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DOI: 10.1148/rg.226025059
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(Radiographics. 2002;22:1532-1536.)
© RSNA, 2002

Plenary Session

Screening CT: Rationale1

Michael N. Brant-Zawadzki, MD

1 From the Department of Radiology, Hoag Memorial Hospital, One Hoag Dr, Newport Beach, CA 92658. From the Plenary Session, Friday Imaging Symposium: Screening for Cancer, at the 2001 RSNA scientific assembly. Received March 18, 2002; revision requested April 24 and received May 22; accepted July 1. Address correspondence to the author (e-mail: ezbz@primenet.com).

Index Terms: Cancer screening


   Introduction
Top
 Introduction
Lung Cancer Screening
 Colon Cancer Screening
 Discussion
 References
 
There can be no better evidence for the utility of computed tomography (CT) in the medical world than the fact that, according to the American College of Radiology, approximately 33 million CT examinations were performed in the United States last year. The explosive growth of this modality arises from its excellent anatomic resolution and its efficiency for diagnosis. Indeed, it is the experience of practicing radiologists that many physicians, particularly those in the emergency department, use CT in lieu of a thorough physical examination.

In the same time frame, a number of medical, social, and cultural trends have combined to fuel the application of modern CT technology to early disease detection in the self-referred, asymptomatic patient. The initial proponents of this application aimed at the high prevalence of heart disease in the population and used the ability of CT to depict calcification within the walls of the coronary artery (a marker of coronary artery disease). The high prevalence of coronary artery disease is widely known. In the United States, approximately 800,000 people die of it each year, and in 180,000 of them, death is the first sign of the disease process. Therefore, a large number of asymptomatic individuals harbor coronary artery disease (Fig 1). Several studies have shown that primary intervention in individuals with traditional risk factors for coronary artery disease produces a decreased prevalence of coronary artery events. More recent studies indicate that traditional risk factors may be inadequate for stratifying individuals at risk for coronary artery disease. These investigators have suggested that use of coronary artery "calcium scores," derived from CT examinations, should be used to modify risk stratification in the asymptomatic population (1–6).



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  		Figure 1.  CT scan of an asymptomatic 65-year-old man shows heavy calcification of the left main and anterior descending coronary arteries. Calcium score was 967.

 
In the remainder of this article, the application of CT to screening for lung cancer and colon cancer is discussed, as well as some of the issues surrounding CT screening.


   Lung Cancer Screening
Top
 Introduction
 Lung Cancer Screening
Colon Cancer Screening
 Discussion
 References
 
Lung cancer is the number one cancer killer in our population, and screening for it with CT has been fueled by a number of preliminary studies (7–9). Such studies suggest that the dismal patient outcome statistics for this disease may be improved by using CT technology for early detection of disease. Particularly, the Early Lung Cancer Action Project (ELCAP) study by Henschke et al (7) and its follow-up report (10) seem to document that CT is much more sensitive than other methods for detecting cancer, that CT enables reliable detection of stage I cancers (patients with stage I cancers who undergo treatment have 5-year survival rates of 80%), and that such a CT screening program can yield cost-effectiveness measures better than those seen with other screening programs such as mammography and colonoscopy (10). Considerable amount of controversy has been raised by this study (11), and lead-time bias and length-time bias arguments have been at the core of the debate. Because there is yet no documentation that early lung cancer detection improves disease-specific mortality, critics have rejected widespread use of lung cancer screening with CT. It has also been argued that two different types of lung cancers exist: the indolent cancer, which matters little to the patient’s prolonged survival, and the highly aggressive cancer, which, despite early detection, will lead to the patient’s death no matter when it is detected. The problem is, one never knows which type of cancer one is dealing with initially, and a fatalistic approach to lung cancer treatment seems contrary to the culture, if not the science, of medical practice, which suggests that early detection gives the best chance for positive patient outcomes (Fig 2).



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  		Figure 2.  CT scan of a 63-year-old man with a history of previous left upper lobectomy for stage IA cancer (5 years prior) reveals a recurrent metachronous lung cancer.

 

   Colon Cancer Screening
Top
 Introduction
 Lung Cancer Screening
 Colon Cancer Screening
Discussion
 References
 
Colon cancer is the second most common cancer killer in the United States, causing approximately 57,000 deaths in the year 2000 (vs approximately 155,000 deaths from lung cancer). Regular colon cancer screening has been recommended by the medical community for individuals over 50 years of age and even over the age of 40 years for those with a significant family history of cancer. Occult blood testing of stool and flexible sigmoidoscopy have proved to be relatively insensitive in the detection of known cancers. The use of barium enema examination has lost favor, with flexible colonoscopy being performed instead. However, the latter procedure is invasive, is costly, and carries potential threats of perforation (one in 1,000), significant bleeding (three in 1,000), and even death (one in 5,000). Only 20%–30% of all individuals at risk have undergone any form of colon cancer screening. It has been stated that the best screening procedure for colon cancer is one that a patient is willing to undergo.

CT colonography has been tested in a number of preliminary trials (12–15). For this procedure, the colon is cleansed and purged as done for conventional colonoscopy. Insufflation of the colon with gas is then followed by thin-section, high-resolution scanning of the patient in prone and supine positions. Software is available to produce axial, coronal, and sagittal views of the colonic wall, with workstation software making the tracking of the colon throughout the abdomen relatively facile (Fig 3). A typical "virtual" colonoscopy can thus be done in approximately 15–20 minutes (which is within the range of a radiologist’s time to perform and interpret a barium enema study). Results of these trials suggest that CT colonography has a sensitivity in the 90%–100% range for polyps larger than 8 mm (14,15). All the abdominal organs are seen when CT colonography is performed; thus, a combination of lung cancer and colon cancer CT screening provides a "whole-body" evaluation.



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  		Figure 3a.  Axial (a) and reformatted coronal (b) images from CT colonography of a 55-year-old man demonstrate a colonic polyp (arrowhead) in the left side of a distended colon.

 


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  		Figure 3b.  Axial (a) and reformatted coronal (b) images from CT colonography of a 55-year-old man demonstrate a colonic polyp (arrowhead) in the left side of a distended colon.

 

   Discussion
Top
 Introduction
 Lung Cancer Screening
 Colon Cancer Screening
 Discussion
References
 
The above applications of CT to early disease detection have dovetailed with the social and cultural trend toward wellness and life-extension. The graying baby boomer population is particularly health-conscious and spends considerable discretionary income on alternative medical strategies. These trends, coupled with the restricted access to physicians and medical technology imposed by managed care, have helped open the dam for a flood of self-referred patients to centers offering CT for early disease detection. Providers of the service welcome the willingness of these patients to pay out of pocket for such studies and have emphasized the customer-oriented quality of services offered both in terms of the technologic and the medical personnel. Partly driven by philosophy and partly by economic realities, use of CT for early disease detection has blossomed to include not just entrepreneurial facilities, but well-established hospital and even university departments.

Critics abound, and many point to the unproved nature of the process, with no patient outcome studies that demonstrate life-prolonging benefit (16). Counter arguments claim that documentation of life prolongation is absent from well-known medical therapies such as coronary an-   gioplasty, spine surgery for disk and other back problems, or even the conventional physical examination performed in a clinician’s office. Critics further decry the exceedingly large number of "false-positive" results. Mostly, they point to the highly prevalent incidental findings such as granulomas that are found in the lungs during lung cancer screening. However, the Early Lung Cancer Action Project (7) and a recent German study (17) indicated that such "incidentalomas" can be excluded in a follow-up CT study without need for invasive biopsy in the vast majority of cases. Follow-up studies are part of any true screening program (eg, mammography, PAP smears), even when results of the test are negative. Radiation hazard is another criticism that has been raised. CT screening facilities have been quite conscious of radiation exposure for the most part and, with use of the newest of technologies, offer considerably lower radiation doses than those incurred from conventional diagnostic CT studies previously done in hospitals (18). The controversy about whether radiation-induced cancer occurs at the low diagnostic doses used (in the 1-rem range) is beyond the scope of this article. Suffice it to say that there is no scientific proof that cancer induction occurs with this dose range. Indeed, the extrapolation of experience from survivors of atomic bombing into the low dose range of diagnostic radiology is fraught with problems, and, despite a considerable amount of research in this area, a logical opinion that a given diagnostic examination can cause cancer is simply untenable (19,20). It is true, however, that repetitive exposure to radiation is not desirable, and undergoing whole-body CT on an annual basis is certainly not a concept that is welcome in the asymptomatic individual. Vendors of CT equipment have become acutely aware of the issues and have introduced new algorithms for dynamic beam attenuation relative to body size and thickness that will further diminish radiation exposure from this technology.

In our preliminary experience, CT screening of self-referred patients over 40 years of age for early disease detection proved professionally satisfying and has provided some interesting insights. Our overall findings are that cancer prevalence is approximately 1% in unselected, self-referred adult individuals (n = 1,500). Lung, renal cancer, and other cancers have been the most common findings. Lymphoma (Fig 4), abdominal aortic aneurysms, and even pancreatic cancer have been detected with such studies (despite the lack of intravenous or oral contrast material administration). The problem of characterizing benign renal and hepatic cysts exists, just as it does when these lesions are found incidentally in patients studied for other reasons. Although some critics believe that such incidental findings cause unnecessary anxiety in the subjects when found, most self-referred individuals are already anxious about their health and, in our experience, find their anxiety relieved by the screening experience, even when an incidental and likely benign abnormality is discovered and properly explained. Polyp detection with CT colonography is improving as the learning curve grows. Others have also demonstrated significant yield of positive findings when whole-body scanning was performed for screening purposes. The Mayo Clinic lung cancer screening project demonstrated early in its process that "significant" findings elsewhere in the body were seen with a prevalence of approximately 14% (Jett J, Mayo Clinic, oral communication, 2001). Abdominal aortic aneurysms were particularly common in their patient population.



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  		Figure 4.  CT scan of a 67-year-old woman self-referred for "full-body" scanning demonstrates a lobulated mass (arrows) lateral to the ascending colon. The mass proved to be mesenteric lymphoma.

 
Obviously, screening for cancer with CT has epidemiologic ramifications that can be addressed only by targeted organ studies. This need is being addressed with the American College of Radiology Imaging Network study for use of CT in lung cancer screening. A similar approach to colon cancer screening will likely be developed. In the meantime, the experience is anecdotal and is likely to remain so for the foreseeable future. However, the consumerism movement in healthcare is quite strong, and the demand for this procedure is likely to remain high. Radiologists, as opposed to other medical specialists or business ventures, are better positioned to satisfy this growing demand responsibly while striving to study the role and value of such a controversial service.

The phrase "...and see your doctor regularly" often accompanies general medical advice for preventive health practice, yet the annual physical examination has no proved cost benefit, and few would argue its sensitivity is greater than that of CT. The "total-body scan" is likely to be more a marketing ploy than a true screening program in the epidemiologic sense; however, targeted organ scanning with CT may yet prove of value in specific, at-risk populations.


   Footnotes
 
See the commentary by Black following this article.


   References
Top
 Introduction
 Lung Cancer Screening
 Colon Cancer Screening
 Discussion
 References
 

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  11. Society of Thoracic Radiology Web Site. A consensus statement of the Society of Thoracic Radiology on screening for lung cancer with helical CT. Available at http//www.thoracicrad.org/str99-consensus_statement.html. 2002; Accessed February.
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