A new study offers the most solid evidence to date that radiation from CT scans increases children's risk of developing leukemia and head and neck cancer.
一项新研究提供了迄今为止最确凿的证据，证明CT扫描产生的辐射将增加儿童罹患白血病和头颈部癌症的风险。
Children and adolescents who received two to three computed tomography scans of the head were three times as likely to develop brain cancer as those in the general population, according to the study of 176,587 children.
6月6日《柳叶刀》（Lancet）杂志网站发表了这项对176,587名儿童所进行研究的结果，研究显示，接受过二至三次CT扫描的儿童和青少年患脑癌的概率是普通人的三倍，接受过五至10次头部CT扫描的儿童患白血病的风险则是普通人的三倍。
The study, published online Wednesday in the journal Lancet, also said the risk of contracting leukemia was three times as great for children who received five to 10 CT scans of the head.
这项研究的作者强调，整体患癌概率仍然非常低，而且CT扫描的直接好处（例如检查头部损伤）仍然大于风险。
The study's authors emphasized that the overall likelihood of getting the cancers remained very small and that the immediate benefits of the scans, such as in detecting head injuries, still outweigh the risks.
这项研究发现，每接受一次CT扫描，患癌风险就随之升高，即使对于只接受过一次CT扫描（以前被称为CAT扫描）的人，风险也有升高。
The report found cancer risk grew with each scan received, and there was an elevated risk even in those who received only one CT, formerly called a CAT scan.
以前的研究提出了对于CT扫描增加致癌风险的担忧；这份报告则提出了第一个关于这种联系强度的直接证据。它研究了英国国民健康服务（National Health Service）对22岁前接受过一次CT扫描的病人记录，并考察了他们平均10年后的病史。
Previous studies raised concerns about cancer risk from the scans; this report offers the first direct evidence of the magnitude of the link. It examined the U.K.'s National Health Service records of patients who had received a CT before age 22 and looked at their medical history for an average of 10 years afterward.
纽卡斯尔大学（Newcastle University）名誉主席、该论文的作者之一艾伦•克拉夫特（Alan Craft）称，"这篇论文证实，辐射，即使是相对低剂量的辐射，也会导致"罹患特定癌症的"风险。没有安全剂量。"
"This paper confirms that radiation, even in relatively low doses, does lead to risk' of certain cancers, said Alan Craft, emeritus chair at Newcastle University and an author of the paper. 'There is no safe dose."
增加的风险意味着，每10,000个做过CT扫描的10岁以下的病人，在第一次做CT扫描10年后患白血病的病例将多出一例，或者每30,000个做过CT扫描的10岁以下的病人，在第一次做CT扫描10年后患脑癌的病例将多出一例。
The increased risk corresponded to one additional case of leukemia in the 10 years after the first scan for every 10,000 patients younger than 10 who were scanned, or to an extra case of brain cancer for every 30,000 such people scanned.
CT扫描结合了一系列不同角度的X光，以提供骨骼和软组织的复合视图。该研究的重点是头部扫描，因为脑部被认为是对辐射高度敏感的部位。研究的考察对象是年轻人，因为他们的组织比成年人更易受影响，而且癌症可能需要很长时间才会形成。
CT scans combine a sequence of X-rays from different angles to offer a composite view of bone and soft tissue. The study focused on head scans since the brain is believed to have heightened sensitivity to radiation. It looked at young people since their tissue is more susceptible than adults' and cancer can take a long time to develop.
克拉夫特博士称，美国病人接受CT扫描的可能性是英国病人的五倍。在美国，2011年使用了8,530万次CT扫描，是20世纪90年代初的四倍。
Patients are five times as likely to receive a CT scan in the U.S. as in the U.K., said Dr. Craft. In the U.S., use of CT scans quadrupled to 85.3 million in 2011 since the early 1990s.
美国放射学会儿科放射成像委员会（American College of Radiology Pediatric Imaging Commission）主席马尔塔•舒尔曼（Marta Schulman）称，该项研究进一步支持了病人只应在必要时进行CT扫描，并且尽可能使用最低剂量辐射的扫描。
Marta Schulman, chair of the American College of Radiology Pediatric Imaging Commission, said the study adds to the impetus that patients should be scanned only when necessary and with the lowest dose of radiation possible.
美国正在努力限制对儿童过度使用CT扫描，包括2009年8月发起的"温柔成像"（Image Gently）倡议，该倡议由一个致力于改变医疗实践的医疗组织联盟牵头。
Efforts are under way in the U.S. to curb overuse of CT scanning in children, including the Image Gently initiative, launched in August 2009 and spearheaded by a coalition of health-care organizations dedicated to changing medical practice.
在6月6日发表的这项研究中，已经被诊断出癌症的病人或在做完CT扫描后二年内患上癌症的病人被剔除，以降低不接受CT扫描也会得癌的病人虚增风险计算结果的可能。
In Wednesday's study, patients who already had cancer diagnoses or who developed cancer within two years of a CT scan were excluded, to decrease the likelihood that risk calculations would be inflated by patients who would have developed cancer without getting a CT scan.
这项研究是五国机构研究人员的合作，它由英国卫生部（U.K. Department of Health）和美国国家卫生研究院（National Institutes of Health）共同资助。