CANCER AND AGE
Part of the explanation for the worldwide differences we’ve noted is how long we live. The longer we’re around the more likely we are to develop cancer (Fig. 1), even though we generally equate longevity with an idyllic lifestyle.
For the latter, according to the US Census Bureau, the place to be is Andorra where you’ll live to be nearly 84 (on average, that is); whereas if you’re a male living in some parts of Glasgow 54 is the cut-off age and in Swaziland the average life expectancy is under 32 years. So you would guess, therefore, that not many Swazis die of cancer – and you’d be right: fewer than 5% compared with about 30% of UK denizens. You might also guess that their spectrum of cancers is a bit odd and indeed we’ve already noted that in countries where the AIDS epidemic is out of control, Kaposi sarcoma is the most common cancer.
Both heart disease and cancers are diseases of old age. Men over the age of 50 who have never smoked are more likely to die of heart disease than anything else and more than two-thirds of cancers are first identified in individuals who are more than 65 years old. In general, the average age of populations is steadily rising both in the developed world and in developing countries. In the Bronze Age, the average lifespan was 18 years; the average worldwide is now 66 – and that has risen from about 35 in the early part of the twentieth century. So, of course, the heart disease and cancer figures are rising and the prediction that the figure of 10 million new cancer cases in 2000 will rise to 16 million in 2020 is perhaps not so surprising. By then there will be 30 million individuals with the disease, 70% of them in the developing world. Other factors are contributing to this progression, notable trends in smoking and unhealthy lifestyles. Of course, not all cancers are so protracted in development. The entire category of childhood cancers, by definition, only appears in the first 14 or so years of life. In addition, most adults will know of someone who has succumbed to a particularly aggressive cancer in early life. So cancers do strike the young but these are mercifully rare manifestations and, from an overall statistical point of view, cancers are indeed one consequence of getting old.
When either incidence or mortality figures are plotted against age as a double logarithmic plot they give straight lines (of the form y = a where ais age). The index n is the gradient of the log-log plot and (n + 1) represents the number of specific events that drive the development of the disease (Fig. 1.11). Between different types of cancer the number varies from four to seven but is typically six, indicating that, on average, acquisition of a major cancer-promoting event requires more than ten years. This type of deduction from cancer statistics was made 60 years ago and was the first evidence that a cancer cell contains a number of mutations that have accumulated over extended periods. The log-log plot shown is for USA white males but similar linear plots can be derived for all major cancers, regardless of country, sex and race. This indicates that although, as we have seen, the incidence of different types of cancer varies widely around the world, similar molecular events drive the development of all cancers.
We now know that cancers are the result of the cumulative effects of mutations in genes that control cell division and repair damaged DNA such that cells are released from the normal controls governing their proliferation and location.
So the major conclusion from this kind of finding is that, to a considerable extent, cancers are self-inflicted in that we have a degree of choice in the way we lead our lives: they are a consequence of our lifestyle – where we live and how we live. The problem with ‘lifestyle’ is that it is a bit tricky to define what is good and what is not. That notwithstanding, in the context of cancer susceptibility there are one or two basic guidelines, namely, be lucky enough to have a decent standard of housing and sanitation, eat well and keep reasonably physically fit. It’s also a good idea to live where airborne pollution isn’t an unseen hazard. That may be easier said than done, particularly given the considerable evidence that links auto emissions to breast and lung cancers. Living near a railway is probably OK – just don’t get a job maintaining any types of diesel that might run on it because at least one study shows that significantly increases your risk of lung cancer.