Tuesday, August 25, 2009

Screening vs. diagnosis of cancer

I listened, with interest, to a CBC Ideas show on early diagnosis, or pre-disease cancerous conditions that have symptoms. In the case of pneumonia, chronic diseases, or dementia, this is true. But if you have no symptoms, the question arises: Do you do prescreening?

They were speaking of PSA and mammography test to identify prostate and breast cancers, respectively. If you have no symptoms is it a screening test. If you have symptoms, it is a diagnostic test. These are regularly scheduled tests, once you reach an age, in many cases. Now PSA screening tests are not paid for through OHIP, diagnostic tests are. One bone of contention is that mammographies are paid for through OHIP. There was some controversy with lobby groups advocating for early screening of prostrate cancer. But that is another post.
Let us deal, firstly, with PSA tests.

Screening vs. Diagnosis PSA Tests*

Dr. Perry Kendle.
Science of medicine and policy conditions, says that informed consent is important. However, in randomized trials they followed those who follow had both screenings, and those who were diagnosed following the exhibition of symptoms. At this point the jury is out. They do not know if prescreening outweighs diagnosis after symptoms for these reasons
There are downsides for being treated for prostate cancer.
  • The positive test result could mean false negatives or false positive tests and over diagnosis.
  • An elevated PSA may result in some other diagnostic procedure which will cause more problems.
  • If cancer is present they don't know if you will find benefits from being treated in terms of mortality outcomes.
  • The treatment may be worse than your current situation, emotionally it can be terribly difficult.
  • Early diagnosis does not guarantee longevity.
  • Prescreening may result in more or less side effects. (Impotence, urinary incontinence)
  • Does screening find more cancer than ordinary care?
Doctor take out samples from the prostate with a needle, a biopsy the size of a wood splinter. Chances for finding prostate cancer are related to the number of times you put the needle into the prostate for a biopsy.

If you sample 6 times, for example, the results show that 20% of men develop cancer.
If you biopsy 12 times, then 40% will develop prostate cancer. The more you biopsy the likelier you develop prostate cancer. How bizarre is this?

More importantly, you need to be looking at the PSA rates in your blood stream and determine baseline data. Finding the antigen does not mean you have prostate cancer.

Randomized Trials
The difficulty is to understand the results of these randomized trials. In this way we can weigh the benefits of screening vs. diagnosis. The standard is to survive five years after a diagnosis, but does this mean a diagnosis or a screening?

They are suggesting that in the cases of these cancers and early screening when one is, say 73, does not promote longevity. If the diagnosis is made at age 75, for example, the patient in the tests still dies at age 79. This in not necessarily success. We must speak of quality of life. To live an extra two years, knowing you have cancer, does not mean a better life. Plus, many more men die with prostate cancer, after age 65, than die due to prostrate cancer. This is called 'lead time bias'.

The doctors are trained to screen and diagnose
We do not know when it is better to wait and see, rather than intervening. In some cases, in slow growing cancers, abnormal cells grow a little and then regress. In other cancers they grow quickly. My father likely lived with his brain tumour for years until it affected him. The radiation, I know, had a horrible impact on him.

Pseudodisease
Many of us have abnormalities. Many of us have lumps and bumps (nodules) found on CT Scans on liver, colons. Some will develop clinical disease. There are two sides...
Overdiagnosis can equal over treatment. My mother, having chemotherapy at her advanced stage of cancer, shortened her life. I have no doubt.

Mammography
Dr. Cornelia Baines (I went to school with her daughter!) says that the Candian National Breast Screening survey trials occured to build on what we knew. Screening reduces breast cancer deaths in people age 50 and over. This is not true for women age 40 - 49. What are the benefits of mammography over clinical breast exams?

In the case of breast cancer, they did demonstrate that after 10 - 11 years 3% decline in mortality, and any decline in mortality was small and not statistically significant. For those over age 50- 59 got a careful clinical, quality breast exam by trained nurses. Early detection did not make a difference in mortality. Stats show that there are less than 2 chances per 1000 of getting cancer. Less than 2 per 1000, a difference of .2 per 1000. For those age 50+ a difference of 1 per 1000.

CBC quoted one physician as saying that there is a 5 - 6% error rate in mammographies. For every one positive case there are 55 other women with diagnostic studies who have nothing.
(Dr. Marla Shapiro)

Mammography Risks
The risk of a false positive, is the same as in PSA false positive tests: anxiety, biopsy, and the stress entrenched in taking the path towards cancer. Our population is taught to screen, rather than to diagnose. Having nodules and lumps is quite predictable over age 50.

Popularity paradox says, the greater number of times we overdiagnosis and overtreat by screening, the more people there are who believe they owe their lives to the prescreening. If you get screening for breast cancer at age 40, she has treatment (and we know how invasive these treatments can be: surgery, radiation), if she has a false positive she will have a positive outcome. Yet, we have found that clinically, early screening does not result in a better outcome. They are the same for screening vs. diagnosis after symptoms, but quality of life is reduced.

In a previous post, once cancer has been diagnosed, I have provided some questions to ask your oncologist, but a big aspect of recent trends is to entertain screening tests. That is to say, without any symptoms, your physician sends you for a test to look for cancerous conditions.
  1. Why are you suggesting this test?
  2. Are there randomized trials for this test (to prove its efficacy)?
  3. What are the options should the test be positive?
For example, once a man has a high level of PSA in his system, the next step is to insert a needle for a biopsy. The more one inserts the needles, the more likely it is that he contracts cancer.
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*You are Prediseased, by Alan Cassels on CBC IDEAS July 29 and August 5, 2009

Why wait until you are diagnosed with cancer, if you can hunt it down before it could kill you? Why not get a simple high tech CT scan to see if you are harbouring signs of pre-disease in your heart, your lungs, your breasts or your bowels?

Know your chances: Understanding health Statistics

Steven Woloshin, MD, MS, Lisa M. Schwartz, MD, MS, and H. Gilbert Welch, MD, MPH
Every day we are bombarded by television ads, public service announcements, and media reports warning of dire risks to our health and offering solutions to help us lower those risks. But many of these messages are incomplete, misleading, or exaggerated, leaving the average person misinformed and confused.

Should I Be Tested for Cancer? Maybe Not and Here's Why
H. Gilbert Welch, M.D., M.P.H.
Getting tested to detect cancer early is one of the best ways to stay healthy—or is it? In this lively, carefully researched book, a nationally recognized expert on early cancer detection challenges one of medicine's most widely accepted beliefs: that the best defense against cancer is to always try to catch it early. Read this book and you will think twice about common cancer screening tests such as total body scans, mammograms, and prostate-specific antigen (PSA) tests.
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*Prostate-Specific Antigen (PSA) Test - National Cancer Institute 18 Mar 2009 ... A fact sheet that describes the PSA screening test for prostate cancer and explains the benefits and limitations of the test.

See also: an excellent post by Musings of a Distractible Mind!

Daily Mail- Clinical breast exams lead to many misdiagnoses‎ -


Some went to facilities that performed only mammograms, while others went to health centres that performed both mammograms and clinical breast exams.


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