In the News focuses on media reports that your patients may have questions about at their next visit. This continuing column will provide summaries of articles in the popular press that may prompt such questions, as well as comments from colleagues in the field.
Computed tomography scans with cumulative doses of about 50 mGy received during childhood can nearly triple the risk of leukemia, and cumulative doses of about 60 mGy can triple the risk of brain cancer, according to a retrospective cohort study of patients who were younger than 22 when they received CT scans and had no previous cancer diagnoses. In their interpretation of the data, the authors of the study published in The Lancet1 noted that even though “the cumulative absolute risks are small,” they should be outweighed by the clinical benefits. “[R]adiation doses from CT scans ought to be kept as low as possible and alternative procedures, which do not involve ionising radiation, should be considered if appropriate,” they added.
“This cohort study provides the first direct evidence of a link between exposure to radiation from CT and cancer risk in children,” said senior author Amy Berrington de González, PhD, Senior Investigator, Division of Cancer Epidemiology and Genetics at the National Cancer Institute (NCI). “Ours is the first population-based study to capture data on every CT scan to an individual during childhood or young adulthood and then measure the subsequent cancer risk.”2
In the time since the study was first published online in June, there has been much “positive feedback about the importance of the study findings from physicians, parents, and researchers,” Dr. Berrington de González commented to The ASCO Post. “However, it is too early at this point to know what the impact might be on practice. We will monitor levels of use and dose levels to assess the potential long-term impact.”
UK Data, U.S. Analysis
Patients were scanned at National Health Service (NHS) Centres in Great Britain between 1985 and 2002. Data for cancer incidence, mortality, and loss to follow-up were obtained from the NHS Central Registry, which “holds computerized records of everyone registered with an NHS general practitioner in Great Britain (most residents),” the study authors noted. “To avoid inclusion of CT scans related to cancer diagnosis, follow-up for leukaemia began 2 years after the first CT and for brain tumours 5 years after the first CT.”
Leading the study were researchers from NCI and the Institute of Health and Society, Newcastle University, England. “The NCI scientists provided expertise in radiation epidemiology, radiation dosimetry, and statistical analysis,” Dr. Berrington de González explained. “The Intramural Research Program of the NCI also cofunded the study along with the United Kingdom Department of Health.”
The study merited news coverage by BBC News as well as major media in the United States, including National Public Radio, The New York Times, the Los Angeles Times, and The Wall Street Journal.
Highly Sensitive Tissues
Among 178,604 patients included in the leukemia analysis, 74 were diagnosed with leukemia. Among 176,587 patients included in the brain tumor analysis, 135 were diagnosed with brain tumors. The investigators chose these two cancer sites because “the red bone marrow and brain are highly sensitive tissues, especially in children.” Dr. Berrington de González noted that “the thyroid and breast are also highly radiosensitive if exposed at a young age. However, these cancers are very uncommon in childhood, so we do not yet have enough cases in our population. Once we do, these will be the next priorities for investigation.”
In the analysis of leukemia risk, 64% of the CT scans involved the head, and the rate was similar for the brain tumor analysis. The next most common sites of CT scans were the abdomen and/or pelvis and chest. These rates are “also very typical of the distribution today,” according to Dr. Berrington de González.
“The risk of leukaemia was positively associated with estimated doses delivered by CT scans to the red bone marrow (P = .0097), as was the risk of brain tumors associated with estimated doses delivered by CT scans to the brain tissue (P = .0001),” the authors stated. While leukemia and brain tumors are common childhood cancers, they are still relatively rare childhood diseases. According to the NCI, annual incidence rates in the United States for children from birth through age 21 are 4.3 per 100,000 for leukemia and 2.9 for brain cancers.
“Because these cancers are relatively rare, the cumulative risks are small: in the 10 years after the first scan for patients younger than 10 years, one excess case of leukaemia and one excess case of brain tumour per 10,000 head CT scans is estimated to occur,” the investigators reported.
Number of Scans
To reach the cumulative dose to the red bone marrow of 50 mGy estimated to nearly triple the risk of leukemia would “broadly speaking” require about 5 to 10 head scans with current scanner settings for children under 15, according to the authors. To reach the cumulative dose to the brain of 60 mGy necessary to triple the risk of brain cancer would generally require about two to three CT scans with current settings.
“Our understanding is that these estimates would also be typical of practice today in the United Kingdom and United States,” Dr. Berrington de González stated. The comparison groups for both analyses consisted of patients with cumulative doses of less than 5 mGy.
An NCI press release about the study noted that while radiation dose levels for CTs have declined over the years, “the amount of radiation delivered during a single CT scan can still vary greatly and is often up to 10 times higher than that delivered in a conventional x-ray procedure.”2
Using vs Not Using CT
“This paper confirms that radiation, even in relatively low does, does lead to risk,” Alan W. Craft, MD, a coauthor of the paper and Emeritus Chair at Newcastle University said in an article in The Wall Street Journal.3 “There is no safe risk.” In a BBC news release, 4 he also noted, “There is a much greater risk of not doing a CT when it’s suggested. This study will push us to be even more circumspect about using it.”
An American College of Radiology (ACR) news release stressed that quality and safety programs help ensure that medical imaging is safe and effective. “The authors correctly note that CT scanners from the period studied (1989–2003) used radiation doses much higher than those of today. Current protocols also allow for use of much lower dose than the study period,” according to the ACR. “As the authors highlight, providers are far more aware of the potential risks involved than in years past and are proceeding as if low doses of radiation cause harm.”5
In addition, the ACR statement noted: “Medical imaging exams are directly linked to greater life expectancy, declines in cancer mortality rates, and are generally safer and less expensive that the invasive procedures that they replace.”
Minimizing Radiation Doses
“Physicians, other pediatric health care providers, CT technologists, CT manufacturers, and various medical and governmental organizations share the responsibility to minimize CT radiation doses to children,” according to Radiation Risks and Pediatric Computed Tomography (CT): A Guide for Health Care Providers,6 which is an educational leaflet for health care providers developed by the NCI and updated the same day as the early online release of the Lancet article (June 7, 2012).
The brochure suggests that “communication between pediatric health care providers and radiologists can determine the need for CT and the technique to be used.”
“There are standard indications for CT in children, and radiologists should review reasons prior to every pediatric scan and be available for consultation when indications are uncertain. When appropriate, other modalities such as ultrasound or magnetic resonance imaging (MRI), which do not use ionizing radiation, should be considered,” according to the leaflet.
Dr. Berrington de González noted, “The radiation dose from conventional x-rays are typically about 10 times smaller than CT. Use of these alternatives would be highly dependent on the clinical situation, but physicians can consult the ACR Appropriateness Criteria (http://www.acr.org/Quality-Safety/Appropriateness-Criteria) to assess what the best test is considered to be for a wide variety of diagnostic scenarios.”
The NCI brochure also suggests adjusting exposure parameters for pediatric CT based on the individual child’s size and weight, limiting the region scanned to the smallest possible area, and considering lower mA and/or kVp settings for skeletal and lung imaging, and for some CT angiographic and follow-up examinations.
The highest-quality images (requiring the most radiation) “are not always required to make diagnoses,” the NCI leaflet notes. “In many cases, lower-resolution scans are diagnostic. Providers should be familiar with the dose descriptors available on CT scanners and minimize the use of CT examinations that use multiple scans obtained during different phases of contrast enhancement (multiphase examinations). These multiphase examinations result in a considerable increase in dose and are rarely necessary, especially in body (chest and abdomen) imaging.”
The article in The Lancet pointed out that CT rates “have been rising rapidly in the developed world.” Asked if at least some of that increase could be due to availability of testing rather than strictly diagnostic need, Dr. Berrington de González responded, “Yes, despite the rises in many developed countries, rates of use still vary widely and seem to be somewhat related to availability. For example, rates of CT scan use are seven times higher in the U.S. than in the UK. Strategies that could prevent rates from rising further are to avoid repeat testing in different facilities by transferring medical records, and using the ACR appropriateness criteria to ensure that an examination is clinically justified.” ■
Disclosure: Dr. Berrington de González reported no potential conflicts of interest.
References
1. Pearce MS, Salotti JA, Little MP, et al: Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: A retrospective cohort study. Lancet 380:499-505, 2012.
2. National Cancer Institute: NIH study finds childhood cancer CT scans linked to leukemia and brain cancer later in life. Press release, June 7, 2012. Available at www.cancer.gov. Accessed July 31, 2012.
3. Wang SS: Children’s CT scans pose cancer risk. Wall Street Journal. June 7, 2012.
4. Dreaper J: CT scans on children ‘could triple brain cancer risk.’ BBC News. June 6, 2012.
5. American College of Radiology: Pediatric CT scans save lives when used appropriately. Press release, June 6, 2012. Available at www.acr.org. Accessed August 22, 2012.
6. National Cancer Institute: Radiation risks and pediatric computed tomography (CT): A guide for health care providers. Available at www.cancer.gov. Accessed July 31, 2012.
