COMMENTARY

CT Imaging and Related Risk for Cancer

David A. Johnson, MD

Disclosures

December 30, 2009

Projected Cancer Risks From Computed Tomographic Scans Performed in the United States in 2007

Berrington de González AB, Mahesh M, Kim KP, et al
Arch Intern Med. 2009;22:2071-2077

Radiation Associated With Common Computed Tomography Examinations and the Associated Lifetime Attributable Risk of Cancer

Smith-Bindman R, Lipson J, Marcus R, et al
Arch Intern Med. 2009;169:2078-2086

CT Imaging and Related Cancer Risk

The use of computed tomographic (CT) scans has increased dramatically over the last few decades. Regardless of the recognized benefits from the radiologic definitions achieved with this technology, there is growing concern about radiation exposure associated with CT imaging, particularly with the lifetime attributable risk for radiation-related secondary cancer development. Conservative estimates are that more than 60 million CT examinations are done yearly in the United States, representing an estimated 70% of all medical x-ray exposure. Since the development of the first commercial CT scanners in 1972, CT has become the major source of medical radiation.

The potentially harmful consequences of ionizing radiation are divided into deterministic and stochastic effects.[1] Deterministic effects of ionizing radiation result from cell and tissue death. Such changes occur only at doses above a certain threshold and are proportional to the dose given. Radiation doses from medical diagnostic procedures are usually far below this threshold, but adverse effects, such as skin burns, have been reported after interventional procedures that use fluoroscopy. The term stochastic radiation effect refers to irradiated cells that are modified rather than killed. Modified cells may develop into cancer after a latency period of years. In principle, stochastic effects have no threshold. Exposed individuals do not incur ill effects with certainty, but they have a higher statistical chance for toxicity, including cancer. The doses delivered in diagnostic procedures are large enough to cause stochastic radiation effects, and the probability increases with the magnitude of the dose.

Although exact risk estimates related to low doses of radiation exposure are difficult to ascertain, the ionizing radiation exposure from a single abdominal or chest CT scan may be associated with elevated risk for DNA damage and cancer formation.[2] The seventh National Academy of Science report on Biological Effects of Ionizing Radiation estimated that a single dose of 10 mSv produces a lifetime risk of developing a solid cancer or leukemia of 1 in 1000.[3]

Two recently published studies[4,5] further addressed the issue of CT-related radiation exposure. A multicenter study sponsored by the US National Institutes of Health and the National Cancer Institute in Bethesda, Maryland, estimated the number of cancers that would arise from all CT studies performed in the United States during the year 2007.[4] Risk models with organ-specific radiation exposures were used to estimate age-specific cancer risks for each type of CT scan. Overall, approximately 29,000 future cancers could be related to CT scans performed in 2007 alone. The largest contributions to incident cancers were from abdominal and pelvic scans (14,000 cancers), followed by chest CT (41,000 cancers), head CT (4000 cancers), and CT angiography (2700 cancers). One third of the projected cancers were the result of CT exposure from ages 35 to 54 years, and 15% were related to exposure in patients younger than 18 years of age.

A second retrospective cross-sectional study evaluated the radiation doses received by 1119 consecutive adult patients who had diagnostic CT.[5] The study authors were surprised to find that radiation doses for common CT exams were higher and far more variable than previous estimates. Within each type of CT study, effective doses varied significantly both within and across institutions, with a mean 13-fold variation between the highest and lowest dose for each study type. The overall median dose of radiation for a routine abdomen/pelvis CT scan with contrast ranged from 12 mSv to 20 mSv, and for multiphase abdomen and pelvis CT scans, the dose ranged from 24 mSv to 45 mSv. These documented figures are considerably higher than those previously cited for abdominal CT radiation exposure (5-10 mSv). The investigators then estimated the number of patients who had each type of CT scan and the development of radiation-induced cancers. The risk for CT-related cancer varied by sex, age, and study type. There was a higher cancer risk in women and in patients with exposure at earlier ages. CT angiography was associated with the highest relative risk (1 cancer for every 150 examinations in 20-year-old women). The relative risks for CT of the abdomen are shown in the Table.

Table. Relative Risks for Cancer After Abdominal CT

CT Study 20 Years 40 Years 60 Years
Female Male Female Male Female Male
Abdomen/pelvis (with contrast) 1/470 1/620 1/870 1/942 1/1320 1/1250
Multiphase abdomen/pelvis 1/250 1/330 1/460 1/498 1/700 1/660
CT = computed tomography

 

Viewpoint

Advances in radiologic imaging have fundamentally changed the approach to diagnostic evaluation of patients with many disorders. With continuing growth in radiography, medical providers should be aware of the potential for radiation risk associated with CT scanning.

The popularity of CT imaging is high for many reasons, ranging from its speed and ease of use, to the relentless development of new imaging techniques, and finally to a growing body of evidence showing its images to be reliable for diagnosis. Unfortunately, drivers of increasing scan volumes also include financial rewards -- particularly for in-office procedures and the practice of defensive medicine -- resulting in unnecessary scans to minimize the threat of a lawsuit resulting from a missed diagnosis.

As these 2 studies made clear, radiation doses are higher than previously thought. They also demonstrated that radiation dose reduction is not yet a universal phenomenon and raised further doubt about whether the risk-benefit ratios and alternatives to CT scanning are seriously considered for every patient and CT scan.

In a recent article,[6] we suggested that patients and physicians need to discuss the choice for radiologic imaging and the relative risks associated with this choice. This is particularly important for discussions of screening in asymptomatic patients. Furthermore, both physicians and patients need to be cognizant of previous radiation exposure as well as the age when the previous exposures occurred. Because radiation-related risks may be cumulative over time, this information should become a key element of the patient's medical history. Most healthcare providers do not think of potential "downstream" radiation-related consequences of diagnostic and screening radiologic imaging that may occur decades later. Therefore, it is time for a paradigm shift in the way in which we approach the use of CT imaging as we balance the risk-benefit profile for our patients!

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