Medical imaging exposes a significant portion of patients to various doses of ionizing radiation, and in some cases, to substantial doses, potentially increasing the associated risk of cancer, according to findings of a retrospective cohort study published in the Aug. 27 issue of the New England Journal of Medicine.

The results are based on an analysis of a database containing records of 952,420 nonelderly adults who were enrolled by a U.S. private health insurance provider, United Healthcare, between Jan. 1, 2005 and Dec. 31, 2007, and living in one of five U.S. markets: Arizona, Dallas, Orlando, South Florida, and Wisconsin.

Roughly 70% of the study population underwent at least one imaging exam during the 3-year study period, “resulting in mean effective doses that almost doubled what would be expected from natural sources alone,” wrote Dr. Reza Fazel, assistant professor of cardiology at Emory University, Atlanta, and her coinvestigators.

While most patients received less than 3 millisievert (mSv) per year – which was considered low exposure – there was a sizable minority of patients who received moderate, high or very high radiation doses, they wrote.

Procedure codes for imaging procedures involving radiation were used to identify claims from hospitals, outpatient facilities, and physicians’ offices. They excluded procedures in which radiation was specifically delivered for therapeutic purposes, such as high-dose radiation for cancer. Procedures were categorized by technique: plain radiography, CT, fluoroscopy (including angiography), and nuclear imaging. They also categorized the procedures by area of focus: chest (including cardiac imaging), abdomen, pelvis, arm or leg, head and neck (including brain), multiple areas (including whole-body scanning), and unspecified.

To account for the possibility of procedure overlap – for example, coronary stent placement and catheterization of the left heart performed at the same time – subjects were limited to one procedure per day that involved the same type of technique and the same anatomical area, selecting the highest dose.

Estimates of typical effective doses from published literature were used to approximate radiation exposure for each imaging procedure. The effective dose is an inexact measure of the overall detrimental biologic effect from radiation exposure.

Patients were stratified by gender and age: 18-34, 35-39, 40-44, 45-49, 50-54, 55-59, and 60-64; 52% were women. The researchers calculated effective doses for the population overall and for each age-based and sex-based group and categorized them by dose: low (no more than 3 mSv/year year, the background level of radiation from natural sources in the United States), moderate (3-20 mSv/year, the upper annual limit for occupational exposure for at-risk workers, averaged over 5 years), high (20-50 mSv/year, the upper annual limit for occupational exposure for at-risk workers in any given year, and very high (greater than 50 mSv/year).

A total of 3,442,111 imaging procedures associated with 655,613 patients were identified in the 3-year period. The average number of procedures per person per year was 1.2 and median number was 0.7/person per year. The mean effective dose was 2.4 mSv/person per year with a median effective dose of 0.1 mSv/year.

The proportion of patients undergoing at least one procedure during the study period increased with age – from 50% in those aged 18-34 years to 86% in those aged 60-64 years. A total of 79% of women underwent at least one procedure during the study period, compared with 60% for men (N. Engl. J. Med. 2009;361:849-57).

Moderate doses occurred at an annual rate of 199 per 1,000 patients. High and very high doses occurred at annual rates of 19 and 2 per 1,000 patients, respectively. “Each of these rates rose with advancing age,” noted Dr. Fazel.

“Generalization of our findings to the United States suggests that these procedures lead to cumulative effective doses that exceed 20 mSv per year in approximately 4 million Americans,” the researchers wrote.

Myocardial perfusion imaging accounted for almost a quarter of the total effective dose (22%). CT of the abdomen, pelvis, and chest accounted for 38% of the total effective dose.

“CT and nuclear imaging accounted for 21% of the total number of procedures and 71.4% of the total effective dose,” the researchers reported. By anatomical site, chest procedures accounted for 45% of the total effective dose. Lastly, the bulk of the total effective dose – 82% – was delivered in outpatient settings, primarily physicians’ offices.

The findings are concerning, particularly for patients who undergo several imaging tests in a short period of time, Dr. Michael S. Lauer wrote in an accompanying editorial (N. Engl. J. Med. 2009;361;841-3).

“Irradiation represents a direct danger imposed by a physician’s decision to refer a patient for imaging. Though the danger may be small, it is cumulative and hence of particular relevance to the small but substantial minority of patients, who ... undergo clusters of tests.”

Despite the cumulative risk associated with radiation exposure, it’s generally not something that is discussed with patients undergoing an imaging procedure, noted Dr. Lauer, who is director of the prevention and population sciences division of the National Heart, Lung and Blood Institute in Bethesda, Maryland. “The issue of radiation exposure is unlikely to come up because each procedure is considered in isolation, the risks posed by each procedure are low and seemingly unmeasurable, and any radiation-induced cancer won’t appear for years and cannot easily be linked to past imaging procedures.

“We have to think and talk explicitly about the elements of danger in exposing our patients to radiation,” wrote Dr. Lauer. Physicians will need to take a careful history to assess the cumulative dose of radiation that a specific patient has already received. This specific risk should be conveyed to the patient.

The study authors acknowledged the long-term risk, but noted that restricting patient dose – as is done for nuclear workers – is not feasible. “The exposure of patients cannot be restricted, largely because of the inherent difficulty in balancing the immediate clinical need for these procedures, which is frequently substantial, against stochastic risks of cancer that would not be evident for years, if at all.”

Dr. Fazel reported that she has no relevant conflicts of interest, though several of her coauthors reported significant relationships with pharmaceutical and medical imaging companies. Dr. Lauer reported that he has no relevant conflicts of interest.

Copyright (c) 2009 Elsevier Global Medical News. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

据发表在8月27日出版的《新英格兰医学杂志》的一项回顾性队列研究发现，医学影像学检查会给众多患者带来不同程度的电离辐射，对于某些患者，其遭受的辐射剂量很大，因此可能会提高其罹患癌症的风险。

 

此结果是对包括952,420位非老年成人的数据库分析后得出的，这一人群均于2005年1月1日至2007年12月31日在美国一家私立医疗保险公司(联合健康保险公司)参保，并来自以下地区：亚利桑那、达拉斯、奥兰多、南卡罗莱那和威斯康星。

 

在过去3年内，约70%的受试者接受过至少一次医学影像学检查，“而平均有效剂量几乎相当于自然放射剂量的两倍，”Reza Fazel和同事写道，她是美国亚特兰大Emory大学的心脏病学助理教授。

 

他们写道，虽然多数患者每年受到少于3 mSv的低度辐射，但仍有少数患者遭受中、高剂量或极高剂量的辐射。

 

对造成辐射的成像过程进行编码，用于识别来自医院、门诊和医生诊室的辐射。它们排除了用于特定治疗目的辐射，如用于治疗癌症的高强度辐射。根据技术不同，这些检查可分为：X线平片、CT、X光透视(包括血管造影)和核成像。编码也常按照射部分分类：胸部(包括心脏造影)、腹部、骨盆、上肢或下肢、头颈(包括大脑)、多部位(包括全身扫描)和未指定部位。

 

如遇到重复照射的可能，如同时进行冠状动脉导管置入和左心插管，则视为每日接受一种(包括同种影像检查技术、同部位的)操作，取最高剂量。

 

将已发表文献中提到的典型有效剂量的估计值，作为每种影像学检查的辐射暴露剂量约值。此有效剂量是一种对辐射暴露的总体有害生物学效应的不精确估测。

 

<根据年龄和性别对患者进行分层：18~34、35~39、40~44、45~49、50~54、55~59和60~64岁，52%为女性。研究者对人群总体、每个年龄段和不同性别组别的患者的有效剂量进行了计算，并根据剂量对其再进一步分组：低辐射组(≤3 mSv/年，美国来自自然界辐射的基础水平)，中度辐射(3~20 mSv/年，有辐射风险工作者的职业暴露的年度上限，平均超过5年)，高度辐射(20~50 mSv/年，有辐射风险工作者的职业暴露的任何一年的年度上限)和极高辐射(＞50 mSv/年)。

 

3年内，655,613例患者共接受3,442,111次影像学检查。平均每人每年接受1.2次影像学检查，中位次数为每人每年0.7次。平均有效剂量为每人每年2.4 mSv，而中位有效剂量为0.1 mSv/年。

 

在受试期间，每年至少接受1次影像学检查的比例随年龄增长而提高，从18~34岁年龄段的50%至60~64岁年龄段的86%。共79%的女性在受试期间接受过至少1次影像学检查，而男性为60%(N. Engl. J. Med. 2009;361:849-57)。

 

中等辐射的年度发生率为199/1,000例，而高度和极高辐射的年度发生率分别为19/1,000例和2/1,000例。“上述发生率亦均随年龄增长而增高，”Fazel博士指出。

 

“将我们的发现推及全美国，提示这些影像学检查导致美国有近400万人的每年累积有效辐射剂量超过20 mSv，”研究者们写道。

 

心肌灌注造影将近占到总有效剂量的1/4(22%)。腹部、盘腔和胸部CT占到总有效剂量的38%。

 

研究者报告：“CT和核成像占到所有影像学检查的21%和总有效剂量的71.4%。”根据解剖部位不同，胸部影像学检查占到45%的总有效剂量。最后，总有效剂量的大部分(82%)是在门诊完成的，主要在医生诊室。

 

这一发现是有意义的，尤其是对于在短期内接受数次影像学检查的患者来说，Michael S. Lauer博士在随后同期发表的编者按中写道(N. Engl. J. Med. 2009;361;841-3)。

 

“医生建议患者接受影像学检查，而辐射可带来的直接危害。虽然这种危害可以很小，但它是累积性的，对于极少数需进行多次影像学检查的患者来说有着特殊的意义。”

 

虽然放射暴露具有累积风险，但患者在接受影像学检查时，医生经常未与其谈及此事，Lauer博士说，他是美国马里兰州Bethesda国立心脏、肺和血液研究所的预防和人口科学部门的主任。辐射暴露的问题不易引起讨论，因为每次检查都被认为是在隔离状态下进行的，而每次检查带来的风险很小，可能都检测不到，另外辐射诱发的癌症数年后才会出现，即使出现也不会轻易与先前进行的影像学检查关联起来。

 

“我们必须对给患者带来的辐射进行认真地思考和讨论，”Leuer博士说。医生需要认真分析患者的病史，以评价每位患者之前所受辐射的累积剂量。这种特殊的风险应告知患者。

 

研究者虽然提出了这种长期风险，但他们也认为，像核工作者那样严格限制患者所受辐射的剂量是不可行的。“患者暴露剂量是无法限制的，这主要是因为要权衡临床急需这些的检查与因辐射诱发(直到数年后才会显现，而且还不一定出现的)癌症风险的难度常常很大。”

 

Fazel博士声明无相关利益冲突，而其共同作者报告与制药和医学影像公司有密切联系。Lauer博士声明无相关利益冲突。

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