To the Editor: Computed tomographic (CT) scans deliver a radiationdose of about 20 mSv. Brenner and Hall (Nov. 29 issue)1 assessthe risk associated with CT radiation exposure by using thelinear no-threshold extrapolation model, which assumes thatcancer induction is proportional to dose even for the smallestdoses. An excess of cancers has never been detected in laboratoryanimals or in humans for doses below 100 mSv. This model isused for analyzing data from cohorts including persons who havereceived doses higher than 100 mSv. This method is exposed tostrong bias.2 Defense mechanisms against radiocarcinogenesisare much more effective at low doses, and the use of the linearno-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 cumulativedose was above 500 mSv. In patients treated with radiotherapy,a threshold was reported for irradiation doses of 0.6 Sv deliveredin 30 sessions.5,6 Overestimation of the risk may deprive patientsof beneficial examinations.
Maurice Tubiana, M.D. Institut Gustave Roussy 94805 Villejuif, France maurice.tubiana{at}biomedicale.univ-paris5.fr
References
Brenner DJ, Hall EJ. Computed tomography -- an increasing source of radiation exposure. N Engl J Med 2007;357:2277-2284. [Free Full Text]
Breckow J. Linear-no-threshold is a radiation-protection standard rather than a mechanistic effect model. Radiat Environ Biophys 2006;44:257-260. [CrossRef][ISI][Medline]
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][ISI][Medline]
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][ISI][Medline]
Rubino C, de Vathaire F, Shamsaldin A, Labbe M, Lê MG. Radiation dose, chemotherapy, hormonal treatment and risk of second cancer after breast cancer treatment. Br J Cancer 2003;89:840-846. [CrossRef][ISI][Medline]
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 theeffects of low levels of ionizing radiation from data on atomic-bombsurvivors. A linear no-threshold hypothesis derived from thedatabase of survivors has been accepted as a policy for radiationprotection. However, the hypothesis is still a subject of controversy1,2— that is, neither proved nor disproved — and theexcessive risk at low doses is a few percent and decreases withage. Favorable ratios of benefits to risk are essential in medicalradiation.
Shigenobu Nagataki, M.D., Ph.D. Japan Radioisotope Association Tokyo 113-8941, Japan nagataki{at}jrias.or.jp
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 allcancers in the United States may be attributable to clinicaluse of CT. This does not comply with any human epidemiologicdata that have not been modeled.1 Doses below about 150 mSvhave never been observed to induce clinical cancer with statisticalsignificance. There is, however, clear evidence that in mammalianorganisms, including humans, low doses and dose rates of x-radiationand gamma radiation can up-regulate, with a delay and temporarilylargely under genetic control, various existing physiologicalmechanisms of protection against induction, propagation, andaccumulation of cellular damage in tissues to evolve into clinicaldisease.2,3,4
Ludwig E. Feinendegen, M.D. Heinrich Heine University Düsseldorf 40225 Düsseldorf, Germany feinendegen{at}gmx.net
References
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][ISI][Medline]
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.)
Feinendegen LE, Neumann RD. Physics must join with biology in better assessing risk from low-dose irradiation. Radiat Prot Dosimetry 2005;117:346-356. [Free Full Text]
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][ISI][Medline]
To the Editor: It is estimated that since 1980, more than 550million CT scans have been obtained in the United States, 75million of them before 1990, as Brenner and Hall mention. Evenassuming multiple CT examinations per patient, this cohort yieldsan extraordinary power for the detection of significant cancer-relatedmortality induced by ionization dose exposure in the 50-mGyrange, as these same authors have claimed in a previous report.1Where are these excess induced cancers?
Dimitri A. Dimitroyannis, Ph.D. Kansas City Cancer Center Kansas City, MO 64154 dimitri.dimitroyannis{at}usoncology.com
References
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. [Free Full Text]
To the Editor: We agree with Brenner and Hall's emphasis onCT use only when appropriate and with correct exposure factors.However, the techniques they cite are well beyond cited standardsfor children,1,2 and the long-range risk estimates are debatable.3,4
The concerns raised are being actively addressed by scientificand educational programs, such as through American College ofRadiology accreditation, ALARA (As Low as Reasonably Achievable)conferences sponsored by the Society for Pediatric Radiology,and the Image Gently campaign (www.imagegently.org), which involvesseveral national medical societies and regulatory agencies,with the participation of more than 400,000 health care professionalspromoting appropriate, high-quality, and safe pediatric CT.
Donald P. Frush, M.D. Duke University Medical Center Durham, NC 27710 frush943{at}mc.duke.edu
Marta Hernanz-Schulman, M.D. Vanderbilt Children's Hospital Nashville, TN 37232
References
Boone JM, Geraghty EM, Seibert JA, Wootton-Gorges SL. Dose reduction in pediatric CT: a rational approach. Radiology 2003;228:352-360. [Free Full Text]
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. [Free Full Text]
Brody AS, Frush DP, Huda W, Brent RL, American Academy of Pediatrics Section on Radiology. Radiation risk to children from computed tomography. Pediatrics 2007;120:677-682. [Free Full Text]
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 increasedradiation dose also may result from new strategies. For example,CT fluoroscopy used as imaging guidance generates a radiationdose of up to 20 times the dose with conventional CT.1 Similarly,routine cardiac CT will presumably result in a substantial increasein the radiation dose in the general population.
In contrast, researchers are working on improving protocolsto substantially reduce the radiation dose, by means of automaticexposure control2 and tailoring of the CT data acquisition tothe patient's body habitus. To decrease the radiation dose inthe general population, magnetic resonance can favorably replaceCT for imaging of the liver, small bowel, vascular structures,urinary tract, and pelvis. Whole-body magnetic resonance unitsprovide a comprehensive evaluation of patients with cancer.The major risk of an increased radiation dose mostly involvesyoung patients scheduled to undergo repeated CT examinations.However, in some cases, such patients can be accurately evaluatedwith magnetic resonace imaging.3
Philippe Soyer, M.D., Ph.D. Hôpital Lariboisière 75010 Paris, France philippe.soyer{at}lrb.aphp.fr
References
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. [Free Full Text]
McCollough CH, Bruesewitz MR, Kofler JM Jr. CT dose reduction and dose management tools: overview of available options. Radiographics 2006;26:503-512. [Free Full Text]
Fidler J. MR imaging of the small bowel. Radiol Clin North Am 2007;45:317-331. [CrossRef][ISI][Medline]
To the Editor: Brenner and Hall voice concern about radiationexposure and the subsequent risk of cancer in the populationbecause of increasing numbers of CT scans, but they focus solelyon postnatal exposure. The risk of childhood cancer per unitdose of radiation is significantly greater for intrauterineexposure than for exposure in the early years of postnatal life.1Case–control studies have shown a 40 to 50% increase inthe risk of cancer after intrauterine exposure to medical diagnosticradiation at doses of a single abdominal CT scan (10 to 20 mGy),2which is equivalent to 500 chest x-rays. Of particular concernis the overall utilization of CT scans in pregnant women, whichhas increased by 25% per year during the past 10 years.3
Heike Varnholt, M.D. University Hospital of Cologne 50924 Cologne, Germany
References
Sadetzki S, Flint-Richter P. Transgenerational effects of parental exposure to ionizing radiation. Harefuah 2006;145:516-521. [Medline]
Wakeford R, Little MP. Risk coefficients for childhood cancer after intrauterine irradiation: a review. Int J Radiat Biol 2003;79:293-309. [CrossRef][ISI][Medline]
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.
The authors reply: The annual number of CT scans in the UnitedStates is now more than 60 million. The concern is that radiationdoses from CT are typically 100 times those from conventionalx-ray examinations such as chest x-rays or mammograms and thatthere is now direct epidemiologic evidence of a small but significantincrease in the risk of cancer at CT doses.1,2,3 Because CTis such a superb diagnostic tool and because individual CT risksare small, the CT benefit–risk balance is generally byfar in the patient's favor.
However, there is general acceptance that perhaps one thirdof all CT scans could be avoided altogether or replaced by adifferent diagnostic tool. So although CT risks are small, asmall risk multiplied by many millions of scans may translateinto a public health concern some years in the future, particularlyin 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 uncertainand depend on extrapolating risks from atomic-bomb survivorswho were exposed to high doses. At very low radiation doses,cancer risks are indeed very uncertain.3 However, at the higherdoses corresponding to a few CT scans (5 to 100 mSv), thereare direct epidemiologic data from about 30,000 atomic-bombsurvivors who were on the peripheries of Hiroshima and Nagasakiand who were exposed in this same low-dose range. This low-dosesubpopulation has been followed for more than 50 years and hasa small but statistically significant increase in the risk ofcancer.1,2 So we do not have to extrapolate CT-associated risksfrom those at higher doses, with all the attendant uncertaintiesthat involves.
Critique 2: No studies of persons having CT scans have shownan increased cancer risk. There have, in fact, not been anyCT-related epidemiologic studies to date, though one has recentlybegun, focusing on pediatric CT.
Critique 3: Many persons who need CT scans will not have thembecause of these cancer-risk estimates. The evidence does notsupport this: for example, in a recently published study,4 whenparents were informed about CT risks, their willingness to havetheir child undergo CT did not significantly change, althoughthey became more willing to consider other imaging options,if they were equally effective. No CT scans were canceled ordeferred after the parents received the risk information.
Critique 4: It will be very difficult to reduce CT usage. Wecompletely agree. Physicians are often subject to significantpressures, from the medical system, the medicolegal system,and the public, to order CT studies, even when they are notreally necessary or even when alternatives exist. Our goal wasto promote already ongoing dialogues among radiologists, emergencydepartment and other physicians, and indeed the public aboutpractical ways to reduce CT usage and CT doses, without compromisingpatient care.
David J. Brenner. Ph.D., D.Sc. Eric J. Hall, D.Phil., D.Sc. Columbia University Medical Center New York, NY 10032 djb3{at}columbia.edu
References
Pierce DA, Preston DL. Radiation-related cancer risks at low doses among atomic bomb survivors. Radiat Res 2000;154:178-186. [ISI][Medline]
Preston DL, Ron E, Tokuoka S, et al. Solid cancer incidence in atomic bomb survivors: 1958-1998. Radiat Res 2007;168:1-64. [CrossRef][ISI][Medline]
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. [Free Full Text]
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. [Free Full Text]
Previous
