PROSTATE CANCER BETWEEN DIFFERENT POPULATIONS
Men living in Europe and North America have a strikingly higher death rate than men living in Indo-China, where the disease is relatively rare, as it is also in most of Africa. Within Europe and North America, there are further interesting variations, with increasing risk of death with increasing distance from the Equator, an effect best seen in the white populations of the United States and Australia where the ethnicity of the white population is fairly uniform. If we look at ethnic effects, there are also striking variations, with men of African origin having, roughly, double the risk of prostate cancer death than white men. In contrast, men of Indo-Chinese descent retain the lower risk of their regions of origin, similar to the effect seen with women and breast cancer.
How can we explain this? The best evidence suggests that the differences between the white and Asian populations are driven by differences in diet plus a difference in racial sensitivity to whatever causes prostate cancer (which is largely unknown). The variation with latitude is much harder to explain by diet and clearly is not explained by race, as it can be observed in Europe, North America, and Australia. The best explanation seems to be exposure to sunlight, with sun exposure being protective. This is a very surprising conclusion, given the widespread public health campaigns aimed at reducing people’s exposure to the sun. How may sun exposure affect the risk of cancer in an internal organ about as protected from the sun as it is possible to be? The answer appears to be vitamin D. Lack of vitamin D leads to rickets and conjures up images of Victorian workhouses and deformed children, but the 21st-century version of the disease may be an increased risk of cancer, as summarized in the box.
If this effect is present with prostate cancer, clearly mediated by circulating factors generated in the skin, could it be seen with other cancers as well? The answer appears to be ‘yes’, and the effect size seems to be similar for pretty much all cancers of internal organs. The only cancers that are increased by sun exposure are those of the skin (specifically melanoma), which actually kill relatively few people. The study of prostate cancer death rates, thus, sheds all sorts of interesting light on the causation of common cancers, and has thrown up a very surprising connection that fundamentally challenges current standard public health advice. In the opinion of the author, the accepted wisdom on sun exposure is overdue for radical revision.
There is a second striking set of differences in the diagnosis and death rates. If we compare, say, the UK and the USA, we see very similar death rates but very different diagnosis rates per 100,000 population, with more than twice as many cases diagnosed per death from prostate cancer in the USA as in the UK. Looked at in another way, a far lower percentage of men, with prostate cancer, die from the disease in the USA than in the UK.
There are a number of possible explanations – prostate cancer may truly be more common in the USA, and the US healthcare system twice as good at treating it as the UK system. Whilst it is true that the UK healthcare system delivers slightly inferior outcomes compared to the US system, these differences for most cancers are of the order of a few percentage points and are unlikely to explain the apparent difference in cure rates. Furthermore, if we look at rates of detection for other common cancers, the UK and USA have generally similar numbers per 100,000 population, suggesting that other factors are operating. The explanation lies in the PSA blood test.
Differences in public policy and availability of PSA tests have resulted in far fewer men being tested in the UK than the USA, with a consequently lower rate of diagnosis of the disease. However, most men diagnosed in the USA, where there are high rates of PSA blood testing, have clinically trivial disease. This may never have troubled them had they not been diagnosed with it, suggesting the large difference in incidence is largely driven by higher rates of diagnosis of low-grade, relatively non-lethal disease in the USA compared to the UK. Both sides of the Atlantic, a smaller number of men are diagnosed and eventually die from more aggressive forms of the disease. Since the late 1990s, death rates have been falling, but whether this is down to screening directly or to other factors is hotly debated.