CME: Radiation and Health Risk

ABSTRACT: Physicians must become more aware of the cumulative effects of radiation from multiple radiographic examinations ordered during a patient's lifetime and assess the risk-benefit ratio of each examination they order.

CME OBJECTIVE: The reader will have a better appreciation of the effects of ionizing radiation during a patient's lifetime and a heightened awareness of the need to assess the relevance of potential information gained from each study, especially from repetitive computed tomography scans.
Lincoln L. Berland, MD, no conflicts of interest; Cheri L. Canon, MD, no conflicts of interest; Stuart A. Royal, MD, no conflicts of interest

Revolutionary advances in diagnostic imaging improve patients' lives. "The immense diagnostic benefits of multidetector row computed tomography (CT) and nuclear imaging studies certainly outweigh the risks," says Lincoln L. Berland, MD, UAB's chief of CT. "The quandary for clinicians is how to assess and monitor the cumulative risk of radiation exposure to patients."

Radiation biologists estimate the population's average annual exposure to natural background radiation is ≤3.0 millisieverts (mSv). About 18% of total exposure in the US population comes from man-made sources. Medical X-rays and nuclear medicine account for 79% of man-made radiation exposure, and CT accounts for two-thirds of overall medical radiation exposure. About 70 million CT exams are performed annually in the United States with 550 million performed since 1980. Concerns about radiation exposure from CT examinations are increasing because multislice scans deliver higher doses of radiation to organs than conventional X-ray procedures, and physicians are increasingly using CT scans as diagnostic tools in asymptomatic adults. Cardiac nuclear medicine scans and interventional scans also confer high radiation doses, but these exams are performed less often than CT scans.

How Much Is Too Much?

Experts have disputed this hotly debated question in venues from The New England Journal of Medicine to USA Today. On one side, substantial evidence indicates exposure to high levels of radiation causes adverse health effects, and some evidence suggests that even low doses, such as from gamma rays and X rays, can compromise health. One study suggests that medical exposure accounts for 1% of cancer in the United States (Lancet. 2004;363:345-351). "That is not high," Berland says, "but it is high enough."

Long-term studies of the carcinogenic effect of radiation of atomic bomb survivors in Japan consistently indicate excess cancers. The large cohort of individuals who received low-dose exposure (5 to 200 milligrays) from the explosions provides statistical power to draw conclusions about cancer risks at low doses. The most recent major analysis confirms that the rate of solid cancers increases with an increasing radiation dose (Rad Research. 2007;168:1-64).

Experts disagree on the relevance of long-term data extrapolated from atomic bomb survivors. Some argue that the known biological effects are not equivalent and that survivors' exposure was an instantaneous whole-body exposure to a combination of radioactive materials, a very different scenario from a CT scan.

The National Research Council's Biologic Effects of Ionizing Radiation (BEIR) reports suggest use of a linear-no-threshold risk model. BEIR-VII (phase 2) states that the risk of cancer proceeds in a linear fashion at lower doses without a threshold and that "the smallest dose has the potential to cause a small increase in risk to humans" (Committee to Assess Health Risks From Exposure to Low Levels of Ionizing Radiation. Health Risks From Exposure to Low Levels of Ionizing Radiation. National Academies Press. 2006). Still, insufficient epidemiological data exist to precisely define the health effects of doses below 100 mSv. Some investigators even argue a protective effect (Exp Hematol. 2007;35[Suppl 1]:37-46).

"This is a legitimate scientific debate with conflicting information, so we opt for the conservative approach," says UAB pediatric radiologist Stuart A. Royal, MD. The more prevalent view is that low-dose radiation—50 to 100 mSv and possibly 10 to 50 mSv—presents a small but significantly increased risk of fatal cancer.

"Physicians and patients need to be aware of certain facts," says UAB radiologist Cheri L. Canon, MD. "Women are at higher risk than men. Girls and women are at higher risk from CT scanning of the chest because breast tissue is more sensitive to cancer development." (A chest CT scan has 10 to 25 times the dose of radiation to the breast as a mammogram.) "The more scans you have, the higher your risk. The younger you are when you are scanned, the higher your risk," she says.

Risk to Pediatric Population

The risk of exposure may be five times as high for children as it is for adults. A longer life expectancy and rapidly growing tissues and organs make children far more radiosensitive. Use of the fast, user-friendly CT scan in children has grown by 10% a year, and 11% of CT scans are performed on children annually. "This trend causes concern because CT scanners are built for adults and calculate radiation dose with factors that are too high for children," Royal says.

Royal has been instrumental in Image Gently, the educational initiative of the Alliance for Radiation Safety in Pediatric Imaging. With a goal of changing practice by increasing awareness of opportunities to lower radiation dose in children, Image Gently launched a multiphase campaign in early 2008 with an initial focus on CT scans. The campaign is based on a simple algorithm: When CT is the right thing to do: child-size the kilovolt peak and amperage; one scan is often enough; and scan only the indicated area.

"Image Gently aims to ensure that physicians, technologists, and nurses are well trained in imaging modalities. The educated pediatrician can determine whether the patient needs a traditional X-ray, MRI, ultrasound, or CT scan and can refer patients to medical centers experienced in pediatric imaging," Royal says. Scanning twice—with and without contrast—often is unnecessary. Radiologists can apply weight-based protocols, perform limited-view studies, and maximize shielding.

Many organizations provide helpful tools for pediatricians. Image Gently offers educational opportunities and a Web site with resources such as protocol guidelines. The Society for Pediatric Radiology holds As Low as Reasonably Achievable (ALARA) conferences. The ALARA doctrine encourages clinicians to make every reasonable effort to reduce radiation exposure (Curr Opin Pediatr. 2008;20:243-247). The American College of Radiology (ACR) Appropriateness Criteria include pediatric conditions and review the utility of various imaging modalities.

Risk Awareness

"The level of knowledge in the physician population and among patients needs significant improvement," Berland says. Some referring physicians are very knowledgeable about radiation issues, but others have had little training. In one recent survey investigators found that only 47% of radiologists, 9% of emergency department physicians, and 3% of patients believed that there was increased risk of cancer with a CT scan (Radiology. 2004;231:393-398).

"Unquestionably, physicians are ordering too many CT scans," Canon says. "We commonly see patients who have had several scans for symptoms in the absence of a diagnosis. Some patients are having repeat scans when previous scans are negative." Also common are follow-up scans to check improvement that are ordered too frequently or at too-short intervals. "In patients with Crohn disease, for example, some physicians use CT for triage instead of a diagnostic tool. A CT exam is such an easy procedure that patients may expect a CT ‘just to make sure.'" Patients who doctor shop may end up having repetitive scans for the same symptoms. No logical gatekeeper or technical system exists that can catalog and track scans on an individual basis.

"As radiologists, we ultimately are responsible, and yet we are rarely armed with all the information we need to make the appropriate decision. Referring physicians have to weigh the many variables and determine the risk-benefit ratio," Canon says. Many radiologists are leading efforts to improve education and minimize radiation dosages. Skilled radiologists undergo rigorous accreditation programs with the ACR, check scanners for accurate emissions and purchase those featuring automatic exposure controls, and design appropriate protocols.

Like the pediatric societies, several other organizations are addressing concerns. In its white paper, Radiation Dose in Medicine, the ACR's blue ribbon panel acknowledges many of the issues and provides 33 specific action recommendations (J Am Coll Radiol. 2007;4:272-284). The ACR offers accreditation of facilities, evidence-based appropriateness criteria to assist prescription of the most appropriate exam for more than 200 medical conditions, and practice guidelines and technical standards.

"Physicians and patients need to be aware of the risks of radiation in imaging, but from a balanced perspective that does not scare the public into avoiding life-saving screening such as mammography," Berland says.

"Every time clinicians order a study, they should stop and think," Canon says. "Ask, Do I really need this? What are the implications of ordering this? What are the consequences to my patient?"

For more information contact Dr. Lincoln Berland, Dr. Cheri Canon, or Dr. Stuart Royal at 1-800-UAB-MIST or at mist@uabmc.edu.

Typical Effective Doses

Fall 2008