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¡¡¡¡Brain cancer: Happy families, hidden dangers

¡¡¡¡IT IS well known that many sorts of cancer run in families; in other words you get them (or, at least, a genetic predisposition towards them) from your parents. The idea that you can get cancer from your brothers and sisters is more surprising. But that is the conclusion of a study conducted by Andrea Altieri and his colleagues at the German Cancer Research Centre, in Heidelberg.

¡¡¡¡Dr Altieri was looking for evidence to support the idea that at least some brain cancers are triggered by viruses and that children in large families are therefore at greater risk, because they are more likely to be exposed to childhood viral infections. This is not a new suggestion, but brain cancer is rare, and its rarity makes it hard to study systematically. In any field of science, small samples lead to spurious results. To find reliable answers to questions about something as infrequent as brain cancer, a whole country¡¯s worth of data is needed.

¡¡¡¡Fortunately, at least one country can provide those data: Sweden. And in the current issue of Neurology, Dr Altieri describes what he discovered when he analysed the records of the Swedish Family Cancer Database. This includes everyone born in Sweden since 1931, together with their parents even if born before that date.

¡¡¡¡More than 13,600 Swedes have developed brain tumours in the intervening decades. In small families there was no relationship between an individual¡¯s risk of brain cancer and the number of siblings he had. However, children in families with five or more offspring had twice the average chance of developing brain cancer over the course of their lives compared with those who had no brothers and sisters at all. Digging deeper, Dr Altieri found a more startling result. When he looked at those people who had had their cancer as children or young teenagers he found the rate was even higher¡ªand that it was particularly high for those with many younger siblings. Under-15s with three or more younger siblings were 3.7 times more likely than only children to develop a common type of brain cancer called a meningioma, and at significantly higher risk of every other form of the disease that the researchers considered.

¡¡¡¡This finding, added to evidence linking viruses to a broad mixture of different cancers, has led Dr Altieri and his team to propose that bugs caught during childhood from younger siblings, may, indeed, lead to brain tumours. The initial premise of the argument, that the more children there are to bite, spit, climb and cuddle together, the more infections they pick up, has been demonstrated in studies on nursery schools. But the mechanisms by which younger siblings have more influence than elder ones are speculative. Experience suggests that snotty-nosed toddlers pass their infections on to family members more frequently than older children do. So it could simply be the frequency of infection that is important. An eldest child will be exposed to all of his siblings¡¯ infections when they are at the snottyª²nosed toddler stage; a second child will be exposed to one fewer; and so on.

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