In terms of new cases, breast cancer is the commonest cancer in women, accounting for 21% of female cancer cases and 14% of female cancer deaths worldwide. The overall survival rate is, however, much better than for lung cancer, with three-quarters of sufferers in Europe and North America surviving 5 years. Even in less developed countries, over half of breast cancer patients will reach this milestone.
A study of the patterns of occurrence of breast cancer also helps to illustrate some of the ways cancer statistics can shed light on the behaviour of the disease.
The risk of getting breast cancer (as for most cancers) increases steadily with age, illustrated in Figure 2 with data from the UK.
Very similar distributions will be found in all developed countries. If we look at the left-hand axis in the figure – the actual numbers for each age – the peak numbers occur in the 50–70 age range – although their risk is higher, there are fewer women in the 70+ age groups due to deaths from other causes. As can also be seen, few women aged fewer than 40 is diagnosed with the disease, although fundraisers often use women from this age group in their promotional materials. Figure 3, looks at the distribution of cases from another angle, that of social class. This demonstrates that wealthier, better-educated women are at significantly higher risk than the less well off. Middle-aged educated women are often formidable campaigners, having both the time and education to lobby effectively. As we shall not see later in the book, cancer research nor treatment access are arranged purely on the basis of need but are often substantially influenced by lobby-group pressure on behalf of particular groups.
The figures on the worldwide risk of breast cancer again show some striking trends. Looking at Figure 3, there is a clear suggestion that breast cancer is in some way associated with affluence – richer countries have higher rates than poorer ones.
For the smoking/cancer link, there is a pretty clear relationship between consumption and risk. It is harder to see why higher average income should increase the risk of an illness – this is the reverse of most public health trends. So why should this be? One factor is the age structure of the population. As seen in Figure 4, risk of cancer increases with age. Hence a woman in a poor country with a low life expectancy may simply not live long enough to get breast cancer, having already died of another disease earlier in life. This does not account for the large range of risk seen, however. There are various theories about the observed underlying difference, and the most likely explanation relates to the effect of hormones on the breast tissue. For example, there are clear effects on cancer risk relating to age of first pregnancy and numbers of pregnancies. Late onset of puberty, early first pregnancy, and more frequent pregnancies are factors that appear to protect against breast cancer. In the West, puberty occurs earlier than in the past due to better nutrition and higher-protein diets, whereas pregnancy occurs later due to effective contraception, the increasing independence of women, and better education. In poorer countries, puberty occurs later and women have less control over their fertility. Whilst this situation of course brings all sorts of potential problems, it does appear to protect against breast cancer. Breastfeeding, which affects hormone levels post-delivery, also appears to protect against breast cancer, and being more prevalent among the better educated in the West may be predicted to skew the trend the other way. Fertility rates tend to drop and age at first pregnancy tends to rise as both national and personal income increases, so it may be expected that, as with lung cancer, increasing development will result in an increase in cases of breast cancer worldwide.