Radiation Exposure From Medical Imaging

Abstract

THE AVERAGE RADIATION DOSE TO WHICH PERSONS IN the United States are exposed has doubled over the past 30 years. Although the average dose from natural background sources has not changed, the average radiation dose from medical imaging has increased more than 6-fold. Medical imaging now contributes about 50% of the overall radiation dose to the US population, compared with about 15% in 1980. The largest contributor to this dramatic increase in population radiation exposure is the computed tomography (CT) scan. In 1980 fewer than 3 million CT scans were performed, but the annual number now approaches 80 million and is increasing by approximately 10% per year. Because CT scanning involves acquiring multiple images, CT scans result in a far larger radiation dose to the patient than most other common radiographic procedures such as chest x-rays or mammograms. Although CT is responsible for most of the rapid increase in population exposure from medical imaging, other radiographic imaging and nuclear medicine procedures are also increasing rapidly, particularly in cardiology. Newer radiographic imaging modalities such as positron-emission tomography CT (PET/CT), singlephoton emission CT (SPECT/CT), and, potentially, CT screening of asymptomatic patients are likely to increase the population radiation exposure still further. This increase in radiological imaging and nuclear medicine certainly has revolutionized medical practice in a fundamental and highly beneficial manner. However, like almost all medical procedures, medical imaging has benefits and risks, and the goal is to provide the public with the optimal benefit/risk balance. The risks associated with radiation doses typical of CT scans are not yet fully quantified, but there is persuasive evidence, at the doses relevant to CT, that the risks of radiation carcinogenesis are real, although small for any individual. The concern arises when an increasingly large population is exposed to small individual risks. Regardless of the actual magnitude, these population risks would undoubtedly be reduced if radiation doses were optimized for each procedure and if medically unnecessary imaging were minimized. Although it is impossible to imagine contemporary medicine without modern medical imaging, there are serious issues of quality control, training, and, particularly of overutilization that can best be addressed through national legislation. In fact, radiation exposure from medical radiographic imaging is comparatively unregulated; this is in striking contrast to radiation exposure in occupational settings, which is stringently regulated despite it contributing a far smaller population exposure. The current US situation is that quality control and quality assurance for x-ray machines and facilities are the responsibility of individual states, and a variety of different standards and rules are in place; accreditation programs, through the American College of Radiology, are currently voluntary. With a single exception, US federal agencies have no legislative authority to regulate usage of x-ray devices. The exception is the 1992 Mammography Quality Standards Act (MQSA), which regulates quality standards at all US mammography facilities. While mammography is an important component of the radiological imaging armory, it contributes much less than 1% to the overall population dose from medical imaging. This patchwork of regulations in the United States stands in contrast to the situation in Europe, where a uniform European medical exposure directive was introduced in 1997, providing wide-ranging requirements that each member state must implement. Should the United States move in this direction or is the current status quo adequate? There are several issues that need to be addressed to optimize the benefit-risk balance for medical imaging. The first is quality control and assurance. Recent incidents in which several hundred patients received radiation overdoses from CT scans suggest that quality control is, at the least, uneven in US medical imaging facilities. Moreover, radiation doses from identical CT procedures can vary by as much as 10-fold from facility to facility. Some recent initiatives by the US Food and Drug Administration, as well as by the American College of Radiology and the Radiological Society of North America, are designed to improve medical imaging quality control. However these initiatives are largely