Scientists at the American Association of Physicists in Medicine (AAPM) are offering additional background information to help the public avoid misinterpreting the findings contained in a report issued March 3 by the National Council on Radiation Protection and Measurements (NCRP), a non-profit body chartered by the U.S. Congress to make recommendations on radiation protection and measurements. The report is not without scientific controversy and requires careful interpretation.
NCRP Report No. 160 updates the 1987 publication, NCRP report No. 93, on the sources of ionizing radiation to the general public in the United States. It examines the various sources of ionizing radiation in the United States, estimates the total amount of radiation delivered in 2006, and compares those amounts to the estimates published in 1987. Among other findings, the naturally-occurring amounts of radiation have changed little in the last two decades. However, a key finding of the report is that there has been a dramatic increase in the amount of radiation from medical imaging procedures, including computed tomography (CT) and cardiac nuclear medicine examinations.
The report does not, however, "attempt to quantify the associated health risks nor specify the actions that should be taken in light of these latest data," and AAPM experts are cautioning that these data do not necessarily indicate that the U.S. population is at any higher risk due to this increased use of medical imaging. They caution that the new report should not deter patients from getting medically-appropriate imaging exams. The NCRP findings on average population dose could be easily misinterpreted if applied to an individual patient's medical situation.
"Tens of millions of CT scans are performed each year in the United States, and their benefits are enormous," says Gerald A. White, M.S., FAAPM, FACR, Chairman of the Board of the American Association of Physicists in Medicine (AAPM), the premiere professional and scientific association of medical physicists, which is comprised of research scientists and board-certified health professionals who specialize in the use of radiation in medicine.
Medical imaging, which includes procedures such as CT scans, cardiac catheterization studies, and nuclear medicine positron emission tomography (PET) exams, has revolutionized medicine in the last few decades. For example, CT scans provide pictures from deep inside a patient's body with unprecedented clarity. These images help doctors diagnose unseen illnesses and injuries, and they guide treatment for millions of patients annually in the United States.
"The medical information derived from CT scans literally saves thousands of American lives on a daily basis," says John M. Boone, Ph.D., FAAPM, FACR, Chairman of AAPM's Science Council and professor and vice chairman of radiology at the University of California, Davis Medical Center. "CT scans are critical for guiding the treatment of people who are in car accidents, people diagnosed with cancer, people who have blood clots in their lungs, and a vast number of other symptoms and conditions."
Even so, in the last few years reports in the medical literature and in the popular press have affected public perceptions of CT scans by raising questions of risk related to the use of X-rays, which in very high doses have the potential to damage cells and cause cancer.
The new NCRP report falls squarely into this controversy because it estimates the total U.S. exposure to all sources of ionizing radiation has increased six-fold since 1980 -- with about half of this increase due to CT scans.
This increase is easily misinterpreted, however, because the report calculates the total radiation dose for all CT scans performed in 2006 and divides that by the U.S. population for that year. What is not considered in this global averaging approach is that CT scans are given disproportionally to certain people and groups -- for example, the elderly, people admitted to hospitals for serious trauma, and cancer patients having scans to evaluate their response to treatment. The vast majority of Americans receive no radiation from medical imaging at all, or they receive imaging exams that do not use ionizing radiation, such as magnetic resonance imaging (MRI) or ultrasound procedures.
"Adding up all the doses and then spreading out the total over the entire population, no matter a person's age, occupation, location, or health status is not appropriate for assessing risk to the general population," says Cynthia McCollough, Ph.D., FAAPM, FACR, who is a professor of radiological physics at the Mayo Clinic and who chaired an AAPM Task Group that issued a CT radiation dose management report last year. "The NCRP report is very clear in this regard. The data summarize the sources and amounts of radiation exposure in the U.S. and the total values are normalized to the total U.S. population. The values reported are not appropriate for estimating potential health effects."
McCollough points out that the medical applications of CT have grown tremendously in the last few decades as the technology has become more and more sophisticated, often replacing more risky invasive or less accurate alternative tests. CT and other medical imaging procedures have nearly eliminated exploratory surgery and enabled minimally invasive surgery both which have shortened or eliminated hospitalization and reduced the risk of surgery related co-morbidity like infection. CT scanners have also reduced the volumes of radiation therapy fields, thereby reducing the probability of radiation harm, including second malignancies.
While the absolute number of CT exams has grown considerably since the 1980s, CT scanners can now tailor the radiation dose to the specific exam type and individual. All modern CT systems are now equipped with automatic exposure control systems that reduce patient dose levels to the minimum necessary for the examination.
In fact, adds McCollough, "The average dose per CT exam has fallen by a factor of 2-3 since the early 1980s. There are simply more people getting CT exams."
The AAPM strongly supports that appropriate utilization standards be applied for all procedures using ionizing radiation. AAPM members contribute to the safety and quality of CT imaging by developing reports such as Dr. McCollough's, which gives the most current standards for CT dose measurement techniques and discusses how facilities can reduce radiation dosages by adjusting the radiation exposure according to each patient's size. Steps like these are taken to ensure the maximum benefit to patients while minimizing their risk. In addition, medical physicists are required to be involved in ACR Accreditation programs, which ensure that radiation exposures are as low as reasonably achievable.
"For an appropriately ordered CT examination, an individual derives much greater benefit than risk," says AAPM past-president Richard L. Morin, Ph.D, FAAPM, FACR, who is a medical physicist and Brooks-Hollern Professor at Mayo Clinic Florida. "Ultimately, people who are scheduled to have CT exams should understand why their doctors have requested the exam – if the test will provide information to assist in their medical care, they should not worry about having the exam. There are likely higher risks associated with failing to have a needed medical test, as the correct diagnosis or treatment decision could be delayed or missed."
NCRP Report No. 160, "Ionizing Radiation Exposure of the Population of the United States" can be found at http://www.ncrponline.org/PDFs/Elec_prepub_160.pdf.
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