Moderator ventana Posted March 10, 2011 Moderator Share Posted March 10, 2011 http://www.jaapa.com/pages/downloadmedia.aspx?articleid=197445&downloadid=5290 From JAAPA Radiology: CT scans and radiation exposure From CSAC, the Clinical and Scientific Affairs Committee of the AAPA March 11 2011 TAKE-HOME POINTS ■ The use of CT scans is increasing rapidly, and while these tests are useful, they carry significant risk. ■ Lifetime attributable risk varies by age, body habitus, and gender. ■ Establishing that the benefit of CT imaging is greater than the risk of its use will help minimize exposure to ionizing radiation. ■ PAs should educate themselves and their patients about the risks associated with CT imaging. ■ Minimizing lifetime exposure remains the most effective way of reducing ionizing radiation effects. WHO SHOULD READ THIS? Any PA that recommends or orders CT-guided imagery. WHY IS THIS IMPORTANT? In the past 2 decades, the use of computed tomography (CT) has increased more than 20-fold. In 2007, CT scanning was performed an estimated 70 million times in the United States, and 4 million of these scans were obtained on patients younger than 18 years.1,2 Because of the widespread availability of CT and its ease of use, many practitioners are ordering CT that may not be warranted. In a straw poll, pediatric radiologists suggested that perhaps one-third of CT studies could be replaced by alternative approaches or not performed at all.3,4 Exposure to ionizing radiation carries risk. In 2009-2010, the overexposure of more than 400 patients to radiation during brain perfusion CT scanning resulted in hair loss and skin redness.5 Concerns over the dose of radiation these patients received led the FDA to update their recommendations for CT scanning.5 While this type of overexposure results in visible changes (deterministic effects), other effects can take years to decades to manifest (stochastic effects) and include cataracts, cancer, and genetic effects. Investigations into the amounts of radiation patients receive have determined this to be much more variable than previously thought. In a 2007 New England Journal of Medicine article, Brenner and Hall noted a 6- to 22-fold increase in radiation exposure between the highest and lowest doses of radiation for each CT study type.6 Even though CT provides "low level" radiation exposure, no evidence suggests that any amount of radiation exposure is safe, and each exposure increases cancer risk in a cumulative fashion.1,2,6 There is also clear evidence that age at time of exposure and sex play a significant role in later development of cancers due to ionizing radiation.7-9 Unfortunately no clear guidelines are currently available, so PAs must take care to order CT only when the benefit outweighs the risk and when less invasive alternatives will not provide adequate clinical information. WHAT ARE SOME COMMON TERMS TO UNDERSTAND? Radiation doses are expressed in grays (Gy) or milligrays (mGy), where one Gy = 1 joule (a measure of ionizing energy absorbed per unit of mass) per kilogram of body mass. Radiation dose is also frequently expressed in sieverts (Sv) or millisieverts (mSv) and is a measure of the "equivalent" radiation dose. Like Gy, one Sv = 1 joule per kilogram of body mass. For x-ray radiation, the type used in CT scanners, 1 mSv = 1 mGy. As a reference point, a typical posterior-anterior chest radiograph provides 0.15 mSv, a mammogram provides 3 mSv, and an abdominal CT provides 10 mSv. These doses are averages; as previously noted, significant variations in doses received exist.2 Depending on the facility, the radiation may be expressed in either mSv or mGy, which are equivalent. WHAT ARE THE RISKS OF RADIATION EXPOSURE? Several large, long-term studies have been done of persons who have been exposed to low levels of radiation at the same dosage level as individuals receiving a single CT scan.1,10,11 These studies include atomic bomb survivors from Hiroshima and Nagasaki, US-based nuclear workers, and inhabitants from Russia's Chernobyl region who have been exposed to a single 5-100-mSv radiation dose. These studies have demonstrated that a linear-no-threshold risk for the development of cancer exists, meaning that the risk of cancer "proceeds in a linear fashion at lower doses without a threshold and that the smallest dose has the potential to cause a small increase in risk."11 The literature also estimates that it may take up to 20 years before the increased risk of cancer manifests. Sex, age, weight, and the area of the body scanned all significantly impact total radiation exposure and resulting risk of cancer development. Those at greatest risk for developing radiation exposure-related cancer later in life are children and women in their 20s. The overall risk for developing a solid cancer or leukemia from a single 10-mSv-dose is 1:1,000.11 For a 20-year-old woman undergoing a single coronary angiography CT, the risk increases to 1:143.7 The increased risk to women is primarily from later development of lung and breast cancer. The estimated risk for a 20-year-old man is considerably lower at 1:686.7 In both sexes, the lifetime attributable risk for one scan decreases with advancing age, although women continue to remain more susceptible than men to the effects of ionizing radiation. In the pediatric population, the increase in cancer risk may be twice as high as in adults. Brenner and colleagues estimated that a 1-year-old receiving a single abdominal CT scan would have an estimated lifetime cancer risk of 1.8:1,000 and 0.7:1,000 for those undergoing head CT.12 This means that out of the 600,000 abdominal and head CTs performed in 2001 (during the study), an estimated 500 persons might die from a cancer attributable to CT radiation.12 Patients who are obese may be at greater risk for later development of cancer because of the increased dose of radiation needed to achieve adequate image quality. In the same respect, pediatric patients may be at increased risk, as equipment may not be recalibrated from adult settings, exposing children to higher levels of radiation than necessary.12 WHAT OTHER CONCERNS SHOULD PAs BE AWARE OF? Radiation doses vary greatly based on center and technician. The median effective radiation dose of an abdominal and pelvic CT (the most common examination performed in the United States) is widely reported to be 8-10 mSv. Yet a study by Smith-Bindman and colleagues found the median dose to be 66% higher than this figure, and the dose for a multiphase CT scan was nearly 4-fold higher.2 In addition, the researchers found a 13-fold variation in dose level (from high to low) in each CT study type they investigated. This investigation involved four medical centers in the San Francisco Bay area and points to the need for everyone involved, including patients, technicians, and ordering providers, to think through the need for the study and to ask questions, including whether the settings and exposures are correct for the patient's age, body type, and procedure, before the examination is performed. WHAT IS THE BOTTOM LINE? It is extremely important that all health care providers be aware of the risks associated with CT scans, especially in the pediatric, obese, and female populations. In order to reduce cancer risk, patients should be educated about the potential risks associated with all radiation technologies, but particularly with CT imaging. Radiologists and technicians should reduce the dose of ionizing radiation or tailor the examinations to obtain optimal diagnostic images while minimizing exposure. The number of CT studies to which a patient is subjected should be minimized by talking with the radiologist about alternatives to the CT exam or by using clinical judgment and physical examination skills to determine appropriate treatment without use of CT. Finally, keeping track of the number of exposures to ionizing radiation that patients receive may help to reduce their overall lifetime exposure and therefore minimize their lifetime risk; however, no clear mechanism for accomplishing this is currently available.2,9,13 JAAPA This article was written by Anthony E. Brenneman, MPAS, PA-C. Contributors included the other members and staff of CSAC 2010-2011: Alison C. Essary, MHPE, PA-C, Chair; Gilbert A. Boissonneault, PhD, PA-C; Marie-Michèle Léger, MPH, PA-C; Mark F. McKinnon, PA-C; Thomas Moreau, PA-C, MS; and Folusho E. Ogunfiditimi, PA-C. The manuscript was edited by Sarah Zarbock, PA-C. REFERENCES 1. Hall E, Brenner DJ. Cancer risks from diagnostic radiology. Br J Radiol. 2008;81(965):362-378. 2. Smith-Bindman R, Lipson J, Marcus R, et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med. 2009;169(22):2078-2086. 3. Slovis TL, Berdon WE. Panel discussion. Pediatr Radiol. 2002; 32:242-244. 4. Donnelly LF. Reducing radiation dose associated with pediatric CT by decreasing unnecessary examinations. Am J Roentgenol. 2005;184(2):655-657. 5. Safety investigation of CT brain perfusion scans: update 11/9/2010. http://www.fda.gov/medicaldevices/safety/ alertsandnotices/ucm185898.htm. Updated November 9, 2010. Accessed February 2, 2011. 6. Brenner DJ, Hall EJ. Computed tomography—an increasing source of radiation exposure. N Engl J Med. 2007;357(22): 2277-2284. 7. Einstein AJ, Henzlova MJ, Rajagopalan S. Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography. JAMA. 2007; 298(3):317-323. 8. Preston DL, Cullings H, Suyama F, et al. Solid cancer incidence in atomic bomb survivors exposed in utero or as young children. JNCI. 2008;100(6):428-436. 9. Sodickson A, Baeyens PF, Andriole KP, et al. Recurrent CT, cumulative radiation exposure, and associated radiation-induced cancer risks from CT of adults. Radiology. 2009;251(1):175-184. 10. Pierce DA, Preston DL. Radiation-related cancer risks at low doses among atomic bomb survivors. Radiat Res. 2000; 154(2):178-186. 11. Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation. Nuclear and Radiation Studies Board. Division on Earth and Life Studies. National Research Council of the National Academies. Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2. Washington, DC: The National Academies; 2006. 12. Brenner DJ, Elliston CD, Hall EF, Berdon WE. Estimated risks of radiation-induced fatal cancer from pedriatric CT. AJR Am J Roentgenol. 2001;176(2):289-296. 13. Rehani M, Frushy D. Tracking radiation exposure of patients. Lancet. 2010;376(9743):754-555. Link to comment Share on other sites More sharing options...
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