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PLENARY SESSION |
1 From the Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 S Kingshighway Blvd, St Louis, MO 63110. Received and accepted August 12, 2002. Address correspondence to the author (e-mail: siegelm@mirlink.wustl.edu).
Index Terms: Cancer screening
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Introduction |
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 Top Introduction General Criteria for Effective...CT ScreeningConclusionsReferences |
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Screening tests differ from diagnostic tests. Diagnostic tests are generally used in patients who come to medical evaluation for symptoms. Results of diagnostic tests are often accurate enough to establish a definite diagnosis, and they can be used as a basis for initiating treatment. By comparison, screening tests are generally offered to asymptomatic populations as a means of determining whether it is probable that they have a target disease.
During the past few years, many institutions have established targeted screening programs for pulmonary cancer and colorectal adenomatous polyps and also whole-body screening programs that use both single-detector and multidetector computed tomography (CT). The Radiological Society of North America recognized the importance of screening tests for cancer and dedicated an afternoon session to this topic at the 2001 annual meeting. The following two articles by Yee and Brant-Zawadzki discuss the role for virtual colonography and whole-body CT in the detection of cancer. Because screening programs for cancer detection are increasing in the United States, we believe that publication of these presentations is appropriate. Radiologists will be asked to make decisions about offering screening tests in their own practices, and thus, they must be aware of the risk/benefit and cost/benefit ratios of screening imaging procedures. These introductory comments, which emphasize the general criteria for a good screening program and the clinical and financial implications of offering CT screening directly to the consumer, provide a foundation for the articles that follow.
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General Criteria for Effective Screening |
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TopIntroductionGeneral Criteria for Effective...CT ScreeningConclusionsReferences |
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CT Screening |
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TopIntroductionGeneral Criteria for Effective...CT ScreeningConclusionsReferences |
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Sensitivity and Specificity Issues
An imaging test for screening must have a high sensitivity and specificity for detecting preclinical disease, and earlier detection should result in alterations in patient management that ultimately improve patient outcome. Negative test results should reassure patients that they are disease free and do not need to worry about cancer. False-negative test results pose problems. Patients who receive false-negative results from a screening test may be falsely reassured that they do not have cancer, which in turn can lead to a delay in both the diagnosis and treatment of disease when symptoms eventually develop. Screening tests also should have low false-positive rates, so that healthy individuals are not exposed to unnecessary tests or procedures, which translate into unnecessary morbidity and additional costs (4). Increasing the specificity of a screening test can improve patient outcomes and increase the cost-effectiveness of screening. Increasing the sensitivity, however, may not be cost-effective because it may lead to an increase in detection of incidental findings or disease that will never affect the patient’s life, as well as additional diagnostic procedures, including invasive ones, that will be necessary to characterize abnormalities seen with CT screening.
At the current time, the accuracy of CT for detecting colorectal cancer (the second leading cause of cancer death in the United States) and for whole-body screening is difficult to estimate, since there are no rigorous published data that address this issue in asymptomatic people with a low risk of disease. Large-scale trials will be needed to determine whether CT screening is beneficial for identifying early disease.
Morbidity and Risks
Screening tests also must have a low likelihood of causing adverse effects. At the time of screening, the patient’s risk of short-term morbidity or death from the target disease is extremely small. A screening test cannot be so risky that it offsets its benefits. A major long-term adverse effect of CT screening is radiation exposure. Concerns about the risks of cancer induction from diagnostic radiology, including CT, have existed for years, but the public and the Food and Drug Administration have taken more interest in this issue since a report appeared in the radiology literature about high radiation exposure levels from CT in the pediatric population (5).
A detailed discussion of risks from radiation exposure is beyond the scope of this article. How-ever, a simplistic analysis based on a downward linear extrapolation hypothesis has been published in a major radiology journal (6). Assuming a risk of cancer death of 0.04% per 1 rem of effective body dose (7), it has been estimated that of every 100,000 people who undergo CT, 40 will develop life-threatening cancer induced by radiation during their lifetimes. Of the same 100,000 people, 23,000 are likely to die from spontaneous cancer. Assuming a very low CT detection rate of 0.005% and resulting cure, 115 people might derive a benefit from CT screening versus the 40 who might develop a screening-induced cancer sometime during their life (6). Critics will undoubtedly argue that this benefit is only theoretical and not an established fact. However, suffice it to say that there is a risk from diagnostic radiation exposure, and radiologists undertaking screening programs must understand the factors under their control that affect CT radiation dose and how to implement a program of radiation dose reduction. Through the collective efforts of radiologists, physicists, and equipment manufacturers, there is now considerable interest in finding strategies for lowering the radiation exposure to patients undergoing CT (8). For more information, the reader is referred to several references that discuss the risks of radiation exposure to patients from CT and methods of minimizing such exposure (5,7–10).
For colorectal cancer screening conducted with CT colonography, there is discomfort for the patient from both the preparation for the examination and the procedure itself. The risk of perforation is virtually nonexistent because the rectal tube is soft and smaller than that used in a standard air-contrast enema study (3). For whole-body screening, the examination is performed without use of intravenously administered contrast material, so there is no short-term adverse effects.
Availability and Cost-Effectiveness
Screening tests must be available to the target population and must reduce costs of medical care. In many aspects, CT is an ideal screening tool. It is fast, particularly with the increasing use of multidetector scanners; comfortable for most examinations; readily available; and not operator-dependent.
On the basis of the currently available evidence, the ability of CT screening to reduce medical care costs has not been proved. In fact, the opposite is often the case, with screening programs actually creating costs for the consumer and the healthcare system. Because whole-body CT screening is not covered by medical insur-ance, the charge for screening is paid out-of-pocket by the patient. The coverage for CT colonoscopy has yet to be decided. Even if patients bear the expense for screening, there are other financial consequences for the remainder of the healthcare system. These consequences include the costs associated with the follow-up of patients with false-positive results and also true-positive results. These costs include those of additional tests or invasive procedures required to evaluate a lesion detected at CT screening and also the costs of managing complications resulting from those interventions (4).
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Conclusions |
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References |
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This article has been cited by other articles:
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  C. T. Kolber, G. Zipp, D. Glendinning, and J. J. Mitchell Patient Expectations of Full-Body CT Screening Am. J. Roentgenol., March 1, 2007; 188(3): W297 - W304. [Abstract] [Full Text] [PDF]
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 C. D. Furtado, D. A. Aguirre, C. B. Sirlin, D. Dang, S. K. Stamato, P. Lee, F. Sani, M. A. Brown, D. L. Levin, and G. Casola Whole-Body CT Screening: Spectrum of Findings and Recommendations in 1192 Patients Radiology, November 1, 2005; 237(2): 385 - 394. [Abstract] [Full Text] [PDF]
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