How do we know that we can prevent cancer?
The evidence that cancer can be prevented has strengthened considerably over the last century, due to the work of scientists around the world.
International differences and time trends
Almost every type of cancer is rare in some part of the world. The four most common UK cancers (lung, breast, bowel and prostate cancers) are very rare in sub-Saharan Africa. In contrast, liver cancer is very common in the developing world but rare in Western countries.
Two lines of evidence suggest that these international variations are mainly caused by differences in people’s lifestyles and environments, rather than their genes.
- Immigrants tend to acquire the cancer rates of their new host country quickly. For example, native Japanese women have low rates of breast cancer, but their risk doubles if they move to the USA, where breast cancer is more common. Within a few generations, both migrant and native populations have the same breast cancer risks.
- Cancer rates can change very quickly within a single country. Lung cancer rates in the UK rose ten-fold in the twentieth century after smoking became increasingly popular, but fell again as more and more smokers started to quit.
Can’t these trends be explained through genes?
Genetic changes cannot possibly account for such large increases in cancer rates over just a few generations.
Identical twins who share the same genes only have a one in ten chance of developing the same type of cancer. This shows that inherited genes have a relatively small impact on the population’s cancer risk.
Identifying the causes of cancer
In the 1950s, Sir Richard Doll and Austin Bradford Hill convincingly showed that smoking causes lung cancer and other diseases, setting the stage for more work on the preventable causes of cancer.
In 1981, Doll, together with Sir Richard Peto, published a landmark paper that concluded that most cancers are caused by lifestyle factors such as smoking, alcohol, unhealthy diets and more. Other environmental factors like pollution affected cancer risk, but to a surprisingly small extent compared to these lifestyle factors.
This work provided a valuable starting point for thousands of studies on lifestyle and cancer. Smoking, alcohol and unhealthy diets were confirmed as causes of cancer, while obesity and physical inactivity were later added to the list.
In 2011, a new study re-estimated the effects and showed that more than four in 10 cancer cases are caused by tobacco, alcohol, diet, overweight, inactivity, infection, radiation, occupation, post-menopausal hormones or breastfeeding.
Different types of studies
Scientists look at large groups of people, using different types of study, to work out if something can increase or reduce the risk of cancer.
- Case-control studies are the most common type - they ask, “Why did these particular people get cancer?” It looks at hundreds or thousands of people with a particular cancer, and compares them with a similar number of people without it. The two groups are matched as closely as possible in other respects, such as age, sex and ethnicity. Researchers then compare their past medical histories and lifestyles to see if there are any differences that might be linked to the disease. These studies are valuable but they can be biased - often, people are asked to remember things that they did many years ago.
- Cohort studies can help scientists to get round these problems. These ask “Which of these people will get cancer?” They recruit large numbers of healthy people, take detailed information about their lifestyle, diet and medical history, and often take blood and urine samples.They then follow these people for a long time to see what happens to their health and how this is linked to their lifestyles. Usually, cohort studies are less biased than case-control ones but they are also more expensive and difficult to set up. Since no one can know which participants will eventually become ill, researchers need to recruit tens of thousands of people or more to begin with, especially if the cancer they are studying is rare.
- Meta-analyses can produce even more reliable answers, by combining the results of several different studies and looking at them together. These studies ask “On the whole, what does the evidence show?” If many individual studies agree on a conclusion, it is more likely to be correct.
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