Clinical and Health Affairs
Significance of Radiation Dose in Medical Imaging
By Ronnell A. Hansen, M.D.
Abstract
Publicized cases of errant high radiation exposure delivered to patients undergoing diagnostic imaging have led to heightened awareness and scrutiny of the costs and benefits of imaging by physicians, the public, and policymakers. The statistical risks associated with the ever-increasing utilization of modalities employing damaging ionizing radiation across the population are compounded by the development of the latest generation of devices, which are capable of delivering greater radiation doses than their predecessors for comparable diagnostic applications. This article reviews the fundamental concepts and risks of medical radiation exposure, trends in imaging utilization, and the role of radiologists and their physician colleagues in managing and appropriately utilizing imaging for patient diagnosis.
Recent publicized cases of high doses of radiation being inadvertently administered to patients undergoing diagnostic CT imaging underscore the importance of balancing risk and benefit for our patients as we seek their diagnoses. Among the latest examples of cases that have made the news are those of a 2½-year-old who received excessive radiation from 65 minutes of repeated head CT for trauma evaluation1 and a group of 206 stroke patients who received six to eight times the typical dose for CT brain perfusion evaluation during an 18-month period (the cases were identified after patients began losing hair in the scanned area).2,3 Although these examples are extreme, physicians should be mindful of how significant increases in our use of diagnostic ionizing radiation in recent years affect both patient safety and health care costs.
The amount of radiation we deliver to our patients varies and can be difficult to calculate accurately.4,5 But several peer-reviewed studies have suggested that, despite using what most physicians and health systems may consider reasonable diagnostic algorithms, delivered doses of radiation are larger than we might anticipate and are poorly tracked.6 Based on mathematical models, some cumulative doses are thought to impart measurable statistical risk for induced cancer and premature cardiovascular disease.7 Patient populations at particular risk include, but are not limited to, those undergoing repeat evaluation for conditions such as urolithiasis and Crohn’s disease,8,9 as well as those undergoing more limited evaluations such as CT coronary angiography and trauma evaluation.10,11
In 2006, James E. Winslow, M.D., an emergency physician at Wake Forest University, retrospectively examined radiation doses in adult patients who had experienced blunt trauma during their first 24 hours of care. Of 100 eligible patients, 86 had available records, with 79 (92%) undergoing a full trauma “pan scan.” The median number of CT scans per patient was three, and the median number of plain radiograph studies was 9.5. The estimated median effective dose of ionizing radiation delivered per patient was 40.2 millisieverts (mSv), the equivalent of approximately 1,005 chest radiographs.11
In an interview with Reuters Health, Winslow noted: “On a population-wide level, these levels likely result in an increased cancer burden. In the United States, most people are typically exposed to roughly 3 mSv of radiation in a year. Our estimate is that for every 100,000 people exposed to 40.2 mSv of radiation, there may be an additional 322 cases of cancer over a lifetime. Other experts in medical radiation have since reviewed our data and believe that we may have underestimated the risk significantly.”
This increased awareness of the need to reduce unnecessary patient exposure has motivated both radiologists and other physicians to initiate reviews of imaging parameters. Such efforts are critical to reducing patient exposure, although for widely used CT scanners, which technology and which protocol modification delivers improved dose with “acceptable noise” for specific studies can be difficult to determine, as scanner design has historically focused on image quality, not dose. A recent study of various CT scanners at Duke University confirmed that CT scanner design, manufacturer, and proprietary method of automated X-ray tube current modulation (a function designed to reduce delivered dose), each contribute significantly to the range of doses delivered by a specific model of machine for a given study type.12 Notably, new scanner selection, usually done in consultation with radiologists, is based on a number of factors beyond cost and dose of radiation delivered, including intended application, physical design, and image quality.
Defining Radiation and Cancer Risk
Radiation may be described as moving energy, which can take a variety of forms including visible light, X-rays, gamma-rays (nuclear medicine), microwaves, and radio waves (MRI). Not all forms of imaging energy cause tissue damage. As far as we know, the magnetic fields/radio frequency employed by MRI and the sound waves used for diagnostic ultrasound do not cause such damage. However, forms of high-energy (ionizing) radiation, which is used for sterilizing equipment and even food products, medical imaging, and cancer therapy can cause damage to tissue. The two main sources of ionizing radiation are natural radiation (cosmic radiation, radon gas, radioactive materials in the body) and radiation from medical tests and procedures. Medical radiation includes X-rays (created using high-energy beam on a target, often tungsten) and high-energy particles from decaying nuclear materials. Exposure to background radiation sources averages 3.1 mSv per year. Average total radiation exposure in the United States from all sources is estimated at 6.2 mSv per year, an increase over 20 years ago, when CT and other types of ionizing imaging were less common and average exposure was 3.6 mSv per year.
Ionizing radiation ejects electrons from atoms, producing positive ions (“free radicals”) that can damage DNA. Ionizing radiation can also damage DNA directly. Such damage can cause cell death (in the case of exposure to very high doses), cell repair, or cell repair with errors. Although inaccurate repair is rare, it can cause abnormal cell function or cancer. Resulting cancer would typically be detected decades after exposure. For that reason, no studies directly link cancer to radiation associated with medical imaging; proof would require nearly 1 million patients followed over decades to detect such a small increased risk with reliability. There are, however, statistical models estimating additional cases of cancer attributable to low-dose radiation exposure.13
When imaging is appropriately used, the information it yields regarding a patient’s diagnosis likely outweighs the small risk of developing cancer many years in the future. Regardless of radiation exposure, we must recognize that the average overall lifetime risk of developing an invasive cancer is 37.5% for women and 44.9% for men, with the majority of cancers occurring later in life and imparting a 25% lifetime risk of death.14 We do know that women who are exposed to radiation are at slightly higher risk of developing cancer as compared with men who are exposed to same dose. This is based on data from survivors of atomic bomb blasts, nuclear accidents, and early use of X-rays.
Pediatric patients are at the greatest risk of developing cancer from radiation exposure, as their rapidly dividing and growing cells are more likely to be damaged, and they have a longer time over which to develop cancer. A retrospective University of North Carolina study of 78,932 pediatric ER patients between 2000 and 2006 correlated 4,138 patients undergoing 6,073 CTs with ER volumes and severity of the diagnosis as defined by a triage-acuity score. During this period, total pediatric ER volume increased by 2%, and triage-acuity scores remained stable. CT scanning of the head increased by 23%, of the cervical spine by 366%, of the chest by 435%, and of the abdomen by 49%. Use of miscellaneous CT scans rose by 96%, with the utilization increase most pronounced in adolescents ages 13 to 17 years.15 Such data are concerning, and most institutions adjust imaging parameters for pediatric patients, affording lower doses and/or shielding sensitive organs. Such optimization of imaging parameters to reduce patient dose is the focus of the Image Gently campaign created by an alliance of the Society for Pediatric Radiology, the American College of Radiology, the American Society of Radiologic Technologists, and the American Association of Physicists in Medicine.
Increased Utilization
Exposure to ionizing radiation from medical procedures grew more than sevenfold between the early 1980s and 2006, primarily because of higher utilization of CT and nuclear medicine. The number of CT scans and nuclear medicine procedures performed in the United States during 2006 was estimated to be 67 million and 18 million, respectively. At the March 2009 National Council on Radiation Protection and Measurements (NCRP) scientific committee meeting, participants noted in their report that “technological advances in CT and the ease of use of this technology have led to many clinical applications that have increased the use of CT at a rate of 8% to 15% per year for the last seven to 10 years.” Market research firm IMV Medical Information Division of Des Plaines, Illinois, indicates that the number of multidetector CT scanners in the United States increased from nearly 51% of total scanners in 2004 to 71% in 2006.16 Scanners of such design have the potential to deliver significant amounts of radiation to patients depending on how they are used and configured. “While the greatest growth in the number of CT procedures occurred in the late 1990s and early 2000s, the use of CT technology in the U.S. is likely to continue to increase over the next 10 years,” the NCRP report stated. “Many new clinical applications are being developed; CT technology can be used with ease, perform a patient scan in a very short time ... and provide high-quality images for diagnosis.”
Between 1982 and 2006, the NCRP’s estimate of radiation exposure to individuals in the United States from in vivo diagnostic nuclear medicine increased by 460%, and the collective effective dose increased by 620%. Between 1972 and 2006, diagnostic nuclear medicine procedures increased by a factor of 5.5, while the U.S. population increased by approximately 50%. And during the past decade, there has been 5% annual growth in the number of nuclear medicine procedures done, while the U.S. population has grown less than 1% annually. The annual number of nuclear medicine patient visits in 2005 was slightly more than 17.2 million, resulting in 19.7 million procedures performed. Cardiac imaging accounted for 85.2% of all nuclear medicine radiation exposure in 2005, whereas the next highest contender, bone imaging, accounted for 9.3% of total radiation exposure. Cardiac interventional procedures across modalities also emerged as a large source of ionizing radiation. A report from the 2009 NCRP Scientific Committee meeting stated: “It is of interest to note that the cardiac procedures comprise only 28% of the total number of procedures, yet the collective effective dose is 53% of the total for all interventional procedures.”
In-Office Imaging and the GAO
One reason for the increase in utilization is the fact that more medical offices have imaging equipment. Medicare data from 1998 to 2005 show that the number of self-referred, in-office CT, MRI, and nuclear medicine scans performed grew at triple the rate of the same exams performed in all settings. Specifically, Government Accountability Office (GAO) and MedPAC reports,17,18 as well as peer-reviewed studies dating back to the 1990s19 have shown that when physicians refer patients to facilities in which they have a financial interest, imaging utilization is significantly increased. Private insurance studies suggest that as much as half of this self-referred imaging is unnecessary.
Public policy also drives excess utilization. The practice of defensive medicine appears to be as important a contributor to excessive radiation exposure as self-referral, as evidenced in a 2008 survey by the Massachusetts Medical Society (MaMS).20 A majority of surveyed physicians indicated that risk of litigation was driving orders for unnecessary tests, procedures, referrals, and even hospitalizations—a phenomenon that is estimated to add at least $1.4 billion to annual health care costs in the Bay State. The MaMS reported that 83% of physicians surveyed practiced defensive medicine, with an average of 18% to 28% of tests, procedures, referrals, and consultations and 13% of hospitalizations ordered to avoid lawsuits. The MaMS findings likely underestimate the total cost, as the 900 physicians surveyed (family doctors, obstetricians, gynecologists, and general surgeons) accounted for only about 46% of the state’s physicians. The findings do, however, mirror the experience reported by physicians in other states. Such defensive medical practices are viewed as a contributor to skyrocketing malpractice insurance premiums for doctors nationwide and heated debate over the need for tort reform.21
Provider financial interest, fear of litigation, advancing technology, and patient demand all contribute to increased imaging use and radiation exposure. The criteria we use to decide whether and how to image our patients is important, as dose reductions of up to 44% were estimated when American College of Radiology appropriateness criteria were applied to patient exam selection.22,23
Conclusion
Clearly, the issue of increasing radiation dose in medical imaging has complex elements, and additional education of physicians, patients, and legislators is key to preventing excess exposure. Ultimately, we may never know the proof of statistical calculations delineating the risks imparted to our patients by the imaging we order, as such effects are decades in the making. As we develop heightened appreciation of the potential for such delayed consequences, we must collectively approach the decision to image—or not to image—in a more circumspect and collaborative manner. Because the United States has no federal regulations regarding medical imaging and radiation dose, it is up to individual radiologists, radiology departments, and ordering physicians to control the amount of radiation patients are exposed to. How we use imaging is a complex issue that—in the face of health care reform, the dangers of excess radiation exposure, and the estimated cost versus benefit—will need to be addressed directly, transparently, and honestly by all of us. If we do not address the way we use imaging, it will be rigidly mandated. As physicians, our primary goal should be to stay on the positive side of the risk/benefit ratio for our patients. MM
Ronnell Hansen is a radiologist with St. Paul Radiology. He is currently president of the East Metro Medical Society and lectures on medical imaging technology, cost, and health policy.
References
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