Ok, let’s work out the statistics on this one.

We need a mix of US and UK populations, but that shouldn’t distort it too much. Let’s start with 1000 good people – 70 of them are going to be diagnosed with colorectal cancer during their lifetime.

Of that 70, 14 (20%) are going to have the late stage (Stage 4 or Dukes D) when they are diagnosed; where the cancer has shown up in a secondary location, often the liver.

Not all of these will be operable (a resection). In fact, only 3.5 (25%) will hear that verdict. These seem to be the relatively lucky ones.

After that it gets harder to interpret, but if I read Simmonds et al (2006) correctly then very roughly 2 of those 3 people will get the ‘all clear’ after 5 years, and the other 1 will find the spread was more virulent than the liver metastasis and they’ll have met a boa morte, are pining for the fjords, etc.

There is a new report  (June 2009) that puts a less rosy glow on matters. This is UK data based on 308,734 cases 1996-2006, but with the survival rate calculated up to 2002 (i.e. 2009 minus 5 years). This data showed an overall survival rate of 50% but with such large variations depending on stage that the average is in this case almost mis-informational. If you are diagnosed with Dukes A, you have a 93% survival rate, but that is only 6.6% if you are diagnosed with Dukes D.

The time series decline over the 5 years is pretty exponential – roughly 40%, 20%, 10%, 9% and 7%. The way I interpret that is similar to above – the metastasis was not as contained as hoped. But then I had a think about what Dukes D means in the context of that data and the fact is that it isn’t more specific than that. For example, it would include those with metastases in the lung, bone, liver, pancreas, etc as well as the unlucky people with instances in >1 place. If I was told I had secondaries in the liver and the bone, for example, I think I’d rate my chances as very low indeed.

It isn’t the fault of the NCIN study I suppose, but it left a bad taste that they wanted to promote one message (early diagnosis good) by using some pretty depressing stats for anyone reading with Dukes D.

As for the title:

You must not change one thing, one pebble, one grain of sand, until you know what good and evil will follow on that act. The world is in balance, in Equilibrium. A wizard’s power of Changing and Summoning can shake the balance of the world. It is dangerous, that power. It is most perilous. It must follow knowledge, and serve need. To light a candle is to cast a shadow.

Ursula K. Le Guin – A Wizard of Earthsea (1968)