COMMENTARY

Is Computed Tomography an Increasing Source of Radiation Exposure?

David A. Johnson, MD, FACG, FACP

Disclosures

January 14, 2008

Computed Tomography--An Increasing Source of Radiation Exposure

Brenner DJ, Hall EJ
N Engl J Med. 2007;357:2277-2284

The earliest occupational groups exposed to ionizing radiation were radiologists and radiologic technologists.[1] Those employed before 1950 and exposed to high levels of radiation exposure from fluoroscopy had increased mortality from cancer. The first commercial computed tomography (CT) scanner was developed in 1972; since then, CT has become the major source of medical radiation.[2]

The potential biological influence of ionizing radiation depends on the energy absorbed per unit mass in a given tissue or organ. This quantity is called the absorbed dose and is expressed in units defined as grays (Gy). When evaluating the radiation risk after partial exposures of the body, the radiosensitivity of the various organs must be taken into consideration. When comparing risks of partial and whole-body exposure, the quantity of effective dose is applied. The radiation dose, a measure of ionizing energy absorbed per unit of mass, is expressed in Gy or milligrays (mGy; 1 Gy = 1 joule per kilogram). The radiation dose is often expressed as the equivalent dose in sieverts (Sv) or millisieverts (mSv). For x-ray radiation, which is the type of radiation used in CT scanners, 1 mSv = 1 mGy.

Conservative estimates reveal that more than 60 million CT examinations were done in 2002 in the United States, representing an estimated 70% of all medical x-ray exposure.[3] It is estimated that 6% to 11% of these exams were performed in children.[4] Although it is a challenge to define precise risk estimates related to low doses of radiation exposure, the ionizing radiation exposure from a single abdominal or chest CT scan may be associated with elevated risk for DNA damage and cancer formation.[5] The 7th National Academy of Science report on Biological Effects of Ionizing Radiation (BEIR VII) is the most recent update on this topic from a respected organization.[6] BEIR VII indicated that a single population dose of 10 mSv is associated with a lifetime attributable risk of 1 in 1000 for developing a solid cancer or leukemia. The overall risk of developing a solid cancer or leukemia from all causes would be 42 in 100.

The radiosensitive tissues are predominantly within the field of view of common chest, abdominal, and pelvic CT scans; the typical abdominal examination dose ranges between 10 and 20 mSv. Unfortunately, many patients are exposed to multiple examinations that increase cumulative dosing. A recent report focused on the effects of multiple exposures to ionizing radiation during CT scanning. The authors found that a subset of patients with renal colic commonly had total exposure rates between 19.5 and 153.7 mSv.[7]

This current review by Brenner and Hall highlights the understanding of the radiation exposure risks associated with x-ray imaging. The typical radiation dose from various radiologic studies is reported as follows:

Study Type Relevant Organ Relevant Organ Dose (mGy or mSv)
Posterior-anterior chest x-ray Lung 0.01
Lateral chest x-ray Lung 0.15
Screening mammogram Breast 3.00
Abdominal CT scan (adult) Stomach 10.00
Abdominal CT scan (neonate) Stomach 20.00
Barium enema Colon 15.00

The authors reported on the risks associated with even low doses of radiation. Most of the quantitative information on radiation risks and radiation-induced cancers comes from studies of survivors of the atomic bombing in Japan in 1945. This cohort of patients has been studied over time. Of note, there was a significant increase in the overall risk for cancer in these patients/survivors who received "low-dose" radiation, ranging from 5 to 150 mSv. The mean dose for this group was approximately 40 mSv, which approximates the relevant organ dose exposure from a typical CT study in which 2-3 scans are done in a patient. The concerns are further corroborated by a recent study involving over 400,000 radiation workers in the nuclear industry.[8] This study also reported a significant association between radiation dose and mortality from cancer. The range of exposure was between 5 and 150 mSv. These risks were quantitatively consistent with those of the atomic bomb survivors. Results of prospective studies for patients who undergo CT scanning will not be available for a long time, but it is possible to estimate the risks of exposure by estimating the organ radiation dose exposure and applying the organ-specific cancer incidence or mortality data derived from the studies of the atomic bomb survivors.

Brenner and Hall estimated the lifetime risk for death from cancer that was attributable to a single "generic" CT scan of the head or abdomen. The risks varied depending on the age of the patient at the time of exposure and the organ-specific dose exposure. Even though the doses are higher for the head CT scan, the risks are higher for abdominal scans because the digestive organs are more sensitive than the brain to development of radiation-induced cancer. Extrapolating from the data provided, the risk for cancer-related death associated with 1 abdominal CT scan is 0.06% for a patient exposed at 25 years of age and 0.02% for a patient exposed at age 50. This risk is striking and apparent when looking at the lifetime radiation risk of two of the most common radiogenic cancers, namely lung and colon cancer. When exposed to as little as 10 mSv at 25 years of age, the risks for death from lung and colon cancer are .025% and 0.0125%, respectively. For a patient exposed at 50 years of age, these associated risks are 0.017% and 0.010% for lung and colon cancer, respectively.

I would suspect that physicians who order x-ray testing for either diagnostic or screening purposes are relatively uninformed as to the radiation risk. To support this premise, referring physicians in the emergency department were largely unaware that there are potential harmful effects from CT radiation exposure, with only 9% aware of increased cancer risk.[9] Radiologists performing CT examinations considered the radiation exposure of limited concern, with only 47% recognizing the increased risk for cancer and many unaware of the dose of radiation delivered to the patient during the examination.[9]

If indeed clinicians are not well versed in radiation risks associated with CT examination, it should not be surprising that these risks might not be explained clearly to patients before obtaining consent for an examination. By comparison, the estimated risk for serious complications and death from receiving iodinated intravenous CT contrast is approximately 1 in 400,000,[10] which is lower than the lifetime attributable risk from a single 10-mSv dose of radiation. Yet considerable attention is given to contrast risk during the consent process. This difference may be accounted for on the basis of a clear causal relation: Contrast is injected and the patient immediately develops symptoms. Radiation effects, however, may not manifest until 5-20 years after the scan, thus causal relations are not apparent on an individual basis.

The US Food and Drug Administration (FDA) has listed medical x-rays as a known carcinogen. It may be necessary for governments to place guidelines on acceptable maximum doses and indications for CT scans. For instance, questionable practices such as whole-body CT screening examinations that expose normal individuals to known risks with unknown benefits may need to be restricted. Concern is particularly relevant at higher dose exposures. A recent report estimating the risk for cancer associated with CT angiography showed that the exposure-related risk was far from negligible.[11] Organ doses ranged from 42 to 91 mSv for the lungs and 50 to 80 mSv for the female breast. Lifetime risks for cancer ranged from 1 in 143 for a 20-year-old woman to 1 in 3261 for an 80-year-old man. The estimated risk for a 60-year-old woman was 1 in 715 and 1 in 1911 for a 60-year-old man. The highest organ lifetime attributable risks were for lung cancer and, in younger women, breast cancer.

Cross-sectional imaging has revolutionized diagnosis and medical practice in the last 30 years. Clinicians, as patients' advocates, are obliged to understand and explain the risks associated with CT radiation and to provide state-of-the-art dose-reduction techniques or to look for suitable alternative testing.

X-rays used in medical diagnostic procedures represent the greatest "man-made" source of radiation exposure to the population, accounting for about 14% of the total annual exposure from all sources. Ionizing radiation from diagnostic procedures has been postulated to cause several hundred cases of cancer per year in the United Kingdom.[12] In the United States, adjusting for the prevalence of CT use, it is estimated that 1.5% to 2.0% of all cancers at present are attributable to radiation exposure from CT scanning.[12]

One might begin to wonder whether clinicians should actually be obtaining informed consent for these procedures involving radiation exposure. If we extrapolate these radiation cancer incidence statistics to standards for what would be applied to a pharmaceutical product evaluation, it is difficult to imagine that the FDA would allow a one-time medication dose (much less potential repetitive dosing) for these associated cancer incident risks.

As more radiologic tests are offered, particularly for screening purposes, it is imperative that the physician as well as the patient recognize the long-term implications of the radiation exposure. This is especially relevant for radiation exposures associated with screening done over repetitive intervals. These screening radiation exposures are recognizably superimposed on top of any diagnostic radiation exposures as a result of any lifetime-specific testing involving radiation. Of note, in some countries (eg, Germany and Switzerland), radiation exposure for screening is forbidden by law. As "noninvasive" testing becomes more of a patient option, the long-term implications of these options will need full disclosure...."caveat emptor."

Abstract

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