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Commentary |
1 Department of Radiology, University Hospitals of Cleveland, 11100 Euclid Ave., Cleveland, OH 44106-5056.
Received February 5, 2001;
accepted after revision February 15, 2001.
This article is a commentary on the preceding article by Ravenel et al.
Introduction
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As with all medical diagnosis and treatment, it is important to evaluate the benefits versus the risks of any procedure and to attempt to maximize the positive and minimize the negative. The rational use of CT relative to patient care involves two components: the appropriate selection of patients and minimization of the radiation dose without compromising diagnostic quality. Appropriate clinical selection of patients has been evaluated by the American College of Radiology, which has widely disseminated indication guidelines; therefore, that topic will not be discussed in this article.
One of the major imperatives of rational CT is to minimize the amount of radiation used to obtain diagnostic scans or to execute interventional procedures. The basic physics principles that were applied for many years with conventional radiography are also valid with CT scanners, both the older devices and the new multislice devices. As with conventional radiography or fluoroscopy, the mottle or noise is directly related to the radiation exposure. One can optimize examinations to determine the appropriate dose required to produce diagnostic images without overapplication of radiation. Determining the optimal dose is especially important during interventional procedures when much repetitive or continuous scanning may be performed.
Factors that have now focused new attention on this topic are the continuing technologic development, refinement of CT scanners, development of new applications, and subsequent increased usage. The remarkable growth in the number and use of CT procedures was not anticipated because many pundits believed that CT technology would stabilize or even decrease with the introduction and refinement of nonradiation devices, including sonography and MR imaging. The facts are that the superlative anatomic detail and diagnostic information provided by the new helical and multislice devices, their ease of use, and their consistently superior results, have dramatically increased the applications of CT techniques.
Looking toward the future, new applications will likely continue to develop with the advent of new sophisticated hybrid devices that integrate other imaging technologies such as nuclear isotopes. Combined CT and positron emission tomography, and combined CT and single-photon emission computed tomography, devices are being built and will soon be introduced in the marketplace. The historic growth and projected growth have been documented by numerous authors such as Mettler et al. [6]. Because of the expected continued growth and the potential risk of radiation exposure of the general population, it is critical that these issues be studied and addressed by current recommendations and future techniques.
The need and ability to adjust milliampere-seconds and radiation dose to suit individual patient size is a no-brainer. Numerous authors have documented the ability to adjust radiation exposure on CT scanners without compromising image quality. Ravenel et al. [7] in this month's journal discuss radiation exposure and image quality. From the earliest reports [1] and those that followed, it has been documented that adult scanning can be adjusted according to body weight (small bodies require a lesser dose and large bodies, a greater dose) and result in excellent diagnostic scans. Clinical articles by Ware et al. [8], Naidich et al. [9], Jurik et al [10], Mayo et al. [11], Rusinek et al. [12], and Diederich et al. [13] have all confirmed this finding and have actively advocated its implementation, especially for chest scanning [14, 15]. Similar observations have been made in regard to head scanning.
I differ from these authors in that I believe the diameter of the patient is a better predictor of the milliampere-second requirement than body weight because the diameter better correlates with the distance of the pathway traversed by the X-ray beam [1]. We believe that using patient diameter will provide the best approximation of tissue length traversed during the examination because body weight alone does not account for body habitus or height, which can vary with identical body weight.
Another useful method for reducing radiation dose with helical scanning is to increase the pitch of the examination. Vade et al. [16] showed that increasing the pitch from 1.0 to 1.5 decreased the radiation dose by 33% without any apparent loss of diagnostic information.
When performing contrast-enhanced studies, one should perform redundant or multiphase studies only when indicated. Numerous authors have shown that the yield for detecting liver lesions is improved by multiple scans taken during different phases of contrast injection. Such multiphasic studies are clearly indicated to evaluate for liver abnormalities; however, it does not seem logical to use such methods in all circumstances. For the evaluation of kidney lesions, triple-phase studies have become the norm, yet such studies should be reserved for patients in whom a question arises on a routine study or other examination, rather than be used as a routine protocol.
Most recently, the topic of radiation dose reduction has been the subject of considerable focus because of the prevalence of CT scanning in pediatric patients. The two most relevant articles, by Paterson et al. [17] and Donnelly et al. [18], clearly show how milliampere-seconds can be reduced by taking into account a patient's body weight. Implementation of these strategies can reduce radiation dose by 30-50% [18].
If equipment calibration permits, these technical steps can, of course, be immediately implemented by radiology imaging centers. Low-dose scanning of children has been used in our facility at Rainbow Babies and Children's Hospital for the past 15 years, since the completion of our first study of image quality and radiation dose (Yulish B, personal communication). Peripherally, this approach to dose management also brings an economic benefit. The lives of our X-ray tubes have been routinely prolonged, even to twice the estimated expectancy, thereby saving money on tube maintenance and replacement.
To refine and facilitate the management of radiation dose, much work remains to be done, especially by our manufacturing colleagues. They will need to modify the CT devices so adjustments of the milliampere-seconds can be performed easily while related technology is being developed and introduced. Research and development should continue with the goal of providing simultaneous measurements and generator adjustments to create "phototimed" CT scanners [19]. Other methodologies should be considered, such as using the ubiquitous "longitudinal scan" (Topogram [Siemens Medical Systems, Iselin, NJ], Scoutview [General Electric Medical Systems, Milwaukee, WI] are examples) as a measurement device for human anatomy in order to preplan and program generator output. Some type of automated device for measuring body size might also be considered.
Finally, I would like to address the most important point of this commentary, which is the benefits and risks of CT scanning in pediatric patients. Being a clinical radiologist and accepting the risk of a "point—counterpoint" discussion with esteemed basic scientists, I am compelled to comment on the recent article that has elevated the public discourse on pediatric CT scanning. This is the article titled, "Estimated risks of radiation-induced fatal cancer from pediatric CT" by Brenner et al. [5], which was referenced in a recent USA Today article [20].
First, I think the choice of the word "fatal" was unfortunate although well-intended, because that word is so intense and in fact the article does not address or discuss fatality. Many tumors are not fatal if found early enough, and remarkable strides in treatment have been made and are occurring on a daily basis—for example, treatments based on human genome or molecular modulation.
Although the article by Brenner et al. [5] is scientifically correct in the information it provides, I believe that it does not fairly discuss or portray the clinical benefits compared with the potential risks of the scanning. In brief, the two concepts that I would like to discuss from the article I will quote: "In the United States, at least 600,000 abdominal and head CT examinations per year are currently performed on children less than 15 years old and, of these individuals, a rough estimate is that approximately 500 will ultimately die from a cancer attributable to the radiation from the CT" [5], and "Of course, in most situations in which pediatric CT is used, the risk—benefit balance is strongly tilted toward benefit..." [5].
Relative to the incidence of cancer occurrence, the authors note in another
paragraph that the same group of 600,000 children would at some point during
their lives develop 150,000 cancers spontaneously or from other causes (both
these 150,000 and the other induced 500 cases assume a normal life span into
the seventh or eighth decade)
[5]. Later they note,
"Thus the estimated projected 500 CT-related deaths represents a small
(
0.35%) percentage increase over this background"
[5]. They also write,
"This small estimated relative risk suggests that detection of an
increased risk in an epidemiologic study would not be easy..."
[5].
Regarding the risk—benefit balance, several comments are germane.
If the pediatric patients to be imaged are properly selected, the benefit would likely exceed the risk. Indeed, Mettler et al. [6], who also published concerning the issue of pediatric CT, noted that their results should not be interpreted to mean that CT is being misused or overused. It is clear that the benefit ratio has not yet been well studied, but the authors concede that "the risk—benefit ratio is strongly tilted toward benefit [6]."
Anecdotal information from our clinical center, where we believe patient selection and scanning is appropriate, intuitively suggests benefit rather than risk. If one uses the numbers of Brenner et al. [5] to predict the death of a child (resulting from a radiation dose from prior CT scan), it would be at a ratio of one in 1200 patients. In the University Hospitals of Cleveland system, which includes Rainbow Babies and Children's Hospital, a 225-bed inpatient facility, we scan about 300 children annually. To offset the risk noted above would require saving the life of only one child in 4 years by a CT scan. In my opinion, THIS is a no-brainer.
Brenner et al. [5] are, of course, to be commended for stimulating the public discussion of this critical topic. Their work will motivate additional research, which should clearly define the benefits relative to the risks and thereby pave the way for future technical refinements. Brenner et al. enthusiastically support the management of radiation dose, noting that the relationship between dose and risk is a linear one. They reaffirm that current suggestions to reduce the dose will reduce the risk by 30-50%.
In summary, CT technology has revolutionized health care. The role and usage of CT continue to expand, dictating that clinicians and scientists study the many clinical applications to ensure that the benefits exceed the risks. Theoretic statistical studies indicate a small link with induced neoplasm, but at this time it appears from the available information that the benefits still far surpass the potential risks. Physicians and patients should be mindful of the available information, but there is no indication that patients, especially children, are being overscanned. Logic dictates that immediate steps should be taken to adapt current technology to reduce the dose, and future technology should be developed to make dose management simpler.
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