• Studies examine radiation exposure, long-term cancer risks of CT scans

    April 2010

      PDF icon Studies examine radiation exposure, long-term cancer risks of CT scans (PDF, 808 KB) »

    Two recent studies concerning radiation exposure associated with computed tomography (CT) scans have raised concerns about long-term cancer risks. Both studies were published in the Archives of Internal Medicine.

    CT scans are an important diagnostic tool helping physicians evaluate trauma, belly pain, chronic headaches and other ailments. The fast, noninvasive CT scan offers a painless way to get three-dimensional images of the inside of the body. Use of the technology since it was introduced in 1980 has jumped from about 3 million to 70 million in 2007.

    “Use of the technology since it was introduced in 1980 has jumped from about 3 million to 70 million in 2007.”

    A CT scan exposes a patient to much higher doses of radiation than does a conventional X-ray. For example, one chest CT scan results in more than 100 times the radiation dose of a routine chest X-ray. Researchers are concerned that with the growing use of CT technology, especially for vascular and cardiac imaging, people are increasingly being exposed to a type of radiation that has been associated with the development of cancer.

    How the studies were conducted

    The first study was a retrospective cross-sectional analysis of radiation doses associated with the 11 most common types of diagnostic CT studies done on 1,119 adult patients at four San Francisco hospitals in 2008. Researchers used a theoretical linear model to estimate lifetime risks for cancer by study type from the measured doses.

    The second study developed computer risk models based on the National Research Council's report on the Biological Effects of Ionizing Radiation. A national survey provided organ-specific radiation doses to estimate age-specific cancer risks for each scan type. Survey and insurance claims data provided age- and sex-specific scan frequencies for 2007 in the United States.

    What the studies found

    The first study found wide variation in the radiation dosage associated with the 11 most common types of CT scans performed at the four participating hospitals. For example, the median effective dose of an abdomen and pelvis scan (the most common type of CT scan in the United States) is generally accepted as being 8 to 10 millisieverts (mSv). The median dose was 66 percent higher at the participating hospitals. Researchers found a 13-fold difference overall between the highest and lowest dose for each study type.

    The number of CT scans researchers projected would lead to the development of cancer varied by the type of scan and the patient's age and sex. For example, for a routine head CT scan it was estimated one in 8,100 women 40 years of age at the time of the scan (or one in 11,080 men) would develop cancer. The risks were projected to double for 20-year-old patients; for 60-year-old patients the risks were about 50 percent lower. Researchers concluded the study findings highlight "the need for greater standardization across institutions."

    The second study projected that out of 29,000 future cancers 2 percent of all cancer diagnoses would result from CT scans performed in 2007. The type of cancer would vary depending on the area of the body undergoing the CT scan. The most commonly caused cancers are expected to be lung (6,200 cases) and colon (3,500), as well as leukemia (2,800). The older the person is at the time of exposure, the less the risk for cancer.

    Rita Redberg, MD, editor of the Archives of Internal Medicine, wrote in an accompanying editorial that, "In light of these data, physicians (and their patients) cannot be complacent about the hazards of radiation."

    Study limitations

    Limitations of the first study included not having enough patients to explore the reasons for the variations in dosages for each study; lack of assessment of the association between image quality and radiation dose; scanners made by one manufacturer; possible misclassifications of the indications for each exam type; use of a theoretical linear model to estimate cancer risk; and imprecise methods for determining the effective dose.

    The second study limitations also included use of a theoretical linear model in which even increases in the lowest doses of radiation are associated with rising cancer risk. This linear model overestimates the actual cancer risk. Projections were made based on assumptions that all patients undergoing a CT scan had the same life expectancy as the general population.

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Page last updated: 8/7/2015 1:37:06 PM
  • What the news means for you

    CT scan is appropriate when need outweighs risks

    Scott D. Stevens, MD

    Wright, Heather, MDThese two studies are important because they raise awareness about exposure to radiation associated with a CT scan. The studies point out the need for patients and their doctors to discuss any test that is ordered to diagnose illness or injury. The doctor should explain why a test is needed, what questions the test is going to help answer and why a particular test is the best choice.

    “Patients and their doctors [need] to discuss any test that is ordered to diagnose illness or injury.” 

    Any test that uses ionizing radiation such as CT, cardiac catheterization or nuclear medicine studies should only be done when clinically necessary, when other tests that don't use radiation will not answer the question, and when the results of the test will change the clinical management of the patient.

    Do benefits outweigh risks?

    A CT scan is the most appropriate modality for many clinical situations. In these situations, the need to obtain information from a CT scan about diagnosis and treatment is greater than the very small risk from the patient's exposure to radiation from the CT. CT is particularly good for these situations:

    • Head CT for acute brain hemorrhage or stroke;
    • Chest CT after an X-ray finds a possible lung cancer; and
    • Abdomen and pelvis CT after trauma, such as an automobile accident.

    The American College of Radiology (ACR), the organization that unifies and educates radiologists nationwide, has published extensive criteria for determining the most appropriate imaging test for specific clinical situations. A patient's doctor can always consult with the radiologist to determine what study, if any, is indicated for a particular patient's situation.

    “You are more likely to die from being struck by a car while walking, by drowning or by radon in the home causing cancer than by the radiation exposure from a CT scan causing a fatal cancer.” 

    What are the risks?

    There is much controversy over whether the low dose of radiation received from a CT scan actually increases a patient's risk of a fatal cancer; the evidence has not conclusively proven this to be the case. However, to maximize patient safety, it is standard procedure to assume that there is a risk associated with the low dose of ionizing radiation exposure.

    You are more likely to die from being struck by a car while walking, by drowning, or by radon in the home causing cancer than by the radiation exposure from a CT scan causing a fatal cancer. In a recent article in the American Journal of Roentgenology by Cynthia McCollough, PhD, and others, it was noted that the lifetime risk of a fatal cancer from all causes is 22.8 percent. A commonly quoted estimate of the additional lifetime risk of dying from cancer is 0.05 percent per 10 mSv of radiation exposure. If you have a 10 mSv exposure from a CT scan, the risk of dying from cancer goes from 22.8 percent to 22.85 percent. Many CT scans deliver less than 10 mSv to the patient. We are all exposed to unavoidable natural background radiation at a dose of about 3 mSv a year. 

    We must pay particular attention to minimizing ionizing radiation exposure in children. 

    Tips for patients

    When a CT scan is ordered, go to a trustworthy institution supervised by board-certified radiologists with certified CT technologists. The facility should have modern CT scanners. A quality and safety program should insure that the scanner is delivering the lowest possible radiation dose. This includes regular checks by certified technologists, a medical physicist and representatives from the scanner's manufacturer. Patients should also keep a log of all their imaging tests. Should you need to go to an emergency room, try to go to one that is associated with your imaging facility so that previous studies are readily available.

    Avoid going to several different hospitals or imaging facilities as this greatly increases the chance of having unnecessary CT scans. If you must go to another facility after a CT scan was done elsewhere, insist on getting a copy of your previous images loaded on a CD. Take that CD with you to the second facility and tell the doctor you want that CD loaded into the imaging library. This is your right and should be standard practice wherever you go. If the images aren't available, the second facility may have to repeat the scan.

    How does UK protect patients?

    UK's radiology department uses the latest Siemens CT technology to generate images of adequate quality at the lowest possible dose of radiation exposure. Protocols standardize what scan is indicated for a particular situation. Our quality and safety program insures proper function of the scanners. Our radiologists consult with ordering physicians to offer alternative studies when possible. In addition, CT scan requests are reviewed by the radiologists and CT technologists to assure appropriateness of the test.

    UK also participates in the national "image gently" campaign for minimizing ionizing radiation exposure to children. We encourage patients to bring their previous scans done elsewhere to us on CD so that we can keep them on file. This helps avoid costly repeat scans and minimizes radiation exposure.

    Dr. Stevens is the division chief of abdominal radiology and assistant professor of radiology in the UK College of Medicine.

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