To Screen or Not to Screen?

Simple messages are not always possible.

By Kari Bohlke, ScD

“Screening for disease control can be defined as the examination of asymptomatic people in order to classify them as likely, or unlikely, to have the disease that is the object of screening. People who appear likely to have the disease are then investigated further to arrive at a final diagnosis. Those people who are found to have the disease are then treated.”

Alan S. Morrison, Screening in Chronic Disease, Second Edition1

At first glance, screening for diseases such as cancer may appear to be a simple topic with intuitive appeal: find the disease early, treat it, and improve survival. The reality, however, is far more complex. Though it’s true that some screening programs—such as Pap testing for cervical cancer—have produced dramatic declines in mortality from the disease being screened for, many other screening tests have a balance of risks and benefits that are far more difficult to assess. Furthermore, even when the balance tips in favor of screening, it’s important that patients have an understanding of the potential risks in order to make a fully informed decision about screening.

One potential risk of cancer screening that has been getting an increasing amount of attention in both the scientific literature and the popular press is overdiagnosis. Overdiagnosis refers to the diagnosis of a cancer that will not cause health problems during the life of the patient. Because it’s generally not possible to tell in advance which early-stage cancers will progress and become lethal without treatment and which will not, the detection of cancer through screening often leads to treatment. For patients with cancers that are very slow growing or that would eventually disappear on their own, this treatment—and the side effects that accompany it—are unnecessary, as is the stress that accompanies a cancer diagnosis.

Although the problem of overdiagnosis may apply to several types of cancer, prostate cancer and breast cancer have been the focus of several studies on overdiagnosis. These are the most commonly diagnosed cancers (other than skin cancer) in men and women, respectively, and together account for more than 386,000 new diagnoses in the U.S. each year.2 Commonly used screening tests include mammography for breast cancer and prostate-specific antigen (PSA) testing for prostate cancer. For women at average risk of breast cancer, the American Cancer Society recommends annual screening mammography starting at the age of 40.3 The American Cancer Society does not recommend routine PSA testing for early detection of prostate cancer, but does recommend that men discuss the pros and cons of prostate cancer screening with their healthcare provider before deciding whether or not to be screened.

A study published in the Journal of the National Cancer Institute reported that during the 30 years since the introduction of PSA testing, the test has resulted in 1.3 million more prostate cancer diagnoses in the U.S. than would have occurred otherwise.4 Roughly one million of these men received treatment for prostate cancer. There has been a decline in prostate cancer mortality during the PSA screening era, but it’s unclear how much of this decline is due to screening and how much is due to improvements in treatment. Nevertheless, even if all of the decline in mortality is due to screening, the results of this study suggest that relatively few men have benefited from early detection.

Overdiagnosis of breast cancer was explored in a study published in the British Medical Journal.5 Researchers analyzed breast cancer trends in the years before and after the initiation of mammography screening programs in Australia, the United Kingdom, Canada, Sweden, and Norway. The results suggested that one in three women who were identified as having breast cancer did not actually need to be treated.

Another potential risk of screening that will eventually affect many people who are screened regularly over a long period is a false-positive test result. In the case of mammography, for example, some women who are cancer-free will have an abnormal mammogram that leads to additional testing. The stress of an abnormal test result and the wait for additional testing and results should not be underestimated.6

In response to the accumulating evidence regarding the limitations of screening for prostate cancer and breast cancer, an article published in the Journal of the American Medical Association notes the importance of developing new approaches to early detection and prevention for both of these diseases.7 The report highlights the need to develop improved methods for distinguishing life-threatening and minimal-risk cancer. Patients with minimal-risk cancers could then be spared the burden of intensive treatment. The report also calls for an increased focus on cancer prevention, particularly among individuals at greatest risk of life-threatening cancer. Although early detection of cancer benefits some individuals, avoidance of cancer altogether would provide even greater benefits.

News about the limitations of existing cancer screening tests should not discourage people from receiving recommended tests such as mammography. In the case of mammography and other recommended screening tests, the risks are thought to be outweighed by the benefits for most people. Furthermore, some screening tests (such as the Pap test for cervical cancer and colonoscopy for colorectal cancer) have the potential to prevent cancer from developing in the first place by identifying precancerous lesions. Nevertheless, we owe it to ourselves to understand the risks in order to make fully informed decisions about screening. And when it comes to tests that still have an uncertain balance of risks and benefits (such as PSA testing for prostate cancer), decisions about screening will need to be made on an individual basis. The key to making the decision that’s right for you is information and open communication with your healthcare provider.

Dr. Otis W. Brawley, chief medical officer of the American Cancer Society, summed up the situation well in a recent letter to the editor of the New York Times: “The American Cancer Society strives to provide clear messages about cancer screening, but simple messages are not always possible, and can do a disservice to the people we serve.”8

References

1 Morrison AS. Screening in Chronic Disease, 2nd Ed. New York and Oxford: Oxford University Press; 1992.

2 American Cancer Society. Cancer Facts & Figures 2009. Available at: http://www.cancer.org/docroot/STT/STT_0.asp. Accessed October 27, 2009.

3 Smith RA, Cokkinides V, Brawley OW. Cancer screening in the United States, 2009: a review of current American Cancer Society guidelines and issues in cancer screening. CA Cancer J Clin. 2009; 59(1):27-41.

4 Welch HG, Albertsen PC. Prostate cancer diagnosis and treatment after the introduction of prostate-specific antigen screening: 1986-2005. Journal of the National Cancer Institute. 2009; 101:1325-1329.

5 Jorgensen KJ, Gotzche PC. Overdiagnosis in publicly organized mammography screening programmes: Systematic review of incidence trends. British Medical Journal [early online publication]. July 9, 2009.

6 Gøtzsche PC, Nielsen M. Screening for breast cancer with mammography. Cochrane Database of Systematic Reviews 2008, Issue 4. Art. No.: CD001877. DOI: 10.1002/14651858.CD001877.pub3.

7 Esserman L, Shieh Y, Thompson I. Rethinking screening for breast cancer and prostate cancer. JAMA. 2009;302:1685-1692.

8 Brawley OW, letter to the editor, New York Times. October 26, 2009.