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An Enhanced Risk of Cancer from CT Scans
Abstract & Commentary
By Toshiki Takenouchi, MD, and Sabiha Merchant, MD Dr. Takenouchi is Fellow in Pediatric Neurology, Weill Cornell Medical College; Dr. Merchant is Assistant Professor of Pediatrics and Neurology, Weill Cornell Medical College Dr. Takenouchi and Dr. Merchant report no financial relationships relevant to this field of study.
Synopsis: There is an increased risk of future cancer as a consequence of performing repeated CT scans, and the younger the patient at the time of scanning, the greater the subsequent risk.
Source: Brenner DJ, et. al. Computed tomography an increasing source of radiation exposure. N Engl J Med 2007;357:2277-2284.
Since the inception of CT scanning in 1970s, its use has increased rapidly and dramatically from approximately 3 million CT scans in 1980 to 62 million scans per year in 2006. At least 4 million of those were performed on children. This is largely due to advances in CT technology that have made it extremely user-friendly for both the patient and the physician, as well as the increased use of CT scan for the screening of asymptomatic patients.
The advent of rapid (spiral) CT scanning allows pediatric patients to be imaged without sedation, resulting in increased CT use for pediatric diagnosis, predominantly for the presurgical diagnosis of appendicitis. The use of CT in screening programs for asymptomatic patients in the adult population also is greatly expanding, including CT virtual-colonoscopy, CT lung screening for current and former smokers, CT cardiac screening, and CT whole-body scanning.
Based on the more than 50-year follow-up of atomic bomb survivors, the enhanced lifetime cancer risks following exposure to low doses of ionizing radiation has become convincing. In particular, there was a significant increase in the overall risk of cancer in the subgroup of atomic-bomb survivors who received low doses of radiation, ranging from 5 to 150 millisieverts (mSv). The mean dose in this subgroup was about 40 mSv, which approximates the relevant organ dose accrued following 2-3 typical CT scans in an adult. Since no large-scale epidemiological studies of this association have yet been reported, the authors calculated the cancer risks for any given CT scan by estimating the organ dose involved and applying organ-specific cancer incidence or mortality data that were derived from the atomic bomb survivors. They conclude that "the estimated lifetime risks of death from cancer that is attributable to a single generic CT scan are highly dependent on age because both the doses and the risks per unit dose are age-dependent." The estimates for lifetime risk of death from brain cancer attributable to a single head CT are as high as 1 in 1,500 for newborns, 1 in 2,000 for 1 year olds, 1 in 3,000 for 5 year olds, and 1 in 10,000 for 15 year olds. Above that age, head CT scans contribute a negligible lifetime risk of death from brain cancer. On the basis of such risk estimates, the authors calculate that currently "about 0.4% of all cancers in the United States may be attributable to the radiation from CT studies," with future estimates being in the range of 1.5 to 2.0%.
In the era of modern medicine, CT scans are ordered in a variety of clinical settings, often as routinely as chest X-rays; this is at times due to our practice of "defensive medicine" or due to a simple lack of communication between patients and doctors. This article suggests that most health care providers underestimate the radiation dose from a CT scan, and a significant number did not believe that CT scans increased the lifetime risk of cancer.
If in fact it is true that about one-third of all CT scans are not justified by medical need, perhaps 20 million adults and, crucially, more than 1 million children per year in the United States are being irradiated unnecessarily.
Randomized controlled studies might be difficult to perform in this setting; however, estimating the lifetime risk of developing cancer in a prospective study of people who were exposed to low-dose diagnostic ionizing radiation is of critical importance. Likewise, establishing a direct association between radiation site and the type of cancer subsequently developed in a group of patients will be an important goal of future studies. Although we do not have evidence-based, randomized, controlled, or prospective cohort studies on diagnostic ionizing radiation-induced carcinogenesis at this point, it is imperative that clinicians take into consideration the relative risk associated with overuse of CT, and not fall into the trap of ordering excessive and unnecessary scans. The data presented underscore the particular importance of performing brain CT scans in newborns and infants only when absolutely necessary. Rather, when available, alternative imaging modalities, such as MRI, should be employed in pediatric populations. The data also emphasize the need for the manufacturers of CT scanners to design machines in the future in which the strength of the settings can be adjusted downward for children whose thinner skull and torso "provide less shielding of organs from the radiation exposure."