A few simple truths...
For screening to prevent people from dying early, simply finding cancers is not enough; we need to find progressive cancers that would kill if left untreated. What’s more, we need effective treatment for these cancers. And the therapy has to be more likely to cure if administered earlier (when cancer may be detected by screening) than later (when cancer may be detected by the patient or doctor without screening).--When Talking about Cancer Screening, Survival Rates Mislead
To detect cancerous cell types upstream from actual disease and then attribute it as an advancement of 5-year survival has little or no impact on overall mortality. Lead time bias advances the diagnosis by comprehensive screening but isn't likely to impact mortality rates. As you can see, if we improve detection of a cancer that is slow growing to begin with or indolent and not likely to progress--we can't say that screening improves mortality rates.
Length bias also impacts the effect of screening programs on overall cancer survival. Length bias identifies indolent or slowly growing tumors that may have a longer pre-symptomatic screen-detectable timeline. Aggressive tumors don't spend much time in the slow growing period therefore detection isn't advanced by routine screening. This contributes to the often misinterpreted "perceived" survival advantage to screen-detected cases. Length bias as described in the graphic may also lead to overdiagnosis--identification of cancers that would not have impacted patient had screening not taken place.
Consider this fact. Cancer cells viewed under a microscope and identifed as cancerous tell us nothing. We are still looking at cells the same way we did 160 years ago--What We Know, What We Don't Know, And What We Believe reveals that we still don't know which cells will be aggressive and which will be indolent. The promise of epigenetics may hold the clue but we do know that just "finding" cancer isn't the key to decreased mortality.
Death rates are improved only where screening has led to a real benefit; they are unchanged where screening has no effect on natural disease progression.
Here’s the bigger problem: screening mammography has failed to reduce the incidence of metastatic disease and it’s created an epidemic of a precancer called DCIS. The premise of screening is that it can find cancers destined to metastasize when they’re at an early stage so that they can be treated before they turn deadly. If this were the case, then finding and treating cancers at an early stage should reduce the rate at which cancers present at a later, metastatic stage. Unfortunately, that’s not what’s happened.--Like Groundhog Day The Mammogram Story that Won't Die
Either mammography isn't sensitive enough to identify these cancers early or they don't fit the Halstedian paradigm of steady progression. The lack of change in the incidence of metastatic disease is consistent with the hypothesis that breast cancer is a systemic disease by the time it's detectable — a paradigm typically attributed to Bernard Fisher.--Trends in Metastatic Breast and Prostate Cancer — Lessons in Cancer DynamicsH. Gilbert Welch, M.D., M.P.H., David H. Gorski, M.D., Ph.D., and Peter C. Albertsen, M.D.
If you are a journalist here is a tip sheet from the Association of Healthcare Journalists' website, Advice on communicating the problems with cancer overdiagnosis.Click here for a PDF of the file.
More to come...Improving Numeracy in Medicine (pre-order for only $5.99--price jumps upon publication). Join discussions about how to translate population level risk to individual risk at the patient level (absolute risk). Hint: the inverse of absolute risk is number needed to treat.
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Bonny is a data enthusiast applying curated analysis and visualization to persistent tensions between health policy, economics, and clinical research in oncology.