Peto’s Paradox

June 3rd, 2009 by Potato

Here’s an interesting question: if there’s some chance that any given cell in your body will turn cancerous per unit time, then if you have more cells, and you live longer, it follows that you have a higher chance of getting cancer. If you extend beyond a human to something big and long-lived, like an elephant or a whale, you wonder: why don’t all whales have cancer?

This is called Peto’s Paradox, and is an interesting one I just heard about.

In fact, cancer is not homogenous across species — humans get it at about 10 times the rate of any wild species. This is partly due to civilization: we don’t die as young from other natural causes, so cancer gets more of a shot to kill us, and of course our penchant for frolicking in toxins (pet dogs and St. Lawrence belugas also get cancer at a higher rate for similar reasons). But even then cancer is not homogeneous: various tissues have different propensities to cancer based in part on genetics, hormones, and environmental exposure (for instance, aside from skin cancers, there aren’t a lot of UV-light caused cancers). So in one sense part of the reason for the paradox is that one of the base assumptions — that any given cell has the same chance to turn cancerous — isn’t quite true.

But the special case of humans (and our domesticated animals) aside, why is it that a wild mouse and a wild whale still have fairly similar rates of cancer? Have whales evolved a resistance to cancer that we should investigate, or could it be related somehow to a fast/slow metabolism (there’s more than one research source that suggests a low-calorie diet for longevity). A recent paper suggests otherwise: that hypertumours (tumours that form inside other tumours) may come into play when you start dealing with larger tumours. After all, a golf-ball sized tumour can kill a person, but would probably go unnoticed in a whale, where it might take something the size of a volkswagen to sink it. In the time it takes that tumour to grow, perhaps a secondary tumour would spring up and feed on the first! It’s an interesting proposal, and the topic of a recent paper.

Complicating this is the interaction between cancer and infectious disease: certain viruses (such as HPV) can increase the likelihood of getting certain cancers. If viruses underlie more cancers than we think, it might explain why the cancer rate is so similar across species of such different sizes.


One Response to “Peto’s Paradox”

  1. Leslie Says:

    Posit that the human race is an organism, and, given a specific activity could be analgous to ‘cancer’, i.e., environmental destruction. Could Peto’s Paradox apply? Could the hypothetical cancer (environmental destruction) grow and cause other ‘cancers’ the longer the human race exists? Just one of those things to discuss after several beers.