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-and 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 snottynosed toddler stage; a second child will be exposed to one fewer; and so on.
　　
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　　脑癌：幸福家庭，危机暗伏

　　癌症会在家族中蔓延是大家熟知的；换句话说，你是从父母那里得到癌症的（或至少是易患癌症的倾向）。而你会从兄弟姐妹那里得到癌症的说法却更让人吃惊。但是位于海德堡的德国癌症研究中心的安德里亚•阿尔提耶里与同事们经过研究得出的结论。
　　阿尔提耶里博士本来是在寻找证据来支持这个观点，即至少部分脑癌是由病毒感染引起的，这让大家庭中的孩子面临更大危险，因为他们更容易暴露在儿童时期的病毒感染中。这个提议并不新鲜，但脑癌是罕见的，这使做系统研究变得很难。在任何科学领域，少量的样本会带来不可靠的结果。要给脑癌这样罕见的疾病找到可靠结论就需要整个国家的统计数据。
　　幸运的是至少有一个国家可以提供这样的数据：瑞典。在本期的《神经病学》上，阿尔提耶里博士描述了他在分析瑞典家庭癌症数据库中的记录之后的发现。其中包含1931年以来在瑞典出生的所有人，甚至还有他们在此前出生的父母。
　　在这之间的几十年中超过13，600瑞典人得了脑癌。在小家庭里一个人得脑癌的风险与他的兄弟姐妹数目没有关联性。然而在有5个以上的孩子的家庭里，这些孩子在他们生命中得脑癌的平均风险是那些没有兄弟姐妹的人的两倍。随着更深入的研究，阿尔提耶里博士有了更为惊人的发现。当他研究那些在儿童时期或青少年时期得脑癌的人时，他发现得病的几率更高--尤其是那些拥有许多弟弟妹妹的人。在15岁以下的拥有3个或更多弟弟妹妹的人比独生子得脑膜瘤的机会高3.7倍，并且研究人员认为他们得其他癌症的机会也明显更大。
　　这个发现为病毒和各种各样的癌症之间的联系补充了证据，也使阿尔提耶里博士和他的团队提出在童年时从弟弟妹妹那里传染来的病菌会导致脑癌。最初的假设是越多孩子在一起嬉闹，越多的身体、唾液和口的接触，感染的机会就越多。这已经在托儿所里证实了。但弟弟妹妹们比年长的兄弟姐妹更有影响力现在还只是猜测。根据经验来说挂着鼻涕的学步顽童比大一些的孩子更容易把疾病传染给家里其他人。所以可能重要的只是感染疾病的次数。最大的孩子要经历所有弟弟妹妹们的幼年阶段；第二个孩子将少经历一个；以此类推。