CT增加致癌风险

CT增加致癌风险

 

作者：鲍勃-海曼

为了更准确地诊断患者，医生正在增加使用计算机断层扫描（CT ）。在英国每年要做300万人次CT，美国人均大约是英国的5倍以上。

做这些扫描的人增加癌症风险的关切增加了。CT给医生提供了比传统X射线更清晰的身体内部的图像。但是机器也送出了更高剂量的电离辐射，不仅导致组织损伤，也增加癌症风险。风险与接受的辐射剂量成正比。

风险随着反复曝光而积累，儿童比成人受的影响更大。因为CT需要身体同一个部位的很多图像，比传统的X射线发送的辐射更多。比如50倍辐射的腹部探测，50倍仍然是一个小数字。估计一个腹部探测发送的电离辐射大约相当于一个人一年接受环境辐射的6倍。

对儿童的风险

英国一个大型研究的作者发现，暴露在高剂量CT辐射下的儿童，比那些接受低剂量的面临发生白血病、脑瘤的风险增加三倍。接受辐射剂量越高的儿童，发生这些癌症的几率越高。

在17年中18万做过扫描的儿童中，有200人被诊断得了癌症。这200名儿童中，大约有170人的癌症是因为高剂量 CT辐射造成的。作者认为他们受辐射影响最大。

总体估计风险是18万分之170，大约千分之一。这里和类似的研究有一个方法论问题。大约3万儿童不能包括在数据分析中，主要因为他们的病历不完整，他们的疏漏可能影响结果。

虽然研究人员试图这样做，我们不能改变因果关系，做更多CT的那些儿童是因为他们有潜在的健康问题，反之亦然。研究人员排除了那些怀疑有癌症而做CT扫描的儿童。

作者认为，这个方法是有局限性的。做 CT扫描的儿童可能不同于那些不是不知道这个方法，使前者更容易发生癌症。彻底排除因果关系的唯一方法是做随机对照实验。这要有大量儿童参加，分为高剂量 组和低剂量组，长期随访，看每组发生癌症的多少。当然，如果这样做的话完全是不道德的。结果也不包括做CT扫描的儿童有得其他疾病更高风险的可能性，或者 比研究认为的时间更长。

逆向因果关系可能导致夸大扫描风险，那些做更多扫描的儿童如果不暴露在额外的辐射中，可能也会更容易得癌症。另一方面，排除其他条件和更长时间，可能导致扫描风险被低估。总体来说，研究得出的风险估计是不确定的。

用与上述英国类似的数据，美国的研究人员总结说，大约有2%的美国癌症患者可能是CT扫描辐射造成的。这个结论与上面详细表述的相同的警告是一致的。美国健康记录中央数据库也没有增加更多并发症风险评估。

把风险转入病例数目

如果估计大体上是正确的，当前英国的CT扫描用量大约是美国的五分之一，那么英国的癌症患者有0.4%是由CT扫描造成的。

读者可能认为这个风险很低，是可接受的，但这意味着英国每年新增的35万癌症患者，有1400人是因为CT扫描造成的。

考虑到医疗增加的趋势，这个比率未来还会增加。大多数专家相信，与CT扫描有关的风险大大超过了它们带来的好处。这个结论看来可能是令人担忧地试探性的。

大多数决策风险需要价值判断，不能仅用自身做比较，在这个案例中，CT扫描的临床好处与那些执行中面临额外癌症风险的人，包括健康的人。

向患者和父母提供CT扫描的医生要解释为什么需要，有没有不暴露于辐射的替代方案可供使用。

 

原文网址 http://www.dailymail.co.uk/health/article-3658669/CT-scans-blamed-causing-cancer-actually-increase-risk.html

By Professor Bob Heyman For The Conversation

Published: 10:51 GMT, 27 June 2016 | Updated: 12:46 GMT, 27 June 2016

Doctors are increasing using computerised tomography (CT) scans in order to more accurately diagnose patients.

In the UK, about three million CT scans are carried out each year, and the rte per person is around five times higher in the US.

However, concerns have been raised about the increased risk of cancer in people who have undergone these scans.

A CT scan (or 'CAT scan') provide doctors with a much clearer picture of what is happening inside the body than conventional X-rays.

But these machines also deliver a much higher dose of ionising radiation

This not only causes tissue damage, but can increase the risk of cancer.

The increase in risk is proportional to the amount of radiation received.

Risk accumulates with repeated exposure, and children are more susceptible than adults.

Because CT scans require many images to be taken in the same body area, they deliver more radiation than a conventional X-ray.

For example, 50 times more radiation in the abdominal area has been detected. But a 50-fold increase in a very small number is still a small number.

It has been estimated that an abdominal scan delivers about six times as much ionising radiation as a person would receive from the environment in a year.

THE RISKS TO CHILDREN

Authors of a large UK study found that children exposed to higher radiation doses from CT scans faced threefold increases in their risks of developing leukaemia and brain tumours compared with those who received lower doses.

The higher the amount of radiation the children received, the greater their chances of developing these cancers were.

The number of children diagnosed with either cancer over a 17-year period was about 200 out of 180,000 who were scanned.

About 170 of these 200 children would, according to the researchers, have developed cancer as a result of having had higher radiation exposure from CT scans.

They were chosen because the authors considered them to be most affected by radiation exposure.

The overall estimated risk was therefore 170 in 180,000, or about one in a thousand.

But there are methodological problems with this and similar research.

Some 30,000 children could not be included in the data analysis, mainly because their medical records were incomplete, and their omission might have affected the findings.

And, although the researchers attempted to do so, we cannot rule out 'reverse causation', that is, some children having had more CT scans because they had underlying health problems, rather than vice versa.

The researchers excluded children who were given CT scans because cancer was suspected.

 

 

Risk accumulates with repeated exposure, and children are more susceptible to damage than adults

And as the authors acknowledge, this approach has limitations.

Children who receive CT scans may differ from those who do not in unknown ways which also make the former more likely to develop cancer.

The only way to definitively exclude reverse causation would be to do a randomised controlled trial.

This would involve randomising children into higher and lower exposure groups and following them up over a long period to see how many in each group developed cancer.

This would, of course, be totally unethical.

Also, the findings don't exclude the possibility that children who have CT scans become at greater risk of experiencing other diseases, or over longer time periods than were considered in the study.

Reverse causation might result in the risk from scans being overstated because the children who had more scans might have been more likely to develop cancers even if they had not been exposed to extra radiation.

 

Most experts believe that the risks associated with CT scans are greatly outweighed by their benefits

On the other hand, exclusion of other conditions and longer time periods might lead to the risk from scans being understated.

Overall, there is considerable uncertainty about the risk estimates generated by the study.

Using UK data similar to that outlined above, researchers in the US concluded that about 2 per cent of US cancers would be caused by radiation from CT scans.

This conclusion is subject to the same cautions as detailed above.

Also, the absence in the US of centralised databases of health records adds further complications to risk estimates.

TURNING THE RISK INTO A NUMBER OF CASES

If the estimate is approximately right, and current UK usage of CT scans in the UK is about a fifth of the US level, then about 0.4 per cent of UK cancers would be caused by exposure to CT scans.

Although readers may view this risk as acceptably low, it would mean that about 1,400 of the 350,000 annual new cancer cases estimated to occur in the UK would result from CT scans.

Given the trend towards increasing medical usage, this rate can be expected to increase in the future.

Most experts believe that the risks associated with CT scans are greatly outweighed by their benefits.

This conclusion might seem worryingly tentative.

However, most decisions about risks entail value judgements because like is not being compared with like, in this case the clinical benefits of having CT scans to the additional cancer risk faced by those who undergo them, including otherwise healthy people.

Patients and parents offered a CT scan should ask their doctor to explain why it's needed, and whether alternatives not involving exposure to radiation could be used.