David J. Brenner, Ph.D., D.Sc.
Higgins Professor of Radiation Oncology and Public Health
Director, Center for Radiological Research
Center for Radiological Research
Columbia University Medical Center
New York City
In 1980, about 3 million CT scans were performed in the USA; in 2006 that number (see Fig. 1 below) was over 60 million . The concern here is that radiation doses from CT exams are typically 100 times greater than those from conventional radiological exams such as a chest x ray - in fact, the radiation doses from CT are in the range where there is direct epidemiological evidence of a small but significant increased radiation-induced cancer risk. This is particularly relevant for children, who are much more sensitive than adults to radiation-induced carcinogenesis.
It is certainly true that, at very low radiation doses, cancer risk estimates are highly uncertain. However at the rather higher doses corresponding to a few CT scans, there are direct epidemiological cancer-risk data from about 30,000 atomic-bomb survivors who were on the peripheries of the two bombed cities, and who were exposed in this low-dose range [2-4]. This low-dose sub-population has been followed for more than fifty years, and shows a small but statistically significant increased cancer risk. Other large-scale epidemiological studies, at doses relevant to CT, have reached the same conclusion . Thus, in the context of CT doses, we have direct evidence of cancer risk, and do not need to extrapolate cancer risk estimates from higher doses, with all the attendant uncertainties that entails.
Because CT is such a superb diagnostic tool and because the individual radiation risks from CT risks are small, the CT benefit/risk balance is generally by far in the patient’s favor. But the long-term public-health impact of the radiation exposure will be related both to the individual risk, and to the number of people exposed to that risk which, as we saw, is millions per year, and increasing rapidly. Could or should anything be done?
There are two ways to reduce potential long-term public-health consequences of increased CT usage: One is to reduce individual CT scan doses, and the other is to reduce the number of CT scans - or at least slow down the rate of increase. In fact there is considerable potential to reduce CT scan doses, using new technologies such as automated exposure control .
In terms of reducing CT usage, even in symptomatic patients, there are probably significant numbers of CT scans, perhaps one third of all scans, which either a) need not be done, or b) could be reasonably replaced with other imaging modalities. Examples of the former are scenarios in the management of blunt trauma, seizures , and chronic headaches.
There are a variety of scenarios where CT might potentially be replaced with other imaging modalities, without significant loss of efficacy. For example, patients with a history of renal colic or kidney stones often have multiple CT exams ; here, combinations of ultrasound and unenhanced abdominal radiography are an appropriate alternative. Another important example is the use of CT as a primary tool for pre-surgical diagnosis of acute appendicitis &ndash a quite recent development. An issue here is that appendicitis is predominantly a young person’s disease, so the radiation risks are higher than for adults. Recent reports  have suggested the practicality of selective imaging strategies for diagnosing pediatric appendicitis, where, depending on the assessed risk, the treatment strategy is appendectomy without imaging (high risk patients), ultrasound followed if necessary by CT (medium risk patients), or in-patient observation (low risk patients). Such selective strategies have the potential to halve the number of CT scans performed for diagnosing pediatric appendicitis .
Overall, the rapid rise in CT usage is unlikely to be slowed without significant education efforts, both within the health care community  and of the public . Regarding public education, there is the concern that, if the public were more aware that there is a small risk associated with CT, many who need CT scans might refuse them. However, the evidence does not support this concern: for example, in a recently-published study , when parents were informed about CT risks, their willingness to have their child undergo a CT did not significantly change, although they became more willing to consider other imaging options, if equally effective; no CTs were cancelled or deferred after receiving risk information.
Overall, reducing CT usage will not be easy. Physicians are often subject to significant pressures, from the medical system, the medico-legal system, and from the public, to prescribe CT, even when not really necessary, or when alternatives exist. Ongoing dialogue is important among radiologists, pediatricians, ER physicians, manufacturers, and indeed the public, to establish practical ways to slow the increase in CT doses and usage, without compromising patient care.
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