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Computed Tomography and Radiation: Weighing the Risks
Jennifer Rossi, MD, Attending Physician, Department of Emergency Medicine, Kaiser Permanente Medical Center, Redwood City, CA.
Grant S. Lipman, MD, Clinical Assistant Professor of Surgery, Division of Emergency Medicine, Stanford University School of Medicine, Palo Alto, CA.
Dan Quan, DO, Attending Physician and Medical Toxicologist, Department of Emergency Medicine, Division of Toxicology, Maricopa Medical Center, Phoenix, AZ.
Utilization of computed tomography (CT) scans has increased markedly in the past decades. It is estimated that more than 75 million scans are performed annually in the United States,1 compared to only 3 million in 1980.
Each of these scans carries a significant dose of radiation, which may have long-term consequences for the patient. In 2007, the paper by Brenner in the New England Journal of Medicine suggested that as many as 2% of cancers in the United States were caused by the radiation exposure of CT scans.2 In my own practice, we saw a significant surge in patients with minor head injuries who demanded a head CT in the weeks after the death of Natasha Richardson, an actress whose sudden death from an epidural hematoma while skiing attracted significant media attention and raised public fears about undiagnosed traumatic brain injuries. Increasingly, patients present to the emergency department (ED) demanding a scan. Unnecessary CT scans carry a risk of radiation that few patients understand.
Sandra M. Schneider, MD, FACEP, Editor
While CT scans are valuable and powerful tools in our diagnostic arsenal, the ionizing radiation used to obtain the images that these studies produce is a possible risk to the patient long term. The potential effects of ionizing radiation in health care settings are a growing concern as experimental and epidemiological evidence has linked such exposure with the development of solid cancers and leukemia.3 Emergency physicians who frequently order CT studies should be aware of this growing public health concern so that they may make fully informed decisions regarding the risks and benefits. Granted, the risk of morbidity and mortality from undiagnosed disease may dwarf the risks of radiation, but long-term oncologic repercussions may cause practitioners to rethink the amount and frequency of ionizing diagnostic studies they order.
Radiation is quantified by several measures of exposure. The gray (Gy) is the "absorbed" dose or the energy absorbed by any given tissue (J) per unit of mass (kg). The organ dose is the amount deposited in the organ of interest and is also expressed in Gy. This is the most sensitive measurement when predicting radiation damage. The "effective" dose is expressed in Sieverts (Sv). This SI-derived unit considers the amount of radiation absorbed by an organ, the organ's radiosensitivity, and all the organs and tissues irradiated by a scan. It is useful when comparing radiation sources but gives only an approximate estimate of the true risk. Two measurements rarely utilized are the rad and the rem ("roentgen equivalent man"). A rad is the certain dose of energy absorbed by one gram of tissue. A rem is calculated by multiplying the rad by a qualifying factor (Q) for each type of radiation to account for the ability of that ray to transfer energy to tissues. The organ dose (Gy) is the preferred unit to use when estimating risk.
Consequence of Radiation
CT scans deliver low-dose ionizing radiation to body organs; typical doses are summarized in Table 1. Radiation causes biological damage due to the creation of hydroxyl radicals that can subsequently alter DNA via strand breaks and base damages. Such genetic alteration is the basis for cancer formation.
This risk of carcinogenesis is not theoretical. In 1945, two atomic bombs exploded in the Japanese cities of Hiroshima and Nagasaki. Approximately 30,000 survivors in those cities and surrounding villages received ionizing radiation doses of approximately 50 mSv (with a range of 5 to 150 mSv). Longitudinal studies on the rates of cancers in this diverse cohort provide most of the current quantitative data on the oncologic risks of such radiation. The major conclusions from this group are that the risk of all solid cancers is linear with increased dose, and the rise is steepest for thyroid and hematologic malignancies. All ages were affected, but children were most vulnerable.
By comparison, the Chernobyl nuclear reactor fire in 1986 released 400 times the radiation of the bombings of Heroshima and Nagasaki but to a less dense population.4 There were 2 deaths from the explosion, and 237 cases of acute radiation sickness (28 of these died in the first few weeks, and another 19 died by 2004). The general population received 10-20 mSv. More than 4000 cases of thyroid cancer have been related to radiation exposure in children and about 30 cases of leukemia. It is suspected that cancer mortality is increased by 1.5% for the general population of the area, and 4-6% for those in the nearby villages.
Utilization of CT Scans
Physicians are utilizing imaging studies more than ever before to aid in diagnosis and treatment decisions. The use of CT scans has increased correspondingly with a documented rise of more than 130% in the past few years. In 2007, there were approximately 75 million scans in the United States (with a population of 300 million).1,5,6 This rise is partially due to better image quality and shorter scan times of newer CT technology. It may also be due to the increasing desire of patients to know what is wrong and to physicians being less willing to miss a diagnosis. In the 25-year period from 1972 to 2006, the per capita radiation dose from medical exposure increased almost 600% to about 3.0 mSv, and the collective dose (sum of all the radiation delivered to patients) increased more than 700%.5 Most of this radiation increase is from CT scans. Despite accounting for only 13% of all radiological studies in both inpatient and outpatient settings, they comprise 70% of the radiation dose to patients.7 To put this in perspective, one chest radiograph delivers 0.02 mSv of radiation, the equivalent of 2.4 days of background radiation a person is exposed to during daily life; an abdominal CT scan delivers on average 500 times that amount or 10 Sv of radiation, which is approximately 3.3 years of background radiation.8
Utilization has increased in all ages but may be particularly problematic in children, who by their lifespan may have increased risk of cancer. Broder reported that CT utilization in adolescents increased 400% between 2000 and 2006.9 CT use for appendicitis increased from 51% to 76% in adults from 1999 to 2004.10 In children this increase was more dramatic. From 1997-2001 CT use for children with suspected appendicitis increased from 1.3% to 58%.11
Several studies have examined the benefit of imaging in the ED. In one such study, the authors determined the absorbed dosage of radiation per clinically actionable result or emergently treatable finding (ETF) for patients who had a CT examination in an ED.12 The records of all patients who underwent a CT examination during one month at an academic medical center were examined. Emergency physicians reviewed radiology reports and determined if the scan had a clinically actionable result (defined as a finding that required further investigation, treatment, or patient observation) or an emergently treatable finding (ETF) (any result that spurred a surgical intervention, special procedure, or ICU admission within 24 hours of the patient's presentation to the ED).
A total of 770 patients (14%) underwent a CT scan, which included 33% of all the trauma patients and 11% of all the other patients. Some patients had multiple examinations; 1094 studies were performed. Actionable results were found on 341 and ETF on 105 studies. The rates of positive findings were greater in trauma (66%) than medical studies (34%) despite the lower rate of scanning in the medical patients. The mean radiation varied between trauma and non-trauma patients: 106.36 mGy vs. 53.27 mGy, respectively, as trauma patients more frequently had multiple scans. The mean radiation dosage was 163 mGy per actionable result and 530 mGy per ETF. There was a non-significant difference between the trauma and medical patients.
The authors found that significant amounts of radiation can be delivered to patients, who receive the equivalent of more than 5000 chest X-rays per ETF. Clearly there are problems with a retrospective study, and some percentage of negative studies is expected and desired. However, issues with ordering CT scans may be present. In a recent study there was a significant variation in the ordering of CT scans. In a recent abstract that reviewed CT ordering, attending physicians ordered scans on 3.5% of patients and residents ordered them on 6.5% of patients.13 In an urban institution, the range for physicians varied between 1.8% to 25%, although in part this may have been due to the staffing pattern.14 Nonetheless, this highlights that there is room for improvement in the ratio of radiation dosage to clinically actionable and emergently treatable findings.
Risk of CT Scans
In 2007, Brenner published the sentinel article in the New England Journal of Medicine analyzing the radiation risk of CT scans.2 He estimated that for every 100,000 people exposed to a dose of 100 mSv, there are an additional 800 cases of cancer. Furthermore, CT scans alone may trigger 1.5-2.0% of all cancers in this country. The majority of diagnostic radiologic procedures involve a small individual risk, which is justified by the medical need. However, no data support whether the potential benefits of CT-based health screening outweigh the risks in asymptomatic patients. A recent study of more than 30,000 patients over 22 years examined the cumulative effects of radiation.15 The authors extrapolated the Lifetime Attributable Risk (LAR) the cancer risk above the baseline rate of a standardized U.S. population using standardized conversion factors derived by the Biological Effects of Ionizing Radiation (BEIR) VII.3 Figure 1 displays the cancer incidence and risk for a given age at exposure from which these calculations were based. It is estimated that full-body CT examinations have a lifetime attributable cancer mortality risk of around 0.08% for a single examination and risk of 1.9% for annual examinations.16 This means a big effect on the population's cancer rate an important and potentially avoidable public health threat.
Prior to 2007, there was little awareness of the potential dangers of CT scanning. A survey done in 2004 reported that emergency physicians and radiologists were unable to provide accurate estimates of radiation doses and cancer risks from diagnostic CT radiation exposure. Only 9% of referring physicians and 47% of radiologists were aware of the increased risk of cancer.7
Yet the emergency physician is often caught between wanting to decrease the radiation risk and determining the appropriate disposition for the patient. The medical-legal implications are significant. Missed appendicitis is the second most common cause for litigation against an emergency physician (missed myocardial infarction is first).17 In more than one-third of those claims, the allegation was misdiagnosis. Patients often pressure their primary care and emergency physicians to perform a CT scan, and at times repeat them. Even when patients were told that the likelihood of a positive study was less than 5%, 28% of patients still wanted the study.18
Case. A 25-year-old male presents with recurrent abdominal pain. The patient has been to this ED 5 times in the past year and each time had a negative CT of the abdomen and pelvis. His complaints are non-specific and identical to his previous visits. His examination shows diffuse abdominal tenderness. He wants another CT scan. He reluctantly agrees to try pain medication first. After 4 hours, he becomes angry. A CT scan is ordered and again shows no abnormalities.
The study of 30,000 patients over 22 years mentioned above also looked at repeat scans.15 Approximately one-third of patients received more than 5 CT examinations over their lifetime. Furthermore, 5% of patients had more than 22 CTs, and 1% had more than 38. After summing the radiation for each scan, 15% of patients were noted to receive more than 100 mSv, and 1% received more than 399 mSv well over the dose at which there is strong evidence for significant risk of carcinogenesis.3,16 For cancer incidence, 7% of patients had an LAR of 1%, and 1% of this cohort had an LAR of 2.7%. For cancer mortality, 3% of patients had an LAR of 1%, and 1% had an LAR of 1.6%. Many of the patients were chronically ill and therefore likely required more CT scans than the rest of the population given their underlying diseases. Further, many of these patients already had known cancer.
An ED-based study described a subpopulation of patients who received recurrent imaging.19 Within one year, 2% of all patients imaged by CT in a single ED had three or more CT scans, and approximately 75% of these were repeats of the same study. This cohort had an estimated increased LAR ranging from 1 in 625 to 1 in 17. Another ED-based study examined the CT use in a subgroup of patients who had three or more visits within a one-year period.20 They then retrospectively reviewed their CT scans in the previous 7.7 years. They reported a median of 10 CTs per patient in that time period, with a maximum number of 70 studies. Cumulative LAR was a median of 1 in 110, and for the maximum, 1 in 17. A retrospective study reported that out of a total of 356 patients evaluated in an ED for urolithiasis, fewer than one-fifth did not undergo a CT scan, whereas almost 80% had two or more scans.20 Other groups that have multiple CT examinations include those with seizure disorders, frequent falls, possible pulmonary emboli, trauma, and bowel obstructions.
Most clinicians are aware that there are subgroups of patients who are more frequently imaged in both the ED and inpatient settings and would agree that these patients have higher attributable cancer rates. Studies identifying the characteristics of these individuals, for example, history of malignancy, or symptoms of renal colic, may focus efforts to decrease scanning of this population. When possible, alternative imaging should be considered. Some patients go to more than one ED in search of a cause for their complaint. A recent paper from an urban ED showed that 4% of patients receiving a CT scan had one at a nearby hospital within 15 days.21 Patient education in the ED is sometimes problematic, particularly when the patient is focused on a specific test. Several multidisciplinary protocols have reported success in decreasing the use of CTs.22,23 Practitioners should take into account the patient's imaging history prior to ordering another similar diagnostic study and consider: What will this fourth (or sixth) recurrent study add to the patient's care? If there is a clear indication for the study, then the benefits outweigh the risks of radiation. When possible, alternative imaging or no imaging may be prudent.
Case 2. A 20-year-old female who is 20 weeks pregnant presents to her private physician with complaints of dyspnea when walking stairs for the past several days. She has no other complaints, and her vital signs are stable. Her physician performs a D-dimer, which was elevated. He sends the patient to the ED to rule out a pulmonary embolism.
Pulmonary embolism (PE) is a potentially fatal disease and the number-one cause of maternal death in the developed world. It has a wide array of presentations. As a result, it is prudent for the emergency physician to have increased suspicion for its presence and a correspondingly low threshold for ordering diagnostic testing. For most EDs in this country, the gold-standard for PE diagnosis is a CT pulmonary angiography.
The pregnant patient carries particular concern. The D-dimer generally increases as pregnancy progresses. A negative D-dimer in the first or second trimester, however, is quite diagnostic. Normal physiologic changes of pregnancy can make a woman feel dyspneic. Doppler ultrasound of the lower extremity is quite sensitive for detecting a clot in the upper leg, but has lower sensitivity in detecting clot in the calf or iliac vein.24 CT scanning gives more radiation to the mother than to the fetus, whereas VQ gives more to the fetus than to the mother due to the fetus' proximity to the bladder where technetium accumulates.24 However, CT carries risks of contrast nephropathy and radiation. The exposure of radiosensitive breast tissue in young women pushes these risks even higher. For each CTA (assuming a dose of 10 mGy), it is estimated that there is a subsequent 1.1% lung cancer and 0.8% breast cancer risk per year of life.3 Pulmonary embolism is a difficult diagnosis in both pregnant patients and those who are not pregnant.
Repeat scans for PE are common. Kline, et al25 conducted a prospective longitudinal follow-up study on a cohort of 675 pregnant and non-pregnant patients who had CT angiography to evaluate for PE between 2001-2002. After adjusting for mortality, the rate of repeated CTA was 37% (226 of 615). The mean age of patients undergoing a repeat exam was 49 years (±16), of which women younger than age 40 comprised 22%. Thirty-one patients with a mean age of 44 had 6 or more scans. Ninety-eight percent of the repeated scans were ordered from an ED. The proportion of positive results was the greatest on the first scan at 5.3%. This rate fell for each subsequent study, with only 3.4% of the second scans positive and even fewer for scans three through six. More than 95% of these repeated scans showed no PE, and three quarters had no significant findings warranting intervention at all. The radiation dose was 17.2±1.9 mGy for the chest CT and 11.9±11.3 mGy for venography of the legs.
Clinical decision rules can help physicians make decisions about whether to order scans and help reduce the rate of negative studies. This may help curtail the subsequent costs to patients and the health care system. The Prospective Investigation of Pulmonary Embolism Diagnosis II (PIOPED II) trial26 has validated an algorithm that integrates the use of clinical judgment and pretest probability rules with D-dimer testing and CT angiography. This can help guide physicians in their work-up, when they otherwise may place too much emphasis on some risk factors such as prior venous thromboembolism and help curtail the ordering of unnecessary CT scans. There is no consideration of the potential harm of repeated CTA in the PIOPED decision guidelines (or other such algorithms). This should be incorporated into future decision rules, especially when evaluating young patients. Unfortunately, those with a long life expectancy have a significant cumulative risk of breast, lung, and other cancers. This study highlights the need to limit repeated scanning in patients to protect them from complications of contrast and radiation and to decrease health expenditures.
Case 3. A 25-year-old male was involved in a motor vehicle accident and taken to a rural hospital. A CT scan preformed there reveals a small fracture of the spleen with no other abnormalities. He is transferred to a tertiary care center where he arrives 4 hours after the accident. He has persistent left upper quadrant abdominal pain. Vital signs are completely normal. A repeat CT scan demonstrates the same finding and no other injuries.
Certain groups of patients, such as trauma victims, undergo more imaging than others because of the nature of their chief complaint and potential to harbor serious injuries not diagnosed through physical and laboratory examinations. Winslow and colleagues reviewed a case series of blunt trauma patients to estimate their dose of ionizing radiation in the first 24 hours of care.27 Only patients with Level II injuries were included. The more critical Level I patients (those with hypotension, traumatic cardiac arrest, or airway compromise) were excluded because they frequently have a limited ED evaluation and go straight to the operating room. Also excluded were trauma patients younger than age 18, victims of penetrating trauma, and those who did not meet the institution's criteria for major trauma. The radiation delivered by each scan was calculated using CT dose indexes on their hospital's machines.
Eighty-six patients were included with a median age of 32, a median Injury Severity Score of 14, and a median effective dose of 40 mSv, with most of the radiation associated with CT scans (the median number of scans was 3). Of these patients, 79 (92%) had a full trauma "pan scan," which included CT of the head, cervical spine, and CTA of the chest, abdomen, and pelvis. The median effective dose for a head CT was 3 mSv and chest/abdomen/pelvis was 26.1 mSv. The median effective dose of ionized radiation was 20.2 mSv, with an intraquartile range of 30.5 to 47.2. This is equivalent to 1005 chest X rays and well above the 3.0 mSv normal background radiation most individuals receive each year.5 The authors concluded that trauma patients receive a statistically significant amount of ionizing radiation during their first 24 hours of care. The results of this study are in concordance with others that have quantified radiation doses to trauma patients.28 While some may question whether all major trauma patients need to be "pan-scanned," such practice is quite common, and occasionally yields an unexpected significant finding. Of more concern is the repeat scanning of such patients. Recent studies suggest that there is a small but significant yield in such scans. In a recent study of more than 2700 blunt trauma patients with a negative initial CT, 8 had significant findings on re-imaging. Of these, most were in the liver and pancreas.29
Trauma patients tend to be younger; age is an important variable in cancer risk, with younger patients more apt to have malignancies. Also, trauma victims receive a higher proportion of CT neck examinations, primarily due to the evaluation for C-spine injuries. These tests irradiate the thyroid along with the cervical spine. One prospective study measured found that trauma patients have a mean thyroid dose of 58.5 mSv, with more than one-fifth having doses above 100 mSv.28 This is particularly concerning as the thyroid is one of the most radiosensitive organs in the body and such doses would result in a 10-fold higher number of thyroid cancers for the study patients over the baseline population.3,28
Repeat re-imaging soon after first scan is more damaging, as the body has not had a chance to upregulate DNA repair mechanisms. This concept is known as the "second hit" phenomenon a second dose of radiation shortly after the first has an exponentially higher chance of causing a significant mutation.
Trauma patients may have critical occult injuries, and an aggressive diagnostic approach is warranted. There is the need to develop decision rules to guide ordering practices and curtail the intense irradiation some trauma patients undergo. Other modalities should be considered as adjuvants or replacements to CT imaging in this population, such as MRI and ultrasound. In addition, providers should work with radiologists to change scan parameters to optimize imaging quality while minimizing overall radiation exposure.
Case 4. A 3-year-old child falls off the couch, hitting his head. For a few seconds he lies "stunned" and then starts to cry. Within a few minutes he is acting normally. His parents bring him to the hospital to have CT scan.
There are three unique considerations of radiation exposure in children: their tissues are more radiosensitive; they have a longer life expectancy, resulting in a larger window of opportunity for expression of radiation damage; and they receive a higher dose per unit of body mass when adult CT settings are used. A recent study examined the age-dependent cancer mortality risks associated with CT scans ("relative risk" above the population's baseline cancer rate).30 The authors multiplied age-dependent lifetime cancer mortality risks (per unit dose) by the estimated doses of various CT exams. Their data suggest that younger patients are at an exponentially greater risk of cancer. (See Figure 2.) They conclude that each year, almost 700 cancer deaths are due to patients undergoing a single head CT, and 1800 are due to a single abdominal CT. A large and disproportionate segment of these patients 170 (25%) and 310 (17%), respectively were exposed before their 15th birthday. In other words, at the current frequency of just pediatric head and body CTs, it is projected that 500 annual radiation-induced cancer deaths will occur. A recent study examined predictors of serious head injury in children younger than 2 years. Children younger than 2 years of age without the following validated predictors have a risk of clinically important brain injury of less than 0.05%: abnormal mental status, loss of consciousness, history of vomiting, severe mechanism of injury or basilar skull fracture, and signs of basilar skull fracture.31
It is clear that the risk of cancer is skewed toward the pediatric population, with rates in children on an order of magnitude higher than those in adults. Pediatric CT is a public health concern, and although the risk to each individual is relatively small (the increased cancer incidence over the natural baseline rate is low), the effect on the population as a whole is significant.
During the past few years there have been several studies that reported increased risk of cancer from CT scans. They generated a flurry of media attention. Because of that intense reaction, there has been an effort to reduce and even eliminate pediatric exposure. The Society of Pediatric Radiology advocates that clinicians follow the principle "As Low As Reasonably Achievable" (ALARA).32 Table 2 lists some methodologies to reduce CT dose.
Case. A 25-year-old female, 8 weeks pregnant, presents with right lower quadrant pain, anorexia, mild leukocytosis, and a low-grade fever. The gynecologic exam is normal. You consult a surgeon, but he is reluctant to operate without a confirmatory CT because of the risk of anesthesia.
Pregnant women experience the same acute illnesses as their non-pregnant counterparts, such as appendicitis, pulmonary embolism, nephrolithiasis, and trauma. CT aids in their diagnosis; however, radiation to the developing fetus must be considered when evaluating this cohort. Chen and colleagues present a set of evidence-based guidelines for the use of CT in pregnancy.33
The potential impacts on fetal health, particularly between 2 and 20 weeks gestastional age, are teratogenesis and carcinogenesis. Some possible birth defects include microcephaly, other CNS defects, and growth retardation. Notably, a typical pelvic CT delivers a radiation dose well below the estimated threshold dose of teratogenic induction (24-46 mGy vs. 50-150 mGy respectively).33 Carcinogenesis, on the other hand, can occur from exposure to any radiation dose, particularly during the first trimester. The risk is dose-related. Based on the existing data, one pelvic CT may double the relative risk of childhood cancer, albeit this overall risk remains very low with an increase from 0.05% to 0.10%.33 Given the course of development, the risk is more significant in the first trimester than in the third.
Table 3 summarizes the guidelines of the diagnostic approaches to pregnant women with suspected pulmonary embolus, appendicitis, or nephrolithiasis.
Imaging of pregnant women is a difficult decision for acute care providers, as the risks and benefits to both the mother and the unborn child must be considered. Often it is more dangerous for both if a serious disease remains undiagnosed. Although risks of carcinogenesis remain low, they are not negligible. Ordering physicians should discuss the risks and benefits of radiation to both mother and fetus with their patients and document these discussions in the medical record.
Conclusions and Recommendations
The diagnosis of cancer for any person is a life-altering occurrence, and it is made even more tragic if the disease may have been preventable. Over the past few decades, there has been a major increase in the collective ionizing radiation dose to the U.S. population from medical procedures. CT scans contribute to most of this increase. The long-term health effects are clear: The ionizing radiation from CT scans increases the lifetime cancer incidence and mortality in patients. Unnecessary CT scans should be avoided. The value to the patient of the diagnostic information should exceed the corresponding radiation risk. Providers should not order potentially harmful tests that don't affect management. In this era of defensive medicine where scans may be ordered to protect against perceived potential litigation stemming from a missed diagnosis, a discussion with the patient of risks and benefits of an unnecessary study is paramount. Furthermore, clinical decision rules reinforce the finding that the physician's clinical suspicions are usually correct, allowing the provider to avoid indiscriminately imaging extremely low-risk patients.
To curtail radiation doses, practitioners should consider alternate modalities such as MRI and ultrasound that have equivalent levels of diagnostic performance without exposing patients to ionizing radiation. This is especially important in the pediatric population. Observation and repeated examinations of a patient for a change in symptoms may preclude the need for imaging altogether.
Emergency physicians should understand that there is a large difference between individual risks vs. population risk. The majority of diagnostic radiological procedures, including CT, in symptomatic patients involve an extremely small individual risk, which is justified by the medical need. In other words, providers must consider the clinical scenario. If the scan is indicated, obtain it. The long-term cancer sequelae of ionizing radiation is very real, and as with any other interventions, alternatives, risks, and benefits should be discussed in detail to optimize patient care.
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