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Wednesday, April 11, 2012




     Last week I wrote about the limited knowledge that most physicians have about cancer incidence, survival rates, and the effects of screening tests. Numerous studies indicate that a majority of patients base their decisions to be screened on their doctors' advice. Unfortunately, if doctors don't know much, your decision won't be very good. The only way to correct this is to try to know a little bit more about cancer yourself. So what do you need to know to make good cancer risk assessments and screening decisions?
     You need to know some basic epidemiologic facts about each type of cancer. In the old days, this information used to be hard to come by, but now it's all available instantly with a simple Google search. Let's consider several of the major cancers that people often get screened for. Here is a table that lists the core information you would like to know in order to make a decision on testing. 

Type of cancer
Test: % of negative test results that are true
Test: % of positive test results that are false
Reduction in relative risk of dying of this cancer after testing.
Effect of testing on chances of being alive in 10 years.
Breast cancer
3 in 100 for each decade after 50 in average risk women
Mammogram: 90%
Mammogram: 2% (each time)
16% with serial testing every 2 years
Colon  cancer
57/100,000 in men at age 50; 150/100,000 at age 80.
Stool blood test: 30%
15% with annual testing
Prostate cancer
8 in 100 at age 50;  37 in 100 at age 70
PSA:  80% (for result > 4)

     If you are considering having a test for cancer, you want to know, first, How common is this type of cancer in persons like me? You also want to know what test are they using and how good is this test. The effectiveness of such tests are rated and compared by telling you how reliable a finding of a negative result is (i.e., How much confidence can you place in a negative test result indicating that you do not have this kind of cancer?) and the chances of having a positive test result that is not due to cancer. The simple fact is that no test rules out your chance of having cancer by 100%, and all tests have a number of false positive results that cause much anxiety but no disease. Finally, you want to know that there have been studies done to assess this screening technique and that it does in fact, after a number of years, reduce your chance of getting one of these cancers.
      In the table above we see that for an average woman, her risk of being diagnosed with breast cancer over any 10-year period after age 50 is about 3%. If she gets a mammogram regularly (at least every 2 years), she can reduce this risk by about 16%;  that is, after testing, her risk for a 10-year period will be 2.5%. This has to be compared to the risk of a false positive test result, which occurs in 2% for every mammogram performed; if you get several mammograms (e.g., the screening program calls for at least 10), then your risk of a false positive result is multiplied, and is 20% over 20 years if you get 10 mammograms. Each woman has to decide for herself whether she can accept this relatively high risk of a false-positive result in comparison to the relatively small benefit (2.5% absolute risk reduction over 20 years). Finally, every one undergoing any cancer screening test has to be aware that getting all the recommended tests is likely (but not guaranteed--note the result for prostate cancer screening with a PSA test) to offer a lower risk of dying of this type of cancer, but will, in fact, not insure that you live any longer overall than a person who does not get the screening test. In technical language, this means that there is no reduction in "all-cause mortality" for any of the common cancer screening tests.
     For women considering having a mammogram they should be aware that very recent studies have suggest that that absolute reduction in deaths from breast cancer is reduced by only 1 person per 2000 by screening, and the chance of having a cancer incorrectly diagnosed and treated is around 20-25%
     A group of widely respected medical experts (the Cochrane Collaboration folks) have put together a nice summary of the basic facts needed to make the breast cancer screening decision--"What you always wanted to know about breast screening". 

     Colon cancer screening poses the problems that the disease is considerably rarer than breast cancer (only about 50 cases per 100,000 persons at age 50, increasing to about 150 cases per 100,000 by age 80), that the test (stool testing for blood) is not very good (it reliably detects only about 30% of actual cancers, and gives a false positive result in 2% of persons tested every time it is used; this is a lot when you screen a large population. For example, if 100,000 persons were screened at age 50, there would be 57 true positive tests, and 2000 false positive tests). All of these false positive results have to be evaluated with additional testing (colonoscopy). The final disadvantage is that people just don't like having the necessary follow-up test for a positive result (the colonoscopy), which requires a full day of bowel preparation (total bowel wash-out) and a full-day lost from work or home on the day of the test, since sedation is used.

     Prostate cancer screening is the most curious case since there really is no good evidence that the test reduces even your relative risk of dying of prostate cancer because this disease is so common in men and often just sits there for many years without causing any problem at all.

     So this is the summary of the kind of basic information you should know before undergoing any cancer screening test. Because there are so many studies demonstrating that most physicians do not actually know or understand these facts, I am afraid I have to suggest that you look them up yourself.


     Above I have discussed the real risks of getting these cancers and your chances of living longer, if you are diagnosed with one. The truth, however, is that most people (including both physicians and consumers) are not driven primarily by the real risks. They are driven by perceived risk--something tantamount to a gut feeling that one is likely to get such a cancer and this risk can be abated or avoided by a screening test. To illustrate how much of a problem the overperception of risk is, consider this chart from a 2010 publication.(1)

     For purposes of this graphic, you are a "participant." What the chart shows is the risk of dying of each cancer and the risk of being diagnosed with each cancer as estimated by the "participants" in a screening program compared to the actual risks as demonstrated by epidemiological studies (SEER). The dark gray bar shows participants estimates of their chance of dying of each cancer, if they have it, and the black bar shows their estimate of their chances of getting each cancer. Compared to the actual risks demonstrated in studies, participants tend to dramatically overestimate their risk both of getting these diseases and of dying from the disease, if they do get it. This fact then induces them to undergo testing whether or not the testing is in fact very effective.
     As I showed in my last blog, physicians are prone to very similar biases. They are inappropriately influenced in favor of testing by reported increases in "5-year survival" even though this means nothing about the actual effectiveness of a test. More than half of physicians are not aware that screening will always lead to an apparent increase in "5-year survival" (because cancers are detected earlier) that does not necessarily imply any reduction in mortality at all. While "5-year survival" is a critically important measure for different treatments for diagnosed cancers, it has no relevance to the assessment of screening effectiveness; a majority of physicians do not know this and will provide inappropriate endorsements of screening tests when told that 5-year survival was noted to increase after starting a screening program. In the referenced study below, only 1 of 65 physicians was aware that screening consistently leads to "overdiagnosis"--imputing a diagnosis of cancer to someone who was never at risk of dying from it.

     So what's the bottom line in cancer screening? There are really only two facts you want to know. First, what is the absolute reduction in your risk of dying of a cancer for which you have been screened?

For some common cancers, here are these numbers:

Breast cancer: The absolute reduction in risk of dying from breast cancer through screening is 1 per 2000.

Colon cancer: The absolute reduction in the risk of dying of colon cancer through screening at age 50 is about 10 cases per 100,000 persons screened. For persons being screened at age 70, the absolute reduction in the risk of dying is about 25 per 100,000 persons screened.

Prostate cancer: The absolute reduction in the risk of dying of prostate cancer through screening is 0.

Cervical cancer: The absolute reduction in the risk of dying of cervical cancer through screening with Pap smears is 3 per 100,000.

Melanoma: There is no evidence that screening programs reduce the absolute risk of dying from melanoma.

The other fact you want to know is, What is your chance of actually living longer due to undergoing these screening tests? Here are the relevant numbers:

Breast cancer: There is no increase in your total expected lifespan from breast cancer screening with mammography.

Colon cancer: There is no increase in your total expected lifespan from colon cancer screening with either the fecal blood test or colonoscopy.

Prostate cancer: There is no increase in your total expected lifespan from prostate cancer screening with the PSA test.

Cervical cancer: There is no increase in your total expected lifespan from cervical cancer screening with Pap smears.

Melanoma: There is no increase in your total expected lifespan from melanoma screening of the skin.

     Well, those are the facts. Now you can make your decision.

References:  Hoffman RM et al. Decision-making processes for breast, colorectal, and prostate cancer screening: results from the DECISIONS study. Med Decis Making 2010; 30(5 Suppl(): 53S-64S. doi: 10.1177/0272989X10378701

1 comment:

  1. Cancer is very bad disease for health. it is the one of the most dangerous disease in the World. Cancer happens when cells that are not normal grow and spread very fast.