Although this is the ideal scenario, it’s also one that currently plays out with only a few cancers and screening tests. This may seem strange since new cancer screening tests grab headlines pretty much every week. Yet, most of these new screening tests fall well short in the real world, where a screening test must meet a number of important criteria before it should be used population-wide.
Saving Lives
It’s not enough that a screening test accurately detects cancer. It must also lead to the patient receiving treatments that can actually change the course of the cancer and reduce mortality. In other words: it must save lives.
For example, imagine that a 45-year old woman is screened and diagnosed with cancer. She has surgery, radiation, and chemotherapy, and the cancer goes into remission. This seems like the screening test saved her life. However, in this case the cancer comes back and she dies at age 60 of cancer. Now imagine that same woman does NOT get screened. At age 50 she notices that something is wrong, goes to the doctor, and is diagnosed with cancer. She undergoes treatment, but dies at age 60 of cancer.
In both cases, this woman died of cancer at age 60. However, in the case where the woman was screened, she lived for five “extra” years—from 45-50 years old—with the physical, social, and emotional consequences of the diagnosis and treatment. Unfortunately for this woman, getting screened did not extend her lifespan. A good screening tool would have delayed her death beyond age 60. Figure 1 below illustrates this concept.
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| Figure 1 |
The good news is that there are good screening tools that do extend people’s lives. For example, in one study 46,551 participants were randomly assigned to undergo yearly Fecal Occult Bloot Testing (FOBT) to screen for colorectal cancer, or to not undergo screening (1). After 18 years, participants who were screened were 33% less likely to die of colorectal cancer than participants who were not screened.
Apart from reduced mortality, several others factors are also necessary for disease screening to be beneficial. First, the disease being screened for must have a phase in which there are no symptoms, but in which detection by screening increases the potential for prevention or cure. Second, the test must be simple inexpensive and easy for people to do. It must also be able to identify people who have the disease as having the disease (high sensitivity), and it must be able to identify people without the disease as not having the disease (high specificity). Third, the test must be performed correctly, interpreted correctly, and be linked to appropriate follow-up tests and treatment. Additional principles of screening outlined by the World Health Organization are summarized in Table 1.
Effective screening is currently available for cancers of the colon, breast, and cervix. Though the best approach to screening for lung cancer is still being developing, it is currently recommended for most people aged 55 - 74 who have a fairly recent history of heavy smoking. Benefits of screening for prostate cancer are still unclear. Some studies show reduced mortality but others don’t.
Table 1. Principles of screening. From Wilson & Jungner (2).
1. The condition sought should be an important health problem.
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2. There should be an accepted treatment for patients with recognized disease.
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3. Facilities for diagnosis and treatment should be available.
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4. There should be a recognized latent or early symptomatic stage.
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5. There should be a suitable test or examination.
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6. The test should be acceptable to the population.
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7. The natural history of the condition, including development from latent to declared disease, should be adequately understood.
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8. There should be an agreed policy on whom to treat as patients.
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9. The cost of case finding should be economically balanced in relation to the possible expenditure on medical care as a whole.
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10. Case finding should be a continuing process and not a once-and-for-all project.
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The Problem of Overtreatment
One issue that makes screening for cancer tricky is that some tumors grow quickly and others grow slowly. This means that some aggressive tumors found by screening may be just as deadly as the same aggressive tumors discovered without screening. Similarly, less aggressive tumors may never become deadly in a person’s lifetime, whether they’re found and treated early or never found at all.
Prostate cancer screening with the PSA blood test is a good example of this. A high PSA level does a pretty good job of identifying men who have prostate cancer, but it doesn’t do a good job identifying the smaller number of men whose prostate cancer is aggressive. Most cases of prostate cancer can grow for years and years and even a lifetime without ever becoming a danger to a man’s health. For these men, a high PSA test may cause biospies, surgeries, and other treatments that have severe side effects but do nothing to improve their health. Yet, for the smaller group of men with aggressive prostate cancer, the PSA test and treatment it leads to may play a key role in their long term health and survival.
It is difficult to find the balance between identifying cancers that could be harmful while trying to limit “overdiagnosis” and “overtreatment” (procedures and treatments done due to screening that have no effect on survival). Nevertheless, it is necessary. As Dr. Stephen Woloshin and colleagues wrote in a JAMA commentary in 2010 during the latest breast cancer screening controversy: “Overdiagnosed women are unnecessarily diagnosed, undergo treatment that can only cause harm, and must live with the ongoing fear of cancer recurrence” (3).
False-Positives and False-Negatives
Every single type of screening test will result in false positives and false negatives. A false positive is telling a person that they may have cancer when they actually don’t. A false negative is telling a person that they do not have cancer when they do. The consequence for people in the false-negative group is missing the opportunity to treat a cancer early. Consequences for those in the false-positive group include the risks of any follow-up diagnostic tests and unnecessary treatments and the emotional toll of being told about the positive test. For example, a false positive prostate cancer test can lead to the removal of the prostate, which can then cause bladder control problems.
Most effective cancer screening tests limit incorrect results, but none can completely eliminate false-positives and false-negatives. False-positives are an issue that can become more common the more screening tests a person has. Since most screening tests are meant to be done regularly, the likelihood of experiencing a false-positive goes up over a person’s lifetime.
In one study of breast cancer screening, a woman’s risk of having a false-positive after undergoing ten mammograms reached almost 50 percent (4). Researchers also estimated that close to 20 percent of women who do not have breast cancer would undergo a breast biopsy after 10 mammograms.
Such findings do not demonstrate that mammograms aren’t beneficial. They do save lives, but they also have their costs, and the key – as with any screening test - is finding which screening methods maximize the benefits while limiting the costs and risks.
Age and Screening
Age is a good example of one way to limit false-positives in screening. Since cancer is rare in young people and more common as people get older, age becomes a general – if imperfect - marker for risk and a broad guide for when screening should start. For example, breast cancer rates begin to rise around age 40 and colon cancer rates around age 50 (Figure 2). The more common a cancer is, the lower the false-positive rate will be for any given test. Though 20 year olds could get mammograms yearly, such testing would cause an unacceptably high number of false-positives, because cancer is unlikely in this age group.
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| Figure 2 |
Screening recommendations tend to vary from organization to organization and are often based on a combination of evidence and/or expert opinion. The majority of the recommendations summarized in the tables below are based on the rigorous evidence-based US Preventive Services Task Force screening guidelines, in addition to the guidelines provided by other well-respected organizations, such as the American Cancer Society.
Bottom Line
Cancer screening saves lives, but only if proven tests are performed and appropriate follow-up is provided. The decision to screen requires that providers and patients discuss the full implications of cancer screening, not just the test but the likely follow-up if tests are positive and the possibility for false positive screening test results.
There is a lot of media hype and industry hype about new screening methods that on their face seem to make good sense, but screening is a complex issue. Therefore, it is important to rely on the recommendations of personal physicians and reputable health organizations who analyze and distill all the science into sound, understandable advice. The American Cancer Society, the National Cancer Institute, and the US Preventive Services Task Force are examples of such organizations.
Web Resources
US Preventive Services Task Force
American Cancer Society
Screening terms
Primary prevention: detection and treatment of a condition before it becomes cancer, thus completely preventing the development of cancer.
Secondary prevention: identifying cancer at its earliest and most treatable stages to prevent as many negative consequences as possible.
False positive: A test result that indicates that a person has a specific disease or condition when the person actually does not have the disease or condition.
False negative: A test result that indicates that a person does not have a specific disease or condition when the person actually does have the disease or condition.
Sensitivity of a test: the proportion of people with disease who test positive
Specificity of a test: the proportion of people without disease who test negative.
Positive predictive value: among those people who test positive, the proportion who truly have disease (proportion of true positives).
Negative predictive value: among those people who test negative, the proportion who truly are free of disease (proportion of true negatives).
Generally Recommended Cancer Screening Tests for Women
Screening For
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Test(s) and General Frequency
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Beginning Age
|
Cervical
Cancer
|
Pap
test every 1-3 years
|
Age 21
or three years after starting sexual activity, which ever comes first
|
Breast
Cancer
|
Clinical
breast exam every 1-3 years
Consider
breast self-exam
|
Age 20
|
Mammography/Clinical
breast exam every year
Consider
breast self-exam
|
Age 40
|
|
Colorectal
Cancer
|
Fecal
occult blood test (FOBT) every year;
or
Fecal
immunochemical test (FIT) every year;
or
Flexible
sigmoidoscopy every 5 years;
or
Fecal
occult blood test every year and flexible sigmoidoscopy every 5 years;
or
Virtual
colonoscopy every 5 years;
or
Colonoscopy
every 10 years; or Stool DNA (unclear how often)
|
Age 50
|
Lung
Cancer
|
Low-dose
CT scan
|
Age 55 –
with history of heavy smoking
|
Sources: ACS; AHA, United States Preventive Services Task Force and Partners HealthCare
Generally Recommended Cancer Screening Tests for Men
Screening For
|
Test(s) and General Frequency
|
Beginning Age
|
|
Colorectal
Cancer
|
Fecal[EW10] occult blood test (FOBT) or Fecal immunochemical test
(FIT) every year;
or Flexible sigmoidoscopy every 5 years; or Fecal occult blood test every year and flexible sigmoidoscopy every 5 years; or Virtual colonoscopy every 5 years; or Colonoscopy every 10 years; or Stool DNA (unclear how often) |
Age 50
|
|
Lung
Cancer
|
Low-dose
CT scan
|
Age 55
– with history of heavy smoking
|
|
Prostate
Cancer
|
Consider
prostate specific antigen (PSA) test or digital rectal exam every year. Talk
to a doctor about risks and benefits.
|
Age 50
(Age 45 for African-Americans)
|
|
Sources: ACS; AHA, United States Preventive Services Task Force and Partners HealthCare
References
1. Mandel JS, Church TR, Ederer F, Bond JH. Colorectal cancer mortality: effectiveness of biennial screening for fecal occult blood. J Natl Cancer Inst. 1999 Mar 3;91(5):434-7.
2. Wilson J, Jungner G. Screening for Disease. Geneva, Switzerland: World Health Organization 1968.
3. Woloshin S, Schwartz LM. The benefits and harms of mammography screening: understanding the trade-offs. JAMA. 2010 Jan 13;303(2):164-5.
4. Elmore JG, Barton MB, Moceri VM, Polk S, Arena PJ, Fletcher SW. Ten-year risk of false positive screening mammograms and clinical breast examinations. N Engl J Med. 1998 Apr 16;338(16):1089-96.
