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Radiation Risks of CT Scans: What Now?

Roberts, James R. MD

doi: 10.1097/01.EEM.0000316461.98657.7d
InFocus

Author Credentials and Financial Disclosure: James R. Roberts, MD, is the Chairman of the Department of Emergency Medicine and the Director of the Division of Toxicology at Mercy Health Systems, and a Professor of Emergency Medicine and Toxicology at the Drexel University College of Medicine, both in Philadelphia.

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All faculty and staff in a position to control the content of this CME activity have disclosed that they have no financial relationships with, or financial interests in, any commercial companies pertaining to this educational activity.

Learning Objectives: After reading this article, the physician should be able to:

1. Discuss the risks and benefits of CT scans in renal colic.

2. Describe the best radiographic test for pulmonary embolism in pregnancy.

3. Summarize the issues of informed consent with regard to radiation exposure from CT scans.

Release Date: April 2008

CT scans have no doubt saved countless lives, but are often ordered for even the most minor complaints, such as vague headache and nonspecific abdominal pain. Some hospitals scan everything based on ridiculous protocols, all spurred by fear of missing even the most minor injury.

Emergency physicians are among the culprits, doling out radiation dwarfing Hiroshima, with the complicit public and lawyers egging them on. Radiologists stoke the fire with their omnipresent recommendations for “further evaluation.” Watch what happens when a 75-year-old patient with a fracture arrives in your ED: He gets two 14-gauge IVs, a Foley catheter, a bevy of housestaff who never even examine the abdomen, and a CT scan of everything, even if it doesn't hurt. CT has replaced serial exams and the obstruction series for abdominal pain, the surgeon's judgment for appendicitis without lowering the unneeded appendectomy rate, the KUB for kidney stones, and plain C-spine x-rays for the boarded and collared EMS arrival. We now go on ambulance divert when the CT scanner is down. Clearly, the CT scan — you name the indication — has become standard of care.

Cost aside, radiation exposure can be quite prodigious, but the risks of diagnostic radiation are underappreciated by most physicians and certainly by patients. The full effect of excessive medical radiation is unknown and often not apparent for 10 to 20 years after the exposure. Recently many cautions, promulgated from various authorities, warn against indiscriminant use of CT scans and the potential risks to the patient, particularly the development of malignancies. Emergency physicians are caught between providing good medical care based on experience and common sense when technology allows for clarification of obscure diagnoses. We are hampered by reflex protocols that other departments force on us and patient and parental demands, not to mention the fractious consultant.

Last month, I tried to sort out some of the issues of this burgeoning dilemma. Bottom line: CT scans irradiate normal tissue and can transform it into cancerous tissue. There are no data, only theoretical conscripts, but it has been calculated that with the current rate of diagnostic radiation exposure, up to two percent of cancers will be related to radiation delivered by clinicians in their quest to find and fix pathology. This month's column continues the discussion of the radiation risk for CT scans by reviewing evaluation for renal colic, pulmonary embolism in pregnancy, and the thorny issue of informed consent.

Radiation Dose Associated with Unenhanced CT for Suspected Renal Colic: Impact of Repetitive Studies

Katz SI, et al

Am J Roent

2006;186(4):1120

Acute nontraumatic flank pain often prompts a CT searching for a possible kidney stone or other mimicking pathology. Many unfortunate individuals suffer urinary calculi as a recurrent problem, with a recurrence rate of about 30 percent to 40 percent over 10 years. The CT scan without contrast has evolved as the preferred method for evaluating such patients. Not only can the tiny calculus be found, but a plethora of renal and surrounding organ problems can be readily clarified. This study from Yale estimated the radiation dose delivered to 4500 patients undergoing 5562 unenhanced CT scans for renal colic over a six-year period.

About 60 percent were women, and three percent involved CT studies of children. Three or more CT studies for suspected renal colic were performed in four percent of the patients. Three or more CT scans for suspected renal colic were administered to four percent of patients, all with chronic nephrolithiasis. Nineteen percent of the patients had six more CT studies, with one patient receiving a total of 18 scans over this period. The total radiation dose with four or more CT scans was in the frequently quoted range of radiation dose delivered to survivors of the atomic bomb in Japan, a subset that had a significant increased risk for fatal cancer. Low-dose protocols can be used to limit radiation, and combining ultrasonography with plain radiographs (KUB study) also may suffice per these radiologists.

Comment: The radiation dose associated with an unenhanced CT for renal colic is high, and it's often recurrent. It's practically a reflex action to obtain a CT scan for suspected renal colic, even if the patient has had similar symptoms with proven stones, has all the classic signs and symptoms, and literally gives you the diagnosis coming in the door. I have always opined that the CT provides the answers to many questions, it's fast, and it's not my money. The first or second kidney stone does not garner much radiation, but one wonders if the patient who had 18 scans glows in the dark. Some of my braver colleagues do not always get a CT scan, but arguments always emerge concerning the number, size, and position of the stone and the degree of obstruction. Try to get a urologist to even talk to you, let alone see the patient in consultation, without this hallowed study. Ultrasound and flat plates simply don't cut it with regard to the information that can be received from a single CT scan. Numerous studies have relegated the KUB to the tar pit of medical antiquity, and no one knows how to get or read an IVP these days. The CT scan trumps everything, even clinical acumen.

It's clinically reasonable to eschew the CT scan on the first ED visit for the patient with a classic story, exam, and urinalysis for renal colic, particularly when he says he's having another stone. Information from prior events, such as the rate of passage of prior stones or surgical interventions, can allow one to be comfortable for at least a few days, as long as your diagnosis is correct and it's not the aorta! I am sure a scan, however, will be performed by any follow-up radiologist or any of your emergency medicine colleagues when the patient returns with persistent pain.

We have the technology to quickly pinpoint the size, shape, and other intricacies of a kidney stone, and even tell the patient if he has a stone on the other side that might pass later on, but the price is excessive radiation to the patient. On my last shift, I saw a 34-year-old with a textbook case of renal colic, and I ordered the requisite CT scan. He had the same small renal calculus in the same spot from the past five scans. Obviously he had a hankering for more Percocet, and had the CT findings to procure a hefty amount of the narcotic from five other physicians in my group. And that's just my hospital.

My current practice of feeding the CT monster will not waver at this juncture based on fears of excessive radiation. I am hoping for technology to develop a scan with less radiation, or for some (so far) gutless national urologic association to take the plunge and advise against routine CT for these scenarios and to create a standard of care that makes some sense. There is, of course, no answer to this dilemma at the current time.

When a Pregnant Patient Has a Suspected Pulmonary Embolism, What Are the Typical Embryo Doses from a Chest CT Scan and a Ventilation/Profusion Study?

Huda W

Ped Radiol

2005;35:452

Pregnancy is a risk factor for pulmonary embolism, and is a leading cause of maternal death. The subtle nature of this potentially fatal process and the many conditions that mimic PE often place the pregnant woman in the path of some radiographic study to rule in or rule out the diagnosis. Traditionally the VQ scan has been recommended for pregnant women, but the typical fetal radiation dose is actually higher than that of a CT scan. The authors state that a radiation dose to the embryo from a typical VQ scan (note both ventilation and perfusion) is 0.9 to 1.8 mGy. xenon gas may give the lower range, but the most commonly used technetium study delivers slightly higher radiation. The radioactive materials used for the VQ scan are tagged to albumin, inhaled and injected into the patient's veins, which stay in the lung where a scanning device detects them. They are then excreted into the bladder. The bladder full of radioactive tracer right next to the uterus gives radiation to the proximate fetus. If you do plan to use a VQ scan, have the patient empty her bladder immediately after the test, and do so regularly so the technetium-laden urine does not sit next to the fetus.

Using thin-slice rapid scanning and 100 mls of iodinated contrast for a CT of the lung delivers a fetal dose of radiation of only 0.14 mGy. The fetal radiation dose from a typical VQ scan is considered to be low, but the fetal radiation dose resulting from a typical CT scan of the chest is considered very low. Of course, the risk to the mother and fetus of not performing a medically indicated study is always greater than the minimal risk for potential harm to the fetus from either study.

Comment: The safest way to diagnose a pulmonary embolism in a pregnant patient to minimize radiation exposure to the fetus and obtain the best information is a chest CT angiogram. Although it is now widely agreed that VQ scans expose the fetus to more radiation than CT, many radiologists will still fight you on this issue. Some simply refuse to do the study. I don't think they read their own literature very closely, and many were trained in a time when the VQ scan was in vogue. There are enough data on the poor validity of VQ scans for PE to convince me that a CT scan, although not perfect in itself, is the better test. Trying to get a reading of a “normal” VQ scan is next to impossible. We also know that a low probability scan has about three percent to five percent incidence of pulmonary embolism. This article is one of many stating that a CT angiogram for PE is the safest study for the fetus from a radiation exposure standpoint.

Relying on D-dimer levels in some pregnant patients can be misleading because there can be a physiological increase in pregnancy. (Br J Radiology 2006;79:441.) One could do a duplex scan of the legs looking for a clot, but this study also can be somewhat abnormal simply because of the pregnancy. One approach is to develop a pretest probability, and if this is low and the D-dimer is negative, it is reasonable to exclude a PE on an academic level. Of course, one pretest probability may be different from another. One can use a variety of scoring systems, but even these will miss a few positive cases. If the D-dimer is elevated, one can perform bilateral duplex scanning of the lower extremities, and consider the patient to have a PE if this study is positive. If the ultrasound is negative and the pretest probability is high enough to be of concern, a CT pulmonary angiogram is suggested. There is a small theoretical risk of contrast-induced hypothyroidism in the infant from iodinated contrast material. Standard CT contrast material is not contraindicated in the pregnant patient.

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Informed Consent

At the current time, there is no standard of care or current practice suggesting that risks from radiation exposure need to be discussed with patients undergoing CT scanning. Occasionally pregnant patients will have the discussion with the radiologist, but trauma patients, those with a suspected PE, kidney stones, or undiagnosed abdominal or chest pain are not routinely told about radiation exposure. I am not sure what patients would do with the information anyway. One assumes that the studies are done for good reasons, but there are always risks and benefits to any test, intervention, antibiotic, or pain medication.

I have found that trying to discuss radiation exposure with many patients actually puts them off. They want the scan or x-ray, and any hesitation on my part seems like I am trying to keep them from getting a study they are convinced they need. This is true even for parents whose children sustain minor trauma. I hate doing a CT scan on a toddler who simply banged his head, and find that a preemptive strike is the best way to go. Before the parents have a chance to ask if I'm going to do a head CT, I tell them that I don't think any x-rays, including a head CT, are necessary. I say, “We can always do it later if things change.” That at least lets them know that I have thought about it, am aware of the test and its abilities, and have made a decision without being asked. If you don't at least mention possible tests that you have ruled out, they wonder if you even thought of it or know about the technology. This is also a good tactic for unneeded antibiotics, blood tests, and admission decisions.

Finally, if one were to give informed consent for radiation exposure for a needed CT scan, how would it go? How about: “I am going to do an x-ray study that's going to expose you to large amounts of potentially harmful radiation. You can decline the test if you want, but I may miss important pathology or fail to diagnose a life-threatening condition. Of course, you may get cancer from the radiation exposure many years from now, but then again, your problem might be cancer to begin with. What would you like me to do?” That's a surefire approach to make you look like a bumbling, flummoxed, befuddled country doc. This is just one decision that most patients can't make; they want you to make it for them.

Bottom line: There are no correct answers, only theories and risk:benefit analyses based on unproven assumptions. I am trying to cut back on the use of CT scans for many patients who really don't need them. I remember practicing medicine for 20 years without a single CT scan. Because we have the technology and there is a very unforgiving public out there, I will usually opt for a CT scan if I think it will give me information on a potentially serious problem. It's a good idea to check old records before you order the 10th CT scan on someone with vague abdominal pain who just had his ninth negative scan last month, but trying to practice either cost-effective or radiation-exposure safeguards is nearly impossible in today's environment.

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Reader Feedback

Readers are invited to ask specific questions and offer personal experiences, comments, or observations on InFocus topics. Literature references are appreciated. Pertinent responses will be published in a future issue. Please send comments to emn@lww.com. Dr. Roberts requests feedback on this month's column, especially personal experiences with successes, failures, and technique.

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IODINATED CONTRAST MEDIA IN PREGNANCY

Animal studies have failed to show an in vivo teratogenic effect of iodinated contrast material used for CT imaging. The iodine content of contrast media has the potential to produce neonatal hypothyroidism. The intravascular use of nonionic contrast media has been reported to have no effect on neonatal thyroid function.

Conclusion: Iodinated contrast seems safe to use in pregnancy.

Source: Eur Radiol 2005;15:1234.

© 2008 Lippincott Williams & Wilkins, Inc.