorough study available remains that of survivors of the atomic bombs in Japan. The study shows that persons who received dose estimates greater than 50 mSv (millisieverts) showed a "significant increase in cancer." (1) Whether lower doses of radiation cause increased cancer risk is still debatable. However, "the International Commission on Radiological Protection has reported that CT doses can indeed approach or exceed levels that have been shown to result in an increase in cancer." (1) Consider that "over a lifetime many individuals will have multiple CT scans for marginal indications, resulting in significant radiation exposure." (1)
According to a controversial study published in the New England Journal of Medicine, based on scans administered today, 1.5 to 2 percent of all cancers could be ascribed to CT scans in the next 10-20 years. (3) Although this study has been criticized, the American College of Radiology (ACR) embraced the article's suggested ways to reduce radiation exposure in patients. These include:
- use imaging choices other than CT;
- use a lower CT dose per patient; and
- prescribe fewer CT studies. (1)
Measuring radiation exposure in patients
Every day people are exposed to low levels of naturally-occurring background radiation, including cosmic radiation and terrestrial radiation. "According to the U.S. Nuclear Regulatory Commission, the average American receives an annual radiation exposure of 3.6 mSv (millisieverts) from all sources assuming no unusual exposures from medical procedures." (4)
The ACR has adopted the term "dose estimate" rather than "dose" to communicate quantitative radiation dose values. Radiation dose estimates are measured as follows.
- Absorbed dose (grays or Gy) is the physical quantity describing energy deposited per unit mass and is the exposure x conversion factor.
- Equivalent dose is measured in sieverts or Sv and is the dose x a quality factor.
- Effective dose is measured in millisievert (mSv), sievert, rad, rem, and roentgen. (4)
The effective dose estimate approximates the dose averaged over the whole body. It considers both organ sensitivity and non-homogenous exposure. It is currently considered to be the best measure of radiation exposure. (4)
While "most radiological examinations produce doses in the range from 3 to 30 mSv," (5) it is difficult to accurately measure the radiation dose a patient may receive during CT scans or nuclear medicine tests. For example, a number of factors are involved when considering the organ radiation dose absorbed by a patient:
- number of scans performed;
- patient's size;
- product of tube current and scan time;
- axial scan range;
- scan pitch;
- maximum tube voltage; and
- scanner design. (6)
Image quality can be compromised when using lower doses of radiation during a scan. As the radiation dose decreases, image "noise" increases, so picture clarity can be affected. (6) While digital x-ray systems or computed tomography may record the dose values delivered by the equipment, it is difficult to measure the amount of radiation absorbed by the body as this will vary by patient and by the organ(s) affected. As stated in the British Journal of Radiology, "typical doses to the lung from a conventional chest x-ray range from about 0.01 mGy to 0.15 mGy, whereas a typical dose to an organ examined with CT, . . . is around 10 mGy to 20 mGy, and can be as high as 80 mGy for 64-slice CT coronary angiography." (6)
As the discussion of scan safety continues, manufacturers of scanning devices are working to develop scanners that deliver lower doses of radiation and that record more information about patient dose. However, these may not be available for 10 years or more. (1, 3) The U.S. Food and Drug Administration is charged with regulating the manufacturers of radiation emitting equipment, but they have no authority over the users of the devices. (1) As the use of CT scans continues to increase, so must concern for the safety of patients undergoing these scans, especially multiple scans during a patient's lifetime.
Reducing exposure in the medical setting
"Radiation exposure follows the inverse square law which makes it dependant on both time and distance from the source." (2) Exposure of medical staff can be managed by ensuring that they follow their organization's established protocols when conducting tests involving radiation. These may include minimizing time spent near the radiation source, using supplied radiation protections such as lead aprons, and generally staying as far from the radiation source as possible. (2)
The single most common source of medical procedure radiation dosing comes from x-ray exams. A chest x-ray may deliver a radiation dose estimate of 0.02 mSv. This approximates the natural-source radiation a person might normally receive over a two-day time period. Improving communication can support improved safety for the patient by reducing the number of x-ray exposures. (4) Simply asking a patient if x-rays were taken at another facility or by another physician can save the patient exposure to additional radiation as well as save the cost of the test. Brigham and Women's Hospital in Boston has taken the step of modifying its electronic medical records system to include how frequently a patient may have received radiation, thus providing physicians with an idea of the patient's cumulative dose. (3)
Clear communication and observing safety precautions are especially important when working with special populations such as pregnant women, women of child-bearing age, and children. Mutations occurring in germ cells as a result of radiation from a medical test can be passed along to prospective generations, so women of child-bearing age (ages 11-55) should be asked if they could be pregnant before any irradiation proceeds. Appropriate shielding should be provided to all patients.
If the patient is a child, nuclear medicine procedures such as those that are used to diagnose disease or dysfunction in the kidney, bladder, or bone might require sedation. Increasingly, physicians are opting for CT scans for young children because the scan can be completed quickly and prevent the need for anesthesia to keep the child from moving during the imaging process. Children undergoing CT scans receive a higher radiation dose than adults because of their smaller size and "because dose is inversely proportional to the patient's diameter." (4) Since they have a longer life expectancy, these children could face a greater risk for cancer mortality in the future. (4)
Imaging by nonradiologists
More nonradiologists — cardiologists, orthopedists, neurologists — are purchasing their own imaging equipment and employing technicians to conduct scans. These specialists who own their own equipment may interpret their own scans or send them to radiologists to read. Some are aware of the issues surrounding radiation safety, but others may have had little training in this area. While the increased use of CTs has kept radiologists busy interpreting scan results, some in the radiology community question whether physician self-referrals may present "a conflict of interest that is driving up health care costs and exposing some patients to unnecessary radiation." (7)
There is also the question of regulation. Currently, there is controversy between radiologists and other specialists who provide imaging services over whether these in-office services should be regulated. The American Medical Association has gone on record "against any effort to ban or discourage the delivery of imaging services in physician offices or that sought to repeal the 'in-office ancillary services exception' under the federal anti-kickback law; this exception allows doctors to provide imaging services in their own offices. The ACR opposed the adoption of both of these resolutions." (8)
According to a recent article in the Austin American-Statesman, the Texas Radiological Society (TRS) will ask the 2009 Texas legislature to gather information on what the organization sees as a growing utilization problem. They are asking the legislature to study who owns imaging equipment in Texas and then consider regulating who can conduct and read the scans. Currently in Texas, accreditation is not required in order to perform scans or read tests. (7) The TRS will also seek to reintroduce a reporting and disclosure bill that was introduced in 2007, but failed to pass. (9) Dr. Josie Williams, president of the Texas Medical Association, has said "a majority of Texas Medical Association members do not favor any legislation." (7) The association's position on self-referral is that "doctors should tell patients of their ownership in scanners and facilities and make referrals only when medically necessary." (7)
Education and patient safety
"When it comes to CT radiation dose, radiologists and referring physicians (specifically emergency department physicians) who request large numbers of CT studies are unaware of the radiation dose delivered during a CT scan and its possible risks. Equally important, the issue of radiation dose remains largely undiscussed with patients." (4) Education may help solve these problems.
Recommendations from the ACR Blue Ribbon Panel on Radiation Dose in Medicine state that raising awareness of radiation exposure issues should occur during medical training. Medical schools could help better educate students about radiation safety by including this information in the curriculum and requiring its study or offering radiology clerkships to students. Additionally, "the issue of radiation exposure associated with diagnostic imaging should be reinforced for nonradiology residents during their postgraduate years" by educating them about the associated risks of imaging studies as well as the "appropriate clinical indications for imaging." (2) To that end, the ACR has developed "Appropriateness Criteria" to help in decision-making at all levels of training. This information is available on the ACR web site.
Emergency departments are heavy users of CT referrals. According to an article in the Annals of Emergency Medicine, "about 9% of all emergency department visits include a CT scan. CT scan use would presumably be highest for complaints such as abdominal pain, headache, chest pain, or major trauma." (10) However, physicians working in emergency situations do not usually have access to the patient's medical history. They are also working quickly to diagnose a patient who is acutely ill or who has experienced trauma. Offering educational resources to emergency departments concerning the "over utilization of medical imaging" and its risks to patients may not achieve effective results. The ACR recommends a meeting of radiology and emergency medicine leadership to jointly develop consensus guidelines. If this collaboration proves to be successful, then this process could be repeated with other specialties. (1)
Risk management considerations
Diagnostic imaging offers great benefit to physicians and patients. Keeping patients safe as they undergo diagnostic tests or treatment is vital. Most patients are unaware of the potential dangers of cumulative doses of radiation delivered through x-rays, CT scans, or nuclear medicine tests and treatments. Physicians and medical personnel who deliver imaging services should try to determine the patient's imaging history. The patient may be able to provide this information or the patient's health insurer may have a record of imaging services. Patients should be provided with information about the potential risk of multiple radiation exposures so that they can keep track of their own exposure.
In an effort to help educate parents who have children scheduled to undergo an imaging procedure involving radiation, the Alliance for Radiation Safety in Pediatric Imaging has introduced the "Image Gently" campaign. Campaign materials include information for parents about their child's procedure and a tracking record so parents can track their child's medical procedure radiation exposure. For more information, please visit the Image Gently web site. Awareness of the potential dangers of cumulative radiation exposure and continuing education in radiation safety for all specialties may also help improve patient safety.
Sources
- Amis ES Jr, Butler PR, Applegate KE, et al. American College of Radiology White Paper on Radiation Dose in Medicine. J Am Coll Radiol. 2007; 4:272-84.
- Ontario Association of Medical Radiation Technologists. AMRT professional practice FAQs - radiation safety. April 9, 2008. Available at www.oamrt.on.ca/profprac/profpracpamqap.shtml. Accessed December 30, 2008.
- Brenner DJ, Hall EJ. Computed tomography—an increasing source of radiation exposure. N Engl J Med. 2007;357:2277-84. Available at www.nejm.org. Accessed January 13, 2009.
- Weiner S. Is medical radiation exposure a public health concern? AHIP Coverage. 2005; 46:52-5.
- Brenner DJ, et al. Cancer risks attributable to low doses of ionizing radiation: Assessing what we really know. Proceedings of the National Academy of Sciences (PNAS). 2003 November; 100(24):13761-66.
- Hall E, Brenner D. Cancer risks from diagnostic radiology. Br J Radiol. 2008 May; 81(965):362-78.
- Roser MA. Radiologists see overuse of patient scans; specialists see turf battle. Austin American-Statesman. January 8, 2009. Available at www.statesman.com/news/content/news/stories/local/01/08/0108radiology.html?cxtype=rss&cxsvc=7&cxcat=52 January 8, 2009. Accessed January 8, 2009.
- Iglehart JK. The new era of medical imaging—progress and pitfalls. N Engl J Med. 2006; 354;26:2822-28.
- Kirk IR.Three issues deserve your TRS PAC support now. The Texas Radiological Society View Box. Available at www.txrad.org/viewbox.html. Accessed February 17, 2009.
- Peter DJ. Radiation risk associated with computed tomography and high frequency emergency department users. Ann Emerg Med. 2007 Jan;49(1):112-3.
Dana Leidig can be reached at
dana-leidig@tmlt.org.
