Last week’s Deep Dive webinar on cancer screening was a pretty wild ride; many EmpowerEd members expressed surprise and outrage after learning about the shockingly poor evidence base for common cancer screening tests.
Fun fact from the webinar: No randomised controlled trial to determine whether colonoscopy is effective for either early detection of colorectal cancer or prevention of mortality (death) from it has been published, and the only one in progress won’t show results until the mid 2020s… which doesn’t stop doctors from recommending it right now. So much for evidence-based medicine.
(Want to watch the webinar? Join my EmpowerEd health and nutrition education program for instant access – your first month is free!)
One of the key concepts that I discussed in the webinar is that of risk, and in particular the difference between relative risk and absolute risk. Understanding these concepts will make you a much more empowered health-care consumer. Chances are it will also dramatically reduce your fear level, which in turn helps you make more clear-headed decisions.
Why is this important? The stomach-churning terror provoked by the mere mention of the word ‘cancer’ persuades people to have screening and diagnostic tests that may have little to no chance of benefitting them, and then, in far too many cases, to undergo treatment that they don’t require, but which may damage their health permanently and even shorten their lives.
Some of my saddest moments in clinical practice have involved clients who were irreparably harmed by unwarranted medical intervention for what Dr Gilbert Welch calls ‘pseudodisease‘ – like the man in his 70s who suffered permanent bowel damage from radiotherapy to his prostate gland, for ‘treatment’ of a prostate-cancer-that-wasn’t – and who have told me, “If I knew then what I know now, I would never have gone for that test in the first place!”
I don’t want you to ever have to say those words, so please, bear with this short primer on risk:
Risk is defined medically as “the probability of developing a given disease over a specified time period“.
Absolute risk is the probability that a specified event will occur in a specified population.
Relative risk is the ratio of the risk of disease among those exposed to a risk or protective factor to the risk among those not exposed.
Let’s consider an example:
For women aged 40-70, the absolute risk of developing metastatic breast cancer or dying of breast cancer in their remaining lifetime is 4% – i.e. 4 out of 100.
Screening mammography produces a 14-32% reduction in the relative risk of breast cancer mortality, depending on age group. This equates to a 0.56% (14/100 x 4) – 1.28% (32/100 x 4) drop in absolute risk; in other words, dutifully attending your mammography screening appointment will reduce the risk of you dying of breast cancer by somewhere between six-tenths of a percent and 1-and-a-third percent.
Impressed? Me neither. Especially when the risk of overdiagnosis and overtreatment – being diagnosed with, and treated for ‘pseudodisease’ (a so-called cancer that would never have actually become a threat to life) – is so high. Here’s how it all stacks up:
Among 1000 50-year-old women undergoing annual mammography for 10 years,
- One woman will avoid a breast cancer death;
- Between 2 and 10 women will be overdiagnosed and treated needlessly (which may result in physical disfigurement and cause their premature death from a complication of treatment; for example radiotherapy and certain chemotherapies and hormonal therapies raise the risk of heart disease);
- Between 5 and 15 women will be told that they have breast cancer earlier than they would otherwise yet have no effect on their prognosis (so they live with the knowledge that they have cancer for longer, but don’t actually live any longer);
- Between 200 and 500 women will have at least one “false alarm” (50–200 will be biopsied), potentially causing prolonged anxiety about their health.
In other words, the risk of suffering some kind of harm from undergoing a screening mammogram, ranging from psychological distress right through to unnecessary treatment-induced death, outweighs the risk of suffering harm if you forego the mammogram.
Now, it’s up to you to weigh up the risks and benefits of any medical intervention, including cancer screening, and make your own decision. But that decision simply cannot be an informed one unless you have a good grasp of risk.
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