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Commentary |
National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ, UK
Early in 2001 the radiology community in the USA was rocked by national newspaper headlines linking CT scans on children to radiation-induced cancer. An article on the front page of USA Today [1] claimed that 1500 cancer deaths would be caused annually by the prevailing rate of CT examinations on American children. Blame was firmly laid at the door of the radiology profession for not being aware of the risks and for using unnecessarily high doses when scanning children by not compensating for their smaller size when setting the CT exposure factors. Media attention had been attracted by a trio of papers in the February 2001 issue of the American Journal of Roentgenology [2–4]. The seminal paper [2] was by David Brenner, Carl Elliston, Eric Hall and Walter Berdon, in which they used the latest Japanese bomb survivor data to estimate that the lifetime cancer mortality risk to a 1-year-old child from the radiation dose associated with a single abdominal CT scan was about one in 550. This was claimed to be an order of magnitude greater than the risk for a mature adult patient having the same CT examination, due to both the higher doses being delivered to children and their predicted higher sensitivity to the carcinogenic effects of radiation. The other two papers provided evidence that it was indeed common American practice for children to be examined using the same CT scanning parameters as for adults [3] and gave some guidance on how the parameters could be adjusted to minimize the dose for smaller patients without loss of important diagnostic information [4].
There was a mixed response to this alarming news from the radiology and medical physics professions. On the one hand, the press was accused of sensationalism and of causing unwarranted anxiety for parents whose children had undergone or were going to have CT scans. On the other hand, it was accepted that the safety issues might well be significant and the publicity would encourage long overdue changes in paediatric CT practice and in the attention paid by CT scanner manufacturers to patient dose reduction. Against this background of heightened awareness and apprehension, the National Council on Radiation Protection and Measurements (NCRP) organized a 2-day symposium in November 2002, at which the subject of excessive patient doses in CT was to be thoroughly discussed. On the first day, key players from radiology and radiation protection organizations in the USA, including radiologists, radiographers, physicists, CT manufacturers and public health officials were invited to outline the problems and to suggest possible solutions. On the second day, the emphasis was on active participation from the audience to develop, in the course of a few hours, a coherent set of recommendations for reducing exposures while maintaining the essential diagnostic value of CT examinations.
Fred Mettler, chairman of the NCRP and ICRP committees dealing with Radiation Protection in Medicine, was the driving force behind the Symposium. In the first session he reviewed the large contribution that CT makes to medical imaging in the USA, admitting that CT now comprised 15% of the examinations and 70% of the collective dose in his radiology department at the University of New Mexico, School of Medicine. Lee Rogers, editor of the American Journal of Roentgenology, described the initial reticence in the USA to accept that such an outstanding imaging modality could be associated with excessively high patient doses. Fortunately attitudes are changing, to the extent that he now believes patient dose to be firmly on the front burner in terms of marketing CT scanners.
The radiation risks from CT exposures were reviewed by Eric Hall, a biologist from Columbia University and co-author of the seminal paper in the AJR [2]. To quell widespread American doubts about the linear, no-threshold model for the dose–effect relationship, he was at pains to point out that organ doses for typical paediatric CT examinations in the USA had now reached levels of 5 mSv to 100 mSv. These doses are within the range for which a statistically significant excess cancer rate has been directly observed in the Japanese atomic bomb survivors. Consequently no models or extrapolations are required to predict that the lifetime fatal cancer risks for children from abdominal CT scans could be as high as 1 in 1000. Hall quoted the UK Royal Society recommendation [5] that such levels of risk are not totally unacceptable as long as the individuals involved know about it, there is some commensurable benefit and everything reasonable has been done to reduce the risk. In the context of paediatric CT, he interpreted these requirements as obtaining informed consent, using CT only when clinically indicated and tailoring the exposure parameters to the size of the patient.
The radiation dose distribution in the patient from CT scans is complex and a bewildering variety of dose concepts and quantities have evolved from both sides of the Atlantic in an attempt to quantify and control the radiation risks to CT patients. Cynthia McCollough, a medical physicist from the Mayo Clinic and chair of the American Association of Physicists in Medicine (AAPM) CT Dose Task Group, stressed the need for international agreement on a consistent set of clearly defined CT dose descriptors. She has played a large part in developing new International Electrotechnical Commission (IEC) Standards for CT dosimetry [6], which reassuringly (for us) closely follow the approach advocated over the past few years by UK and European physicists. American CT users and manufacturers are now being encouraged to adopt this common approach. Later in the day, Cynthia also reviewed current technical initiatives for patient dose reduction in CT. All the latest scanners provide some form of X-ray tube current modulation based on a determination of patient thickness from previous scans or earlier parts of the same scan. However, the ultimate goal is for complete automatic exposure control (AEC), where the detector signals are instantaneously and continuously used to modulate the X-ray tube output to produce a desired noise level in the final image. Presentations by the major CT manufacturers indicated that the technology to do this is being rapidly developed and all that remains is for radiologists to agree on acceptable levels of noise, i.e. image quality, for different CT imaging tasks.
The regulation and control of medical imaging facilities in the USA is carried out by a combination of federal and state agencies. The Center for Devices and Radiological Health (CDRH) of the US Food and Drug Administration (FDA) sets technical safety requirements for new X-ray imaging equipment including CT scanners and organizes national surveys of medical imaging practice. Stanley Stern from CDRH described the recent Nationwide Evaluation of X-ray Trends (NEXT) survey of patient doses from CT examinations. Specially trained inspectors from 39 States surveyed 263 randomly selected CT facilities, measuring doses in a standard CT head phantom and collecting information on examination protocols and frequencies. Average effective doses for adult patients of 2 mSv for head scans and 6–15 mSv for body scans were not dissimilar to those seen in earlier surveys in the UK [7]. However, about 1 in 5 Americans had a CT examination during the survey year (2000–2001), which is 7 times the current rate in the UK.
The initiative taken by the American College of Radiology (ACR) in setting up an accreditation programme for CT facilities throughout the USA was described by James Brink, a radiologist from Yale University School of Medicine. Although not a regulatory requirement, accreditation is likely to be essential for those facilities seeking reimbursement from health insurance schemes and highly attractive to those wishing to reduce the opportunities for patient litigation in the USA. The accreditation programme will evaluate staff qualifications, examination protocols and equipment performance by assessing phantom images and comparing measured doses with reference doses for adult and paediatric CT examinations. The reference doses have been based initially on earlier European guidelines [8] and a 3 year old NRPB publication [9], but will be adapted to the American situation as soon as sufficient USA-wide data have been collected and analysed.
On the second day of the Symposium the 130 participants could choose between four working sessions to develop recommendations in the areas of education, equipment, policy/regulations and clinical matters. After a couple of hours of intense discussion within these groups, the participants re-assembled in a plenary session to be presented with a summary of the recommendations drafted by each group and the opportunity for further discussion and possible improvement. After, at times, fairly lively debate, a list of key recommendations was drawn up by Fred Mettler and will be published in full by NCRP towards the middle of 2003. As a flavour of what is to come, some of the recommendations that engendered most debate but appeared to be finally agreed upon, were as follows.
Regarding "education", great importance was given to improving the understanding of CT doses and risks by both radiology professionals and patients, despite misgivings expressed by a few participants about the reliability of the available quantitative risk estimates. The ACR was consequently recommended to collaborate with the AAPM to prepare a booklet that physicians could use when counselling patients, which put the radiation risks from CT into perspective with other everyday risks. The need to avoid scaring patients away from justified CT examinations by placing too much emphasis on the risks and not enough on the benefits, was raised many times throughout the discussions. Three of the key recommendations on "equipment" were to develop patient-size based technique charts for all existing scanners, to provide standard clinical protocols with target CT dose index (CTDIvol) values for common examinations on new scanners and to introduce true, real-time AEC as soon as possible. There had obviously been a turf battle in the past between physicists and manufacturers over the "invention" of new CT dose descriptors and terminology, but now this had been resolved through the auspices of the IEC [6], a recommendation to "put a moratorium on new dose terms" was accepted in spirit, if not in this exact wording. The recommendations on "policy/regulations" endorsed the ACR accreditation programme and the organization of CT dose data collection at a federal level, but much of the discussion reverted to the question of how to clearly present CT dose and risk information without scaring patients off. In relation to "clinical matters", the importance of continuing professional education was emphasised, for both radiographers and radiologists. Regularly scheduled in-service training programmes to keep them up-to-date with CT dose-reduction techniques, particularly for smaller patients, and with the amazing capabilities of the latest multislice CT scanners, were considered essential. It was stressed that radiologists need more help from the manufacturers than just the occasional visit from an applications specialist.
By organizing this Symposium, American radiologists and radiation protectionists have risen to the challenge posed by the high doses and enthusiastic use of CT scanning on children in their country. Whether they will be successful in managing this situation to the maximum benefit of patients, depends on the effectiveness and practicality of the final list of recommendations and ultimately on the enthusiasm with which they are implemented by radiologists, radiographers, medical physicists and CT manufacturers throughout the USA. While we can take comfort in the generally more cautious approach to paediatric CT on this side of the Atlantic, we will watch developments in the USA closely and should be grateful to our American colleagues for grasping the nettle and giving this subject the thorough airing it so clearly deserves.
Footnotes
Published with permission of the National Radiological Protection Board. 
Received for publication March 7, 2003. Revision received July 31, 2003. Accepted for publication August 6, 2003.
References
This article has been cited by other articles:
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  W. E. Muhogora, N. A. Ahmed, A. Beganovic, A. Benider, O. Ciraj-Bjelac, V. Gershan, E. Gershkevitsh, E. Grupetta, M. H. Kharita, N. Manatrakul, et al. Patient doses in CT Examinations in 18 countries: initial results from international atomic energy agency projects Radiat Prot Dosimetry, September 1, 2009; 136(2): 118 - 126. [Abstract] [Full Text] [PDF]
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 D. B. Larson, S. B. Rader, H. P. Forman, and L. Z. Fenton Informing Parents About CT Radiation Exposure in Children: It's OK to Tell Them Am. J. Roentgenol., August 1, 2007; 189(2): 271 - 275. [Abstract] [Full Text] [PDF]
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