1mSvミリシーベルトが平均的な米国人が1年で受ける放射線と概算されている。ＵＣＳＦのDr. Rebecca Smith-Bindmanらの研究により、頭部スキャンでの2mSvからmultiphase腹部骨盤スキャンの31mSvまでと大きな開きがある。10mSvの低容量被爆は広島・長崎の原爆被爆生存者の間で増大した発がんリスクとなっている。過去20年で、1980年の300万回から2007年の7000万回へとＣＴスキャン施行は急増した。
検査部位や年齢・性別などにより将来のリスクは大きく異なる。冠状動脈血栓症CT血管造影の場合、40才女性では270人に1人がガンを発症し、男性では595人に1人となる。20才だとリスクが2倍となり、60才では1/2となる。Berrington de Gonza'lezらの研究は全国研究協議会の「電離放射線の生物学上の効果」リポートに基づいたリスクモデルを使って、概算された。
ＭＧＨの放射線部長Dr. James Thrallによれば、人口の1/4は放射線被曝にかかわらずガンになるので、この分析のうちの約1,500万人はどっちみちガンになるだろうという。
医療画像検査による放射線被爆と発ガン性／米国医療事情 ＣＴスキャン ＰＥＴ
CT Scan Radiation May Lead to 29,000 Cancers, Researchers Warn
Popular Diagnostic Scans May Be Overused, Some Worry
By TODD NEALE MedPage Today Staff Writer
Dec. 15, 2009
Radiation doses from computed tomographic (CT) scans are higher than previously thought and vary widely, even among the same types of examinations, two studies showed.
Radiation from medical tests is generally measured in units called millisieverts, or mSv. One mSv is equivalent to the estimated dose of background radiation the average American absorbs in a year.
In the first study, based on information from four San Francisco-area hospitals, median effective doses ranged from 2 mSv for a routine head scan to 31 mSv for a multiphase abdomen and pelvis scan, according to Dr. Rebecca Smith-Bindman of the University of California San Francisco, and her colleagues.
Radiation doses as low as 10 mSv have been linked to an increased cancer risk among survivors of the Hiroshima and Nagasaki atomic bomb blasts, the researchers reported in the latest issue of the journal Archives of Internal Medicine.
The issue is important, investigators say, because over the past two decades, the use of diagnostic CT has skyrocketed from 3 million scans nationwide in 1980 to 70 million in 2007.
In the latest study, exposure varied widely both within and between hospitals. In many cases, a 13-fold difference existed between the highest and lowest dose for each study type, "highlighting the need for greater standardization across institutions," the researchers wrote.
In addition to standardizing procedures, they suggested reducing the number of unnecessary CT scans and tracking dose information for individual patients to lower overall radiation exposure from medical imaging.
A second study, by Amy Berrington de Gonza'lez of the National Cancer Institute in Bethesda, Md., and colleagues, projected that 29,000 future cancers will be directly attributable to CT scans performed in 2007.
In an accompanying editorial, Dr. Rita Redberg of the University of California San Francisco, editor of Archives of Internal Medicine, said the two studies "make us question if we have gotten carried away in our enthusiasm" for the use of CT.
It's becoming clear, she said, that the large doses of radiation from CT scans will lead to additional cancers, which must be taken into account when physicians consider use of CT for their patients.
"In light of these data, physicians (and their patients) cannot be complacent about the hazards of radiation or we risk creating a public health time bomb," she wrote.
In a statement, Robert Smith, director of cancer screening for the American Cancer Society, urged caution when interpreting the results of the studies.
"As striking as these numbers are, we need to address this issue with common sense," he said. "It is important that patients not refuse a CT scan when one is indicated."
But he said the results have to be taken seriously.
"They remind us that the potential benefits of a CT scan have to outweigh the possibilities of harm."
The projected impact on future cancer risk varied by exam type, age, and sex. For example, an estimated 1 in 270 women who undergo CT coronary angiography at age 40 will eventually develop cancer directly related to that scan, the researchers said; for men, the estimated rate was 1 in 595.
Risks were roughly doubled in 20-year-olds and halved in 60-year-olds.
"CT is generally considered to have a very favorable risk-to-benefit profile among symptomatic patients," Smith-Bindman and her colleagues wrote. "However, the threshold for using CT has declined so that it is no longer used only in very sick patients, but also in those with mild, self-limited illness who are otherwise healthy. In these patients, the value of CT needs to be balanced against this small but real risk of carcinogenesis resulting from its use."
In the study by Berrington de Gonza'lez and colleagues, the risk of future cancers from CT scans in 2007 was estimated using risk models based on the National Research Council's "Biological Effects of Ionizing Radiation" report.
Of the 29,000 projected cancers -- or about 2 percent of all cancer diagnoses -- most were expected to be caused by scans of the abdomen and pelvis (14,000), chest (4,100), and head (4,000). About 2,700 would be caused by CT coronary angiography.
The most commonly caused cancers would be those of the lung (6,200) and colon (3,500), as well as leukemia (2,800).
Acknowledging the estimates involved several assumptions, the researchers wrote that "further work is needed to investigate the balance of the risks and benefits from CT scan use and to assess the potential for dose or exposure reduction."
But in an interview, Dr. James Thrall, radiologist-in-chief at Massachusetts General Hospital and chair of the American College of Radiology's board of chancellors, said the projections in the Berrington de Gonza'lez study are likely overestimates, he cautioned, which could scare patients and physicians away from using necessary CT scans.
"Radiation should never be given unnecessarily," said Thrall, who noted that he, too, believes there are too many CT scans performed each year.
"If a scan is necessary, it should be done with the lowest dose possible," he said, "and before a scan is done the reasons for doing it should be challenged and subject to appropriateness review."
He also attempted to put the 29,000 projected cancers into context. Because about a quarter of the population will develop cancer with or without radiation exposure, about 15 million people in the analysis would develop cancer anyway.
"In that context, 29,000 is a very small number when you put it up against the immediate benefits to the patients from the scans they receive," he said.
Cancer Risks and Radiation Exposure From Computed Tomographic Scans
How Can We Be Sure That the Benefits Outweigh the Risks?
Rita F. Redberg, MD, MSc
Arch Intern Med. 2009;169(22):2049-2050.
The introduction of the computed tomographic (CT) scanner ushered in a new era of internal medicine diagnosis. Conditions that once required laparoscopy for diagnosis could now be diagnosed on the radiology reading board. The previously opaque anatomy of the living brain could now be visualized. The best part was that the test was "noninvasive," fast, and painless. With the exception of patients who were allergic to dye or had renal insufficiency, CT was considered completely safe. No wonder it had such a significant effect on the practice of medicine.
Two articles in this issue of the Archives make us question if we have gotten carried away in our enthusiasm. Every day, more than 19 500 CT scans are performed in the United States, subjecting each patient to the equivalent of 30 to 442 chest radiographs per scan. Whether these scans will lead to demonstrable benefits through improvements in longevity or quality of life is hotly debated. What is becoming clear, however, is that the large doses of radiation from such scans will translate, statistically, into additional cancers. With CT scan use increasing annually, it is imperative that clinicians take into account the radiation risks when assessing the benefit to their patients.
The number of CT scans is remarkable: a recent study of nearly 1 million nonelderly adults showed that 70% received CT scans during the 3-year period of study (2005-2007).1 There were an estimated 72 million CT scans conducted in 2007 alone.2 The doses of radiation from them also are eye opening. Although most patients receive relatively low doses from their scans, nearly 20% of the study's population received "moderate" exposures of between 3 and 20 mSv, and some 2% (translating to as many as 1.4 million patients nationwide) were exposed to "high" and "very high" doses of 20 mSv to more than 50 mSv.
What risks, then, are posed by radiation exposure from CT scans, and are such risks justified?
Two studies in this issue of the Archives help inform this discussion by providing actual effective radiation doses in the most commonly used CT scans and the cancer risks associated with this radiation. Smith-Bindman and colleagues3 collected actual data on radiation dosages for the most commonly used CT scans at 4 institutions in the San Francisco Bay area in California in 2008. They found a surprising variation in radiation dose―a mean 13-fold variation between the highest and lowest dose for each CT type studied (range, 6- to 22-fold difference across study types). There was no discernable pattern to the variation, which occurred within and across institutions. The investigators found a median effective dose of 22 mSv from a typical CT coronary angiogram and 31 mSv for a multiphase abdomen-pelvis CT scan. At one institution, exposure was a staggering 90 mSv for a multiphase abdomen-pelvis CT scan.
Even the median doses are 4 times higher than they are supposed to be, according to the currently quoted radiation dose for these tests. Just 1 CT coronary angiogram, on average, delivers the equivalent of 309 chest radiographs. From their data, Smith-Bindman et al3 estimated the risk of cancer, taking into consideration age, sex, and study type. By their calculations, 1 in every 270 forty-year-old women undergoing a CT coronary angiogram will develop cancer from the procedure.
In a second study, Berrington de Gonza'lez and colleagues2 determined CT scan use frequency using data from a large commercial insurance database, Medicare claims data, and IMV Medical Information Division survey data. They estimated there were 72 million CT scans performed in 2007. Excluding scans conducted after a diagnosis of cancer and those performed in the last 5 years of life, Berrington de Gonza'lez et al2 projected 29 000 excess cancers as a result of the CT scans done in 2007. These cancers will appear in the next 20 to 30 years and by the authors' estimates, at a 50% mortality rate, will cause approximately 15 000 deaths annually.
In other words, 15 000 persons may die as a direct result of CT scans physicians had ordered in 2007 alone. Presumably, as the number of CT scans increase from the 2007 rate, the number of excess cancers also will increase. In light of these data, physicians (and their patients) cannot be complacent about the hazards of radiation or we risk creating a public health time bomb.
The effort to avoid unnecessary excess cancers must be multifaceted. First, radiation protocols should be improved to eliminate the 13-fold difference in radiation dose for the same CT scan; exposures will be significantly reduced if all institutions were to use the lowest-dose technique. Smith-Bindman and colleagues3 found, for example, that the "usual" protocol sometimes unwittingly increased radiation. The authors offer several techniques to improve the quality of CT scans. In addition, patients should be fully informed about the radiation risk; it is unlikely that many patients now appreciate that a single CT scan may represent the radiation equivalent of hundreds of chest radiographs.
A popular current paradigm for health care presumes that more information, more testing, and more technology inevitably leads to better care. The studies by Berrington de Gonza'lez et al2 and Smith-Bindman et al3 counsel a reexamination of that paradigm for nuclear imaging. In addition, it is certain that a significant number of CT scans are not appropriate. A recent Government Accountability Office report on medical imaging, for example, found an 8-fold variation between states on expenditures for in-office medical imaging; given the lack of data indicating that patients do better in states with more imaging and given the highly profitable nature of diagnostic imaging, the wide variation suggests that there may be significant overuse in parts of the country.4 For example, a pilot study found that only 66% of nuclear scans were appropriate using American College of Cardiology criteria―the remainder were inappropriate or uncertain.5
The articles in this issue make clear that there is far more radiation from medical CT scans than has been recognized previously, in amounts projected to cause tens of thousands of excess cancers annually. Also, as these scans have become more sensitive, incidental findings lead to additional testing (and often more radiation), biopsies, and anxiety. Although a guiding principle in medicine is to ensure that the benefit of a procedure or therapy outweighs the risk, the explosion of CT scans in the past decade has outpaced evidence of their benefit. Although there are clear instances when CT scans help determine the treatment course for patients, more and more often patients go directly from the emergency department to the CT scanner even before they are seen by a physician or brought to their hospital room. To avoid unnecessarily increasing cancer incidence in future years, every clinician must carefully assess the expected benefits of each CT scan and fully inform his or her patients of the known risks of radiation.
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