On things that "cause cancer"

6/16/2014 07:55:00 PM
New study shows that not being a Dwarf elevates cancer risk.
NBC News is claiming that sitting causes cancer:
"For every two hours spent sitting in front of the computer or television, the average person raises his or her risk of colon cancer by 8 percent, of endometrial cancer by 10 percent and of lung cancer by 6 percent."
They go so far ast o say that "sitting is the new smoking."

Ok, WTF? Hold on. Sitting is just like smoking? No. This has gone way, way too far.

It seems like the news is claiming that pretty much everything these days causes cancer. Being fat causes cancer. Having big breasts causes cancer. There's even a column at the New York Times claiming that being tall causes cancer. What's going on here?

None of these things actually cause cancer. At least, not in the way people seem to think. To be sure, these things are correlated with getting cancer, and those correlations are "causal" in the statistical sense--increasing your weight will increase your risk of getting cancer. But it's simply perverse to compare sitting to smoking.

Cancer follows a kind of Markov process: the body is made up of billions of cells, and each cell has some non-zero probability of becoming cancerous and spreading. But we don't measure the incidence of cancer as the number of malignant cancerous cells in society, but rather as the number of people who have cancerous malignant cells. A person is said to "have cancer" if at least one cell in their body has become malignant and cancerous, which means that your probability of having cancer is the union of the probabilities of each individual cell becoming cancerous--the more cells in your body, the higher the probability is that you'll get cancer. Tall people have more cells than short people, fat people have more cells than skinny people, large-breasted people have more cells than small-breasted people, and that's the only reason why any of these things are correlated with getting cancer.

But here's the thing. Suppose you are a doctor trying to diagnose someone with cancer, and you have a patient with a lump you suspect might be cancer. As Aaron Carroll has pointed out, no clinical test for cancer is going to be 100 percent accurate, so this is a largely a Bayesian process such that whether you believe the test result depends in part on your prior beliefs about their probability of having cancer. Suppose your patient is tall, has big breasts, and is fat. All of these things "cause" cancer, so should they affect your prior beliefs about whether the lump is cancer? NO! While these things are correlated with a person's over all chance of having cancer, they do not affect individual cells' chances of becoming cancer--hence, these studies say nothing whatsoever on the probability that that lump in your patient is cancer. And that's what is so strange about all these new studies claiming that these kinds of things cause cancer. If you follow their logic, the conclusion would seem to be that we should all endeavor to be as small as possible to minimize our risk of getting cancer. The logic leads to absurdities like amputating body parts that are no more likely than any other body part to have cancer in it, simply because that leaves you with fewer body parts to potentially get cancer.

Which brings me back to the NBC news article. I'm fairly certain that 100 percent of the effect that study detected was due to the fact that sitting is correlated with weight--people who sit less are getting more exercise, burn more calories, weigh less, and therefore have fewer cells overall, which is why their probability of developing cancer is lower. But, if someone who sits a lot also ate less and exercised more, they'd be able to completely avoid this added risk. That's in stark contrast to smoking, which elevates the risk of individual cells becoming cancerous, a risk which is present regardless of any compensating behaviors they adopt, which is to say that smoking is actually carcinogenic, while sitting is not. If a patient with a lung problem tests negative for lung cancer, then the fact that they smoke is a huge reason for doctors to ignore the negative test result.

What I'm trying to say is that sitting only causes cancer in the same sense that having all four limbs causes cancer.