Correspondence
Computed Tomography and Radiation Exposure
N Engl J Med 2008; 358:850-853February 21, 2008
- Article
To the Editor:
Computed tomographic (CT) scans deliver a radiation dose of about 20 mSv. Brenner and Hall (Nov. 29 issue)1 assess the risk associated with CT radiation exposure by using the linear no-threshold extrapolation model, which assumes that cancer induction is proportional to dose even for the smallest doses. An excess of cancers has never been detected in laboratory animals or in humans for doses below 100 mSv. This model is used for analyzing data from cohorts including persons who have received doses higher than 100 mSv. This method is exposed to strong bias.2 Defense mechanisms against radiocarcinogenesis are much more effective at low doses, and the use of the linear no-threshold model in this dose range is highly debatable3,4; it greatly overestimates the risk. After repeated x-ray examinations, induction of cancer has been observed only when the cumulative dose was above 500 mSv. In patients treated with radiotherapy, a threshold was reported for irradiation doses of 0.6 Sv delivered in 30 sessions.5,6 Overestimation of the risk may deprive patients of beneficial examinations.
6 ReferencesMaurice Tubiana, M.D.
Institut Gustave Roussy, 94805 Villejuif, France
maurice.tubiana@biomedicale. univ-paris5. fr 1
Brenner DJ ,Hall EJ . Computed tomography -- an increasing source of radiation exposure. N Engl J Med 2007;357:2277-2284
Full Text | Web of Science | Medline2
Breckow J . Linear-no-threshold is a radiation-protection standard rather than a mechanistic effect model. Radiat Environ Biophys 2006;44:257-260
CrossRef | Web of Science | Medline3
Tubiana M ,Aurengo A ,Averbeck D ,Masse R . The debate on the use of linear on threshhold for assessing the effects of low doses. J Radiol Prot 2006;26:317-324
CrossRef | Web of Science | Medline4
Tubiana M ,Aurengo A ,Averbeck D ,Masse R . Low-dose risk assessment: comments on the summary of the International Workshop. Radiat Res 2007;167:742-744
CrossRef | Web of Science | Medline5
Rubino C ,de Vathaire F ,Shamsaldin A ,Labbe M ,Le MG . Radiation dose, chemotherapy, hormonal treatment and risk of second cancer after breast cancer treatment. Br J Cancer 2003;89:840-846
CrossRef | Web of Science | Medline6
Le Pogam MA, Rubino C, Diallo I, et al. Radiation dose fractionation and second cancer risks after breast cancer treatment. Radiat Prot Dosimetry (in press).
To the Editor:
The report by Brenner and Hall is based on the effects of low levels of ionizing radiation from data on atomic-bomb survivors. A linear no-threshold hypothesis derived from the database of survivors has been accepted as a policy for radiation protection. However, the hypothesis is still a subject of controversy1,2 — that is, neither proved nor disproved — and the excessive risk at low doses is a few percent and decreases with age. Favorable ratios of benefits to risk are essential in medical radiation.
2 ReferencesShigenobu Nagataki, M.D., Ph.D.
Japan Radioisotope Association, Tokyo 113-8941, Japan
nagataki@jrias.or. jp 1
Tubiana M, Aurengo A. La relation dose-effet et l'estimation des effets cancérogènes des faibles does de rayonnements ionisants. Paris: Report of National Academy of Medicine, March 2005. (Accessed February 1, 2008, at http://www.academie-sciences.fr/publications/rapports/pdf/dose_effet_07_04_05.pdf.)
2
Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, National Research Council. Health risks from exposure to low levels of ionizing radiation: BEIR VII phase 2. Washington, DC: National Academies Press, 2006. (Accessed February 1, 2008, at http://www.nap.edu/catalog.php?record_id=11340.)
To the Editor:
Brenner and Hall state that 1.5 to 2% of all cancers in the United States may be attributable to clinical use of CT. This does not comply with any human epidemiologic data that have not been modeled.1 Doses below about 150 mSv have never been observed to induce clinical cancer with statistical significance. There is, however, clear evidence that in mammalian organisms, including humans, low doses and dose rates of x-radiation and gamma radiation can up-regulate, with a delay and temporarily largely under genetic control, various existing physiological mechanisms of protection against induction, propagation, and accumulation of cellular damage in tissues to evolve into clinical disease.2-4
4 ReferencesLudwig E. Feinendegen, M.D.
Heinrich Heine University Düsseldorf, 40225 Düsseldorf, Germany
feinendegen@gmx.net 1
Cardis E ,Vrijheid M ,Blettner M , et al. The 15-country collaborative study of cancer risk among radiation workers in the nuclear industry: estimates of radiation-related cancer risks. Radiat Res 2007;167:396-416
CrossRef | Web of Science | Medline2
Aurengo A, Averbeck D, Bonnin A, et al. Dose-effect relationships and the estimation of the carcinogenic effects of low doses of ionizing radiation. Paris: National Academy of Medicine, March 30, 2005. (Accessed February 1, 2008, at http://www.radscihealth.org/rsh/Papers/FrenchAcadsFinal07_04_05.pdf.)
3
Feinendegen LE ,Neumann RD . Physics must join with biology in better assessing risk from low-dose irradiation. Radiat Prot Dosimetry 2005;117:346-356
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Feinendegen LE ,Pollycove M ,Neumann RD . Whole-body responses to low-level radiation exposure: new concepts in mammalian radiobiology. Exp Hematol 2007;35:Suppl 1:37-46
CrossRef | Web of Science | Medline
To the Editor:
It is estimated that since 1980, more than 550 million CT scans have been obtained in the United States, 75 million of them before 1990, as Brenner and Hall mention. Even assuming multiple CT examinations per patient, this cohort yields an extraordinary power for the detection of significant cancer-related mortality induced by ionization dose exposure in the 50-mGy range, as these same authors have claimed in a previous report.1 Where are these excess induced cancers?
1 ReferencesDimitri A. Dimitroyannis, Ph.D.
Kansas City Cancer Center, Kansas City, MO 64154
dimitri.dimitroyannis@usoncology. com 1
Brenner DJ ,Doll R ,Goodhead DT , et al. Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl Acad Sci U S A 2003;100:13761-13766
CrossRef | Web of Science | Medline
To the Editor:
We agree with Brenner and Hall's emphasis on CT use only when appropriate and with correct exposure factors. However, the techniques they cite are well beyond cited standards for children,1,2 and the long-range risk estimates are debatable.3,4
The concerns raised are being actively addressed by scientific and educational programs, such as through American College of Radiology accreditation, ALARA (As Low as Reasonably Achievable) conferences sponsored by the Society for Pediatric Radiology, and the Image Gently campaign (www.imagegently.org), which involves several national medical societies and regulatory agencies, with the participation of more than 400,000 health care professionals promoting appropriate, high-quality, and safe pediatric CT.
4 ReferencesDonald P. Frush, M.D.
Duke University Medical Center, Durham, NC 27710
frush943@mc.duke. edu Marylin J. Goske, M.D.
Cincinnati Children's Hospital Medical Center, Cincinnati, OH 45229Marta Hernanz-Schulman, M.D.
Vanderbilt Children's Hospital, Nashville, TN 372321
Boone JM ,Geraghty EM ,Seibert JA ,Wootton-Gorges SL . Dose reduction in pediatric CT: a rational approach. Radiology 2003;228:352-360
CrossRef | Web of Science | Medline2
Cody DD ,Moxley DM ,Krugh KT ,O'Daniel JC ,Wagner LK ,Eftekhari F . Strategies for formulating appropriate MDCT techniques when imaging the chest, abdomen, and pelvis in pediatric patients. AJR Am J Roentgenol 2004;182:849-859
Web of Science | Medline3
Brody AS, Frush DP, Huda W, Brent RL, American Academy of Pediatrics Section on Radiology
CrossRef | Web of Science | Medline4
Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, National Research Council. Health risks from exposure to low levels of ionizing radiation: BEIR VII phase 2. Washington, DC: National Academies Press, 2006. (Accessed February 1, 2008, at http://books.nap.edu/catalog.php?record_id=11340.)
To the Editor:
Radiation dose is a critical issue. An increased radiation dose also may result from new strategies. For example, CT fluoroscopy used as imaging guidance generates a radiation dose of up to 20 times the dose with conventional CT.1 Similarly, routine cardiac CT will presumably result in a substantial increase in the radiation dose in the general population.
In contrast, researchers are working on improving protocols to substantially reduce the radiation dose, by means of automatic exposure control2 and tailoring of the CT data acquisition to the patient's body habitus. To decrease the radiation dose in the general population, magnetic resonance can favorably replace CT for imaging of the liver, small bowel, vascular structures, urinary tract, and pelvis. Whole-body magnetic resonance units provide a comprehensive evaluation of patients with cancer. The major risk of an increased radiation dose mostly involves young patients scheduled to undergo repeated CT examinations. However, in some cases, such patients can be accurately evaluated with magnetic resonace imaging.3
3 ReferencesPhilippe Soyer, M.D., Ph.D.
Hôpital Lariboisière, 75010 Paris, France
philippe.soyer@lrb. aphp. fr 1
Silverman SG ,Tuncali K ,Adams DF ,Nawfel RD ,Zou KH ,Judy PF . CT fluoroscopy-guided abdominal interventions: techniques, results, and radiation exposure. Radiology 1999;212:673-681
Web of Science | Medline2
McCollough CH ,Bruesewitz MR ,Kofler JM Jr . CT dose reduction and dose management tools: overview of available options. Radiographics 2006;26:503-512
CrossRef | Web of Science | Medline3
Fidler J . MR imaging of the small bowel. Radiol Clin North Am 2007;45:317-331
CrossRef | Web of Science | Medline
To the Editor:
Brenner and Hall voice concern about radiation exposure and the subsequent risk of cancer in the population because of increasing numbers of CT scans, but they focus solely on postnatal exposure. The risk of childhood cancer per unit dose of radiation is significantly greater for intrauterine exposure than for exposure in the early years of postnatal life.1 Case–control studies have shown a 40 to 50% increase in the risk of cancer after intrauterine exposure to medical diagnostic radiation at doses of a single abdominal CT scan (10 to 20 mGy),2 which is equivalent to 500 chest x-rays. Of particular concern is the overall utilization of CT scans in pregnant women, which has increased by 25% per year during the past 10 years.3
3 ReferencesHeike Varnholt, M.D.
University Hospital of Cologne, 50924 Cologne, Germany1
Sadetzki S ,Flint-Richter P . Transgenerational effects of parental exposure to ionizing radiation. Harefuah 2006;145:516-521
Medline2
Wakeford R ,Little MP . Risk coefficients for childhood cancer after intrauterine irradiation: a review. Int J Radiat Biol 2003;79:293-309
CrossRef | Web of Science | Medline3
Lazarus E, DeBenedectis C, Mayo-Smith W, Spencer P. Utilization of radiological examinations in pregnant women: a ten-year review 1997-2006. Presented at the 93rd Annual Meeting of the Radiological Society of North America, Chicago, November 25–30, 2007.
Author/Editor Response
The annual number of CT scans in the United States is now more than 60 million. The concern is that radiation doses from CT are typically 100 times those from conventional x-ray examinations such as chest x-rays or mammograms and that there is now direct epidemiologic evidence of a small but significant increase in the risk of cancer at CT doses.1-3 Because CT is such a superb diagnostic tool and because individual CT risks are small, the CT benefit–risk balance is generally by far in the patient's favor.
However, there is general acceptance that perhaps one third of all CT scans could be avoided altogether or replaced by a different diagnostic tool. So although CT risks are small, a small risk multiplied by many millions of scans may translate into a public health concern some years in the future, particularly in the case of pediatric CT.
Critiques of these notions have fallen into four main categories. Critique 1: Cancer risks at very low doses are very uncertain and depend on extrapolating risks from atomic-bomb survivors who were exposed to high doses. At very low radiation doses, cancer risks are indeed very uncertain.3 However, at the higher doses corresponding to a few CT scans (5 to 100 mSv), there are direct epidemiologic data from about 30,000 atomic-bomb survivors who were on the peripheries of Hiroshima and Nagasaki and who were exposed in this same low-dose range. This low-dose subpopulation has been followed for more than 50 years and has a small but statistically significant increase in the risk of cancer.1,2 So we do not have to extrapolate CT-associated risks from those at higher doses, with all the attendant uncertainties that involves.
Critique 2: No studies of persons having CT scans have shown an increased cancer risk. There have, in fact, not been any CT-related epidemiologic studies to date, though one has recently begun, focusing on pediatric CT.
Critique 3: Many persons who need CT scans will not have them because of these cancer-risk estimates. The evidence does not support this: for example, in a recently published study,4 when parents were informed about CT risks, their willingness to have their child undergo CT did not significantly change, although they became more willing to consider other imaging options, if they were equally effective. No CT scans were canceled or deferred after the parents received the risk information.
Critique 4: It will be very difficult to reduce CT usage. We completely agree. Physicians are often subject to significant pressures, from the medical system, the medicolegal system, and the public, to order CT studies, even when they are not really necessary or even when alternatives exist. Our goal was to promote already ongoing dialogues among radiologists, emergency department and other physicians, and indeed the public about practical ways to reduce CT usage and CT doses, without compromising patient care.
4 ReferencesDavid J. Brenner. Ph.D., D.Sc.
Eric J. Hall, D.Phil., D.Sc.
Columbia University Medical Center, New York, NY 10032
djb3@columbia.edu 1
Pierce DA ,Preston DL . Radiation-related cancer risks at low doses among atomic bomb survivors. Radiat Res 2000;154:178-186
CrossRef | Web of Science | Medline2
Preston DL ,Ron E ,Tokuoka S , et al. Solid cancer incidence in atomic bomb survivors: 1958-1998. Radiat Res 2007;168:1-64
CrossRef | Web of Science | Medline3
Brenner DJ ,Doll R ,Goodhead DT , et al. Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc Natl Acad Sci U S A 2003;100:13761-13766
CrossRef | Web of Science | Medline4
Larson DB ,Rader SB ,Forman HP ,Fenton LZ . Informing parents about CT radiation exposure in children: it's OK to tell them. AJR Am J Roentgenol 2007;189:271-275
CrossRef | Web of Science | Medline
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