The usefulness of cancer survival statistics

This is just a quick hit sort of a post.  I have another, longer one in the offing, which might go up later today, or another day, or never – it depends if I ever manage to get the ideas in my head onto the screen in an intelligible way, which is always a struggle for me at this time of year.  Anyway, the point is this stray thought is buzzing around in my head and distracting me from the other thing, so I want to get it set down and out of the way.  Right, here goes…

The BBC are reporting statistics newly compiled by Macmillan Cancer Support, which show that median survival times for cancer have increased from one to six years over the last four decades (though this headline figure of course disguises a lot of variability, with survival times for some cancers having barely changed, and others improving a great deal).  Ciaran Devan, the charity’s chief executive, is quoted as saying the following:

It is median survival times that give an accurate new picture of how long people might expect to live with different cancers.  Finally we can answer the big question: “How long have I got”?

Now, to be clear, I’m not a doctor, or an epidemiologist, or a statistician, or any kind of professional person who has expertise in interpreting information like this.  But I’m really not at all sure that median data like this actually make it possible to answer the question “How long have I got?”

The problem, as I see it, is that survivability for some cancers is heavily dependent on the time of diagnosis.  Some cancers, if diagnosed and treated early enough, have a very good prognosis, but are usually fatal if not diagnosed until after the disease has progressed.  Other cancers, while not fully curable, can be treated much more effectively if they are caught early, and so enable the patient to live longer after diagnosis than if they are caught later.  The problem with the median data from Macmillan (at least as it’s presented by the BBC) is that this factor seems to have been entirely excluded.

As a measure of the overall effectiveness of cancer treatment in England and Wales these figures are very interesting, useful, even, but for answering a specific individual’s question – “How long have I got?” – they don’t seem at all useful.  Knowing, for example, that the median survival time for colon cancer is 10 years doesn’t alter the fact that an individual with advanced disease may have a limited prognosis, but another individual may live very much longer (may, in fact, live long enough to die of something unrelated to their cancer).  I’m sure median survival times factor into an oncologist’s thinking when she is trying to answer a patient’s question of how long he has to live, but I’m equally sure other things factor in as well – not just the stage of the disease, but how well the patient has responded to treatment, their overall level of health, and so on.  It seems highly unlikely that any medical professional would base a prognosis on median survival times not broken down by stage, or recommend that a patient rely on such figures if they come across them in the course of their own research.

I also worry about the effect on patients of the chief executive of a respected cancer charity seeming to announce that these figures can be relied on by individuals trying to answer the question “How long have I got?”  I’m particularly concerned that people who actually have a very good prognosis might stumble across this and be persuaded by it that they have only a limited number of years to live when they in fact may survive to a ripe old age.  The particular irony is that one of Macmillan’s primary goals (via their helpline and network of support nurses) is to provide cancer patients with accurate and reliable information about their cancer.  It’s unfortunate that this particular announcement – or, at least, it’s coverage by the BBC – seems to run counter to that goal.

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One Response to The usefulness of cancer survival statistics

  1. Kapitano says:

    It sounds like Ciaran Devan either knows nothing about statistics, or cares more about getting his charity in the news than spreading information about cancer.

    Perhaps he thinks the median income is a way to answer the question “How rich am I?”.

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