Why two thirds of cancer cases are not preventable

The lead

One of the most significant Science papers on cancer came out this month (Jan 2015). It shows that regardless of all future discovery about the nature of cancer, 65 percent of all people who develop cancer do so simply because we age.

Cell division is imperfect. That’s part of life. It if were perfect, evolution could not occur, and we wouldn’t be here trying to cure cancer. It is an unfortunate side effect of the process that created us, and it cannot be “cured.”

The argument

Vogelstein and Cristian Tomasetti used a mathematical formula to explain the genesis of cancer. Here’s how it works: Take the number of cells in an organ, identify what percentage of them are long-lived stem cells, and determine how many times the stem cells divide. With every division, there’s a risk of a cancer-causing mutation in a daughter cell. Thus, Tomasetti and Vogelstein reasoned, the tissues that host the greatest number of stem cell divisions are those most vulnerable to cancer. When Tomasetti crunched the numbers and compared them with actual cancer statistics, they fell along a straight line on the chart.

The statistics

There was a good linear correlation with strength r=0.81. The square of the correlation (r²) is the amount of Y predicted by X . In this case, Y is the risk of getting cancer and X is the number of cell divisions of this tissue in your lifetime. This, they concluded that this theory explained two-thirds (65%) of all cancers:

There is a linear pearson correlation of r=0.81 with an r-squared of 0.65, meaning variation in this chart explains two-thirds of cancer risk.

There’s more to that collection of dots than just a trend line. Look again:

Some types of cancer appear to occur more often than expected due to our environment. The red lines show how elevated each type is, presumably by our habits and environment. For example, smokers and non-smokers both get lung cancer, but smokers are much more likely to get lung cancer by the length of the red line connecting the two dots. The non-smokers’ lung cancer risk falls along the trend line, as expected.

The green lines are a little less intuitive. Cancers that fall below the trend line appear less often than expected because some body process is working to remove tumors in these tissues as they appear. The intestine, duodenum, and colon constantly slough off tissue as food passes through the digestive tract, so maybe any fledgling cancers leave the body this way. Likewise, AML and CLL are types of leukemia – white blood cell cancers. And something about blood cells in fluid make this type of cancer less frequent than expected.

All the cancers with ‘osteo’ with them are huddled in the lower left – low risk and low cell division – because bone cells rarely divide.

All of these deviations from the trend line represent logical explanations and strategies for preventing cancer. They only add up to 35% of the total explanation. But they’ll account for 99% of all the TV news headlines about cancer. That’s the sad part. Headlines focus on all the small deviations from the trend line, and ignore the existing of this trend line all together.

The message

The good news is that you can reduce your lifetime risk of getting cancer by up to 35% by eating well, exercising, avoiding smoking and drugs, and having a positive attitude about life and death. But we are never going to prevent cancer the way we ended small pox and (until recently) measles. That would require reversing the ageing process entirely, which is generally a bad thing.