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Mass Screening With CT Colonography: Should the Radiation Exposure Be of Concern?

2005; Elsevier BV; Volume: 129; Issue: 1 Linguagem: Inglês

10.1053/j.gastro.2005.05.021

1528-0012

David J. Brenner, Maria A. Georgsson,

Advanced X-ray and CT Imaging

Background & Aims: Computed tomography colonography (CTC), particularly using noncathartic techniques, has the clear potential to increase compliance for colorectal cancer screening. Because the geometry for CTC is highly advantageous, it can be performed with lower radiation doses than almost any other CT examination. If CTC were to become a standard screening tool for the population age 50 years and older, the potential market in the United States would soon be over 100 million people. Therefore, it is pertinent to consider the radiation exposure and any potential radiation risk to the population from such a mass CTC screening program. Methods: Organ doses from CTC examinations can be estimated with standard techniques. These doses can be applied to organ- and dose-specific radiation cancer risk estimates to estimate the excess cancer risk resulting from the radiation from a paired (supine and prone) CTC examination. Results: The cancer risks associated with the radiation exposure from CTC are unlikely to be zero, but they are small. A best estimate for the absolute lifetime cancer risk associated with the radiation exposure using typical current scanner techniques is about 0.14% for paired CTC scans for a 50-year-old, and about half that for a 70-year-old. These values probably could be reduced by factors of 5 or 10 with optimized CTC protocols. Conclusions: In terms of the radiation exposure, the benefit-risk ratio potentially is large for CTC. Background & Aims: Computed tomography colonography (CTC), particularly using noncathartic techniques, has the clear potential to increase compliance for colorectal cancer screening. Because the geometry for CTC is highly advantageous, it can be performed with lower radiation doses than almost any other CT examination. If CTC were to become a standard screening tool for the population age 50 years and older, the potential market in the United States would soon be over 100 million people. Therefore, it is pertinent to consider the radiation exposure and any potential radiation risk to the population from such a mass CTC screening program. Methods: Organ doses from CTC examinations can be estimated with standard techniques. These doses can be applied to organ- and dose-specific radiation cancer risk estimates to estimate the excess cancer risk resulting from the radiation from a paired (supine and prone) CTC examination. Results: The cancer risks associated with the radiation exposure from CTC are unlikely to be zero, but they are small. A best estimate for the absolute lifetime cancer risk associated with the radiation exposure using typical current scanner techniques is about 0.14% for paired CTC scans for a 50-year-old, and about half that for a 70-year-old. These values probably could be reduced by factors of 5 or 10 with optimized CTC protocols. Conclusions: In terms of the radiation exposure, the benefit-risk ratio potentially is large for CTC. There is no doubt that colonoscopy-driven polypectomy can result in a significantly decreased incidence of colorectal cancer,1Winawer S.J. Zauber A.G. Ho M.N. O’Brien M.J. Gottlieb L.S. Sternberg S.S. Waye J.D. Schapiro M. Bond J.H. Panish J.F. et al.The National Polyp Study WorkgroupPrevention of colorectal cancer by colonoscopic polypectomy.N Engl J Med. 1993; 329: 1977-1981Crossref PubMed Scopus (3830) Google Scholar, 2Citarda F. Tomaselli G. Capocaccia R. Barcherini S. Crespi M. Efficacy in standard clinical practice of colonoscopic polypectomy in reducing colorectal cancer incidence.Gut. 2001; 48: 812-815Crossref PubMed Scopus (611) Google Scholar and that there is suboptimal compliance with current guidelines for colorectal cancer screening.3Seeff L.C. Nadel M.R. Klabunde C.N. Thompson T. Shapiro J.A. Vernon S.W. Coates R.J. Patterns and predictors of colorectal cancer test use in the adult U.S. population.Cancer. 2004; 100: 2093-2103Crossref PubMed Scopus (452) Google Scholar, 4Subramanian S. Amonkar M.M. Hunt T.L. Use of colonoscopy for colorectal cancer screening evidence from the 2000 national health interview survey.Cancer Epidemiol Biomarkers Prev. 2005; 14: 409-416Crossref PubMed Scopus (60) Google Scholar Screening using computed tomography colonography (CTC), sometimes referred to as virtual colonoscopy, first was suggested in 1983,5Coin C.G. Wollett F.C. Coin J.T. Rowland M. DeRamos R.K. Dandrea R. Computerized radiology of the colon a potential screening technique.Comput Radiol. 1983; 7: 215-221Abstract Full Text PDF PubMed Scopus (43) Google Scholar but only recently has become a potential option for mass screening.6Hara A.K. Johnson C.D. Reed J.E. Ahlquist D.A. Nelson H. Ehman R.L. McCollough C.H. Ilstrup D.M. Detection of colorectal polyps by computed tomographic colography feasibility of a novel technique.Gastroenterology. 1996; 110: 284-290Abstract Full Text PDF PubMed Scopus (203) Google Scholar, 7Pickhardt P.J. Choi J.R. Hwang I. Butler J.A. Puckett M.L. Hildebrandt H.A. Wong R.K. Nugent P.A. Mysliwiec P.A. Schindler W.R. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults.N Engl J Med. 2003; 349: 2191-2200Crossref PubMed Scopus (1689) Google Scholar, 8Iannaccone R. Laghi A. Catalano C. Mangiapane F. Lamazza A. Schillaci A. Sinibaldi G. Murakami T. Sammartino P. Hori M. Piacentini F. Nofroni I. Stipa V. Passariello R. Computed tomographic colonography without cathartic preparation for the detection of colorectal polyps.Gastroenterology. 2004; 127: 1300-1311Abstract Full Text Full Text PDF PubMed Scopus (237) Google Scholar Figure 1 shows that CTC is of increasing interest to the clinical community. In the most common current usage of CTC, after bowel preparation, the colon is inflated and the colon is scanned by CT. The resulting data then can be analyzed for polyps based on 2-dimensional images, or by using a 3-dimensional endoluminal view. In general, CTC is an excellent application of CT because of the radiologic contrast exhibited by colonic polyps projecting into an air- or CO2-filled lumen (Figure 2).5Coin C.G. Wollett F.C. Coin J.T. Rowland M. DeRamos R.K. Dandrea R. Computerized radiology of the colon a potential screening technique.Comput Radiol. 1983; 7: 215-221Abstract Full Text PDF PubMed Scopus (43) Google Scholar, 6Hara A.K. Johnson C.D. Reed J.E. Ahlquist D.A. Nelson H. Ehman R.L. McCollough C.H. Ilstrup D.M. Detection of colorectal polyps by computed tomographic colography feasibility of a novel technique.Gastroenterology. 1996; 110: 284-290Abstract Full Text PDF PubMed Scopus (203) Google Scholar, 9Hara A.K. Johnson C.D. Reed J.E. Ahlquist D.A. Nelson H. Ehman R.L. Harmsen W.S. Reducing data size and radiation dose for CT colonography.AJR Am J Roentgenol. 1997; 168: 1181-1184Crossref PubMed Scopus (146) Google Scholar CTC may well have the potential to increase colorectal cancer screening compliance, largely because of the possibility that it can be performed with noncathartic pre-examination bowel preparation. Current compliance with screening guidelines clearly is poor—at most, about one third of adults over age 50 in the United States have had an endoscopic examination within the past 10 years.3Seeff L.C. Nadel M.R. Klabunde C.N. Thompson T. Shapiro J.A. Vernon S.W. Coates R.J. Patterns and predictors of colorectal cancer test use in the adult U.S. population.Cancer. 2004; 100: 2093-2103Crossref PubMed Scopus (452) Google Scholar, 4Subramanian S. Amonkar M.M. Hunt T.L. Use of colonoscopy for colorectal cancer screening evidence from the 2000 national health interview survey.Cancer Epidemiol Biomarkers Prev. 2005; 14: 409-416Crossref PubMed Scopus (60) Google Scholar From a technologic perspective, CTC is not quite ready for use in mass-screening programs. The 3 main outstanding issues, all of which seem relatively close to solution, are as follows. CTC sensitivity and specificity for lesions greater than 10 mm in diameter generally are well over 90%—about as good as those for optical colonoscopy.10Macari M. Bini E.J. Xue X. Milano A. Katz S.S. Resnick D. Chandarana H. Krinsky G. Klingenbeck K. Marshall C.H. Megibow A.J. Colorectal neoplasms prospective comparison of thin-section low-dose multi-detector row CT colonography and conventional colonoscopy for detection.Radiology. 2002; 224: 383-392Crossref PubMed Scopus (201) Google Scholar There is evidence that a well-designed CTC screening program can achieve at least 90% sensitivity and specificity in the size category from 7 and 10 mm,7Pickhardt P.J. Choi J.R. Hwang I. Butler J.A. Puckett M.L. Hildebrandt H.A. Wong R.K. Nugent P.A. Mysliwiec P.A. Schindler W.R. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults.N Engl J Med. 2003; 349: 2191-2200Crossref PubMed Scopus (1689) Google Scholar, 11Johnson K.T. Johnson C.D. Anderson S.M. Bruesewitz M.R. McCollough C.H. CT colonography determination of optimal CT technique using a novel colon phantom.Abdom Imaging. 2004; 29: 173-176Crossref PubMed Scopus (22) Google Scholar but not all studies have achieved this.12Cotton P.B. Durkalski V.L. Pineau B.C. Palesch Y.Y. Mauldin P.D. Hoffman B. Vining D.J. Small W.C. Affronti J. Rex D. Kopecky K.K. Ackerman S. Burdick J.S. Brewington C. Turner M.A. Zfass A. Wright A.R. Iyer R.B. Lynch P. Sivak M.V. Butler H. Computed tomographic colonography (virtual colonoscopy) a multicenter comparison with standard colonoscopy for detection of colorectal neoplasia.JAMA. 2004; 291: 1713-1719Crossref PubMed Scopus (581) Google Scholar In general, it may be less the invasive nature of conventional colonoscopy that results in poor compliance, but more the necessity for cathartic bowel preparation.13Weitzman E.R. Zapka J. Estabrook B. Goins K.V. Risk and reluctance understanding impediments to colorectal cancer screening.Prev Med. 2001; 32: 502-513Crossref PubMed Scopus (183) Google Scholar, 14Ristvedt S.L. McFarland E.G. Weinstock L.B. Thyssen E.P. Patient preferences for CT colonography, conventional colonoscopy, and bowel preparation.Am J Gastroenterol. 2003; 98: 578-585Crossref PubMed Scopus (171) Google Scholar, 15Akerkar G.A. Yee J. Hung R. McQuaid K. Patient experience and preferences toward colon cancer screening a comparison of virtual colonoscopy and conventional colonoscopy.Gastrointest Endosc. 2001; 54: 310-315Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar, 16Harewood G.C. Wiersema M.J. Melton 3rd, L.J. A prospective, controlled assessment of factors influencing acceptance of screening colonoscopy.Am J Gastroenterol. 2002; 97: 3186-3194Crossref PubMed Google Scholar, 17Gluecker T.M. Johnson C.D. Harmsen W.S. Offord K.P. Harris A.M. Wilson L.A. Ahlquist D.A. Colorectal cancer screening with CT colonography, colonoscopy, and double-contrast barium enema examination prospective assessment of patient perceptions and preferences.Radiology. 2003; 227: 378-384Crossref PubMed Scopus (253) Google Scholar CTC offers the potential for noncathartic bowel preparation through the use of barium or iodinated tagging agents that impart a high density to both stool and residual fluid, allowing increased contrast with soft-tissue polyps. Recent results with noncathartic CTC (Figure 2B) have been very encouraging.8Iannaccone R. Laghi A. Catalano C. Mangiapane F. Lamazza A. Schillaci A. Sinibaldi G. Murakami T. Sammartino P. Hori M. Piacentini F. Nofroni I. Stipa V. Passariello R. Computed tomographic colonography without cathartic preparation for the detection of colorectal polyps.Gastroenterology. 2004; 127: 1300-1311Abstract Full Text Full Text PDF PubMed Scopus (237) Google Scholar, 18Callstrom M.R. Johnson C.D. Fletcher J.G. Reed J.E. Ahlquist D.A. 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An overview of the regulations and guidance.Radiol Clin North Am. 2000; 38: 759-772Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar so CTC should be optimized and standardized if it is to be used for mass screening. Particularly until the previous 2 points are settled, it probably is premature to consider standardizing CTC scanner parameters. If CTC were to become a standard screening tool for patients over age 50 years, the potential market in the United States soon would be greater than 100 million people. Even if the recommended CTC frequency were to be that currently recommended for optical colonoscopy (every decade), this would imply that several million CTC scans might be performed each year. Should the relative simplicity of the CTC tests result in the recommended examination frequency being increased, then several tens of millions of these CTC scans might be expected to be performed in the United States each year. Therefore, it is pertinent to consider the radiation exposure and any potential radiation risk to the population from such a mass screening program. Some typical low doses of societal relevance are shown in Table 1. Radiation dose is a measure of ionizing energy absorbed per unit mass and has units of Gy (Gray) or mGy; it often is quoted as an equivalent dose, in units of Sv (Sievert) or mSv. For x-rays, which is the radiation produced in CT scanners, 1 mSv = 1 mGy.Table 1Typical Mean Doses Relevant to Some Low-Dose Radiation Exposures to Different PopulationsRadiation exposure scenarioApproximate mean individual dose (mSv)aAll doses are effective whole-body doses with the exception of the medical exposures (mammography, CT scan), which are to specific organs.Round-trip flight, New York-Seattle57Barish R.J. In-flight radiation exposure during pregnancy.Obstet Gynecol. 2004; 103: 1326-1330Crossref PubMed Scopus (40) Google Scholar0.06Single-screening mammogram (breast dose)21Kruger R.L. Schueler B.A. A survey of clinical factors and patient dose in mammography.Med Phys. 2001; 28: 1449-1454Crossref PubMed Scopus (54) Google Scholar3Background dose caused by natural radiation exposure58NCRPNational Council on Radiation Protection and Measurements.Exposure of the Population in the United States and Canada from Natural Background Radiation. 1988; (NCRP Report 94)Google Scholar3/yAdult CT scan (stomach dose from abdominal scan)59Brenner D.J. Elliston C.D. Hall E.J. Berdon W.E. Estimated risks of radiation-induced fatal cancer from pediatric CT.AJR Am J Roentgenol. 2001; 176: 289-296Crossref PubMed Scopus (2491) Google Scholar10Excess dose (>15 y) to 4 million individuals in Ukraine in the vicinity of the Chernobyl accident60Likhtarev I.A. Kovgan L.N. Jacob P. Anspaugh L.R. Chernobyl accident retrospective and prospective estimates of external dose of the population of Ukraine.Health Phys. 2002; 82: 290-303Crossref PubMed Scopus (59) Google Scholar, 61Likhtarev I.A. Kovgan L.N. Vavilov S.E. Perevoznikov O.N. Litvinets L.N. Anspaugh L.R. Jacob P. Prohl G. Internal exposure from the ingestion of foods contaminated by 137Cs after the Chernobyl accident—report 2. Ingestion doses of the rural population of Ukraine up to 12 y after the accident (1986–1997).Health Phys. 2000; 79: 341-357Crossref PubMed Scopus (39) Google Scholar13Typical dose to an A-bomb survivor at a distance of 2.3 km from the explosion hypocenter at Hiroshima25Preston D.L. Pierce D.A. Shimizu Y. Cullings H.M. Fujita S. Funamoto S. Kodama K. Effect of recent changes in atomic bomb survivor dosimetry on cancer mortality risk estimates.Radiat Res. 2004; 162: 377-389Crossref PubMed Scopus (342) Google Scholar13Dose range over 20-block radius from hypothetical nuclear terrorism incident62Kelly H. Dirty bombs: response to a threat. Public Interest Report: Journal of the Federation of American Scientists. NLMID: 0410524.FAS Public Interest Rep. 2002; 55: 1-10Google Scholar3–30Radiation worker annual exposure limit43ICRP1990 Recommendations of the International Commission on Radiological Protection Publication 60. Pergamon, Oxford1991Google Scholar20/yExposure on international space station63Benton E.R. Benton E.V. Space radiation dosimetry in low-Earth orbit and beyond.Nucl Instrum Methods Phys Res B. 2001; 184: 255-294Crossref PubMed Scopus (361) Google Scholar170/ya All doses are effective whole-body doses with the exception of the medical exposures (mammography, CT scan), which are to specific organs. Open table in a new tab The biological effects of low-dose x-ray exposure have been investigated and debated for more than a century.23Brenner D.J. Doll R. Goodhead D.T. Hall E.J. Land C.E. Little J.B. Lubin J.H. Preston D.L. Preston R.J. Puskin J.S. Ron E. Sachs R.K. Samet J.M. Setlow R.B. Zaider M. Cancer risks attributable to low doses of ionizing radiation assessing what we really know.Proc Natl Acad Sci U S A. 2003; 100: 13761-13766Crossref PubMed Scopus (1263) Google Scholar There is little question that intermediate and high doses of ionizing radiation, for example, greater than 100 mSv, given acutely or over a prolonged period, produce deleterious effects in humans, the most significant being cancer induction.24Preston D.L. Shimizu Y. Pierce D.A. Suyama A. Mabuchi K. Studies of mortality of atomic bomb survivors. Report 13: solid cancer and noncancer disease mortality: 1950–1997.Radiat Res. 2003; 160: 381-407Crossref PubMed Scopus (891) Google Scholar At lower doses, however, the situation is less clear. Compared with higher doses, the risks for low doses of radiation are lower, and progressively larger epidemiologic studies are required to quantify the cancer risk to a useful level of precision. This is because as the dose decreases, the signal-to-noise ratio (radiation risk to natural background risk) decreases. Most of the quantitative information that we have regarding radiation-induced cancer risks comes from studies of A-bomb survivors. A-bomb survivor cohorts generally are used as the basis for predicting radiation-related risks for a general population because (1) they are the most thoroughly studied (over many decades) large exposed population, (2) the cohorts are not selected for disease, (3) all age groups are covered, and (4) a substantial subcohort of about 25,000 survivors, typically those who were approximately 2–2.7 km from the explosion hypocenters,25Preston D.L. Pierce D.A. Shimizu Y. Cullings H.M. Fujita S. Funamoto S. Kodama K. Effect of recent changes in atomic bomb survivor dosimetry on cancer mortality risk estimates.Radiat Res. 2004; 162: 377-389Crossref PubMed Scopus (342) Google Scholar received radiation doses comparable with those of concern here. The key questions here are: (1) What is the lowest dose of x-rays for which there is convincing evidence of significantly increased cancer risks in humans? (2) What is the most appropriate way to extrapolate these risks to even lower doses? (3) What is the dependence of cancer risks on age at exposure? These issues recently have been reviewed extensively.23Brenner D.J. Doll R. Goodhead D.T. Hall E.J. Land C.E. Little J.B. Lubin J.H. Preston D.L. Preston R.J. Puskin J.S. Ron E. Sachs R.K. Samet J.M. Setlow R.B. Zaider M. Cancer risks attributable to low doses of ionizing radiation assessing what we really know.Proc Natl Acad Sci U S A. 2003; 100: 13761-13766Crossref PubMed Scopus (1263) Google Scholar In summary, there is good epidemiologic evidence of increased cancer risk for children exposed to acute doses of 10 mSv (or greater), and for adults exposed to acute doses of 50 mSv (or greater).23Brenner D.J. Doll R. Goodhead D.T. Hall E.J. Land C.E. Little J.B. Lubin J.H. Preston D.L. Preston R.J. Puskin J.S. Ron E. Sachs R.K. Samet J.M. Setlow R.B. Zaider M. Cancer risks attributable to low doses of ionizing radiation assessing what we really know.Proc Natl Acad Sci U S A. 2003; 100: 13761-13766Crossref PubMed Scopus (1263) Google Scholar As we discuss later, relevant organ doses for a paired (supine and prone) CTC examination are of the order of 15 mSv or less. The issue here is how to estimate risks at doses somewhat (although not a great deal) lower than those for which there is statistically significant evidence of increased cancer risks. The current consensus26NCRPEvaluation of the linear-nonthreshold dose-response model for ionizing radiation, Report No. 136. National Council on Radiation Protection and Measurements. NCRP, Bethesda, MD2001Google Scholar is that the measured risks reasonably can be extrapolated linearly to somewhat lower doses, although as the dose of interest becomes progressively lower, the uncertainties inherent in this extrapolation become progressively greater. Relatively small extrapolations from epidemiologic data are required (eg, from 50 to 15 mSv), however, to estimate cancer risks at the doses relevant to CTC examinations. Regarding age at exposure, as can be seen in Figure 3, radiation risks generally decrease markedly with age. This is because sensitivity is related to the proportion of dividing cells in an organ, which decreases with increasing age, and other competing risks play an increasing role with increasing age. At present, medical X-rays are responsible for about 17% of all the ionizing radiation exposure to which an average US resident is exposed (Figure 4). Within this fraction of the total radiation pie, about two-thirds is from CT examinations.27Mettler Jr, F.A. Wiest P.W. Locken J.A. Kelsey C.A. CT scanning patterns of use and dose.J Radiol Prot. 2000; 20: 353-359Crossref PubMed Scopus (538) Google Scholar This large proportion is despite the fact that only about 1 in 10 of all radiologic examinations are CT scans, and reflects the fact (Table 2) that CT scans produce a much larger radiation dose than conventional radiographs such as dental radiographs, chest radiographs, or mammograms. This is inherent in the nature of a CT scan, which essentially involves the generation of multiple X-ray images.Table 2Typical Organ Doses From Various Radiologic ExaminationsExaminationRelevant organRelevant organ dose (mSv)Dental radiographBrain0.005Posteroanterior chest radiographLung0.01Lateral chest radiographLung0.15Screening mammogramBreast3Adult abdominal CTStomach10Neonate abdominal CTStomach25 Open table in a new tab The basic principle of helical, or spiral, CT scanning is shown in Figure 5. Essentially, the patient is moved through a continuously rotating x-ray source/detector combination. A more modern version is the multidetector CT, which gives the advantage of short scan times, coupled with potentially very thin slice widths. A relatively new CT dose-reduction technique is automatic tube current modulation (Figure 6),28Lehmann K.J. Wild J. Georgi M. Clinical use of software-controlled x-ray tube modulation with “Smart-Scan” in spiral CT.Aktuelle Radiol. 1997; 7: 156-158PubMed Google Scholar, 29Hundt W. Rust F. Stabler A. Wolff H. Suess C. Reiser M. Dose reduction in multislice computed tomography.J Comput Assist Tomogr. 2005; 29: 140-147Crossref PubMed Scopus (49) Google Scholar, 30Keat N. CT scanner automatic exposure control systems.Medicines and Healthcare Products Regulatory Agency, Report 05016. MHRA, London, England2005Google Scholar, 31Kalra M.K. Maher M.M. Toth T.L. Schmidt B. Westerman B.L. Morgan H.T. Saini S. Techniques and applications of automatic tube current modulation for CT.Radiology. 2004; 233: 649-657Crossref PubMed Scopus (561) Google Scholar now available from all the major scanner manufacturers. These systems continuously lower or raise the x-ray tube current to compensate for different instantaneous levels of attenuation of the x-ray beam by the patient. For example, when the beam is aimed in the posteroanterior direction, fewer x-rays are needed (for the same image quality) compared with the lateral-medial direction; or when the beam is passing through the region of the transverse colon, fewer x-rays are needed compared with the pelvic bone region. For helical CT scans, the speed that the patient table moves relative to the rotation speed of the x-ray tubes/detectors is an important determinant of the radiation dose; it is defined through the pitch, which is the linear table motion feed per 360° rotation, divided by the total beam width (the slice width × the number of detectors). The radiation dose from CT depends on a number of factors. The most important are the tube current, the scan time, the pitch, the tube voltage, the number of detectors, the slice thickness, and the particular scanner design.32McNitt-Gray M.F. AAPM/RSNA physics tutorial for residents: topics in CT. Radiation dose in CT.Radiographics. 2002; 22: 1541-1553Crossref PubMed Scopus (483) Google Scholar For a given CT scanner operating at a given voltage, the organ dose is proportional to the mAs (current [mA] × rotation time) and is inversely proportional to the pitch. It is always the case, however, that the relative noise in CT images will increase as the radiation dose decreases; thus, there always will be a trade-off between the need for low-noise images and the desirability of using low radiation doses.33Martin C.J. Sutton D.G. Sharp P.F. Balancing patient dose and image quality.Appl Radiat Isot. 1999; 50: 1-19Crossref PubMed Scopus (63) Google Scholar Because of the advantageous geometry of a CTC scan, the dose/noise trade-off can be very much weighted toward low-dose, higher-noise images.9Hara A.K. Johnson C.D. Reed J.E. Ahlquist D.A. Nelson H. Ehman R.L. Harmsen W.S. Reducing data size and radiation dose for CT colonography.AJR Am J Roentgenol. 1997; 168: 1181-1184Crossref PubMed Scopus (146) Google Scholar, 10Macari M. Bini E.J. Xue X. Milano A. Katz S.S. Resnick D. Chandarana H. Krinsky G. Klingenbeck K. Marshall C.H. Megibow A.J. Colorectal neoplasms prospective comparison of thin-section low-dose multi-detector row CT colonography and conventional colonoscopy for detection.Radiology. 2002; 224: 383-392Crossref PubMed Scopus (201) Google Scholar, 34van Gelder R.E. Venema H.W. Serlie I.W. Nio C.Y. Determann R.M. Tipker C.A. Vos F.M. Glas A.S. Bartelsman J.F. Bossuyt P.M. Lameris J.S. Stoker J. CT colonography at different radiation dose levels feasibility of dose reduction.Radiology. 2002; 224: 25-33Crossref PubMed Scopus (161) Google Scholar, 35Hara A.K. Johnson C.D. MacCarty R.L. Welch T.J. McCollough C.H. Harmsen W.S. CT colonography single- versus multi-detector row imaging.Radiology. 2001; 219: 461-465Crossref PubMed Scopus (159) Google Scholar, 36Iannaccone R. Laghi A. Catalano C. Brink J.A. Mangiapane F. Trenna S. Piacentini F. Passariello R. Detection of colorectal lesions lower-dose multi-detector row helical CT colonography compared with conventional colonoscopy.Radiology. 2003; 229: 775-781Crossref PubMed Scopus (123) Google Scholar Several studies have examined systematically the various scanner parameters discussed earlier, and generally have come to the conclusion that more noise can be accepted in a CTC scan compared with other CT scans, while still maintaining sensitivity and specificity, at least for polyps greater than approximately 7 mm in diameter.10Macari M. Bini E.J. Xue X. Milano A. Katz S.S. Resnick D. Chandarana H. Krinsky G. Klingenbeck K. Marshall C.H. Megibow A.J. Colorectal neoplasms prospective comparison of thin-section low-dose multi-detector row CT colonography and conventional colonoscopy for detection.Radiology. 2002; 224: 383-392Crossref PubMed Scopus (201) Google Scholar, 11Johnson K.T. Johnson C.D. Anderson S.M. Bruesewitz M.R. McCollough C.H. CT colonography determination of optimal CT technique using a novel colon phantom.Abdom Imaging. 2004; 29: 173-176Crossref PubMed Scopus (22) Google Scholar, 34van Gelder R.E. Venema H.W. Serlie I.W. Nio C.Y. Determann R.M. Tipker C.A. Vos F.M. Glas A.S. Bartelsman J.F. Bossuyt P.M. Lameris J.S. Stoker J. CT colonography at different radiation dose levels feasibility of dose reduction.Radiology. 2002; 224: 25-33Crossref PubMed Scopus (161) Google Scholar, 37Taylor S.A. Halligan S. Bartram C.I. Morgan P.R. Talbot I.C. Fry N. Saunders B.P. Khosraviani K. Atkin W. Multi-detector row CT colonography effect of collimation, pitch, and orientation on polyp detection in a human colectomy specimen.Radiology. 2003; 229: 109-118Crossref PubMed Scopus (44) Google Scholar, 38Wessling J. Fischbach R. Meier N. Allkemper T. Klusmeier J. Ludwig K. Heindel W. CT colonography protocol optimization with multi-detector row CT—study in an anthropomorphic colon phantom.Radiology. 2003; 228: 753-759Crossref PubMed Scopus (41) Google Scholar To estimate the radiation dose to different organs from adult CTC scans, we used the ImPACT CT Dosimetry Calculator39Jones D.G. Shrimpton P.C. Survey of CT practice in the UK. National Radiological Protection Board, Harwell, UK1991Google Scholar (London, England), for a given CT scanner with given scanner settings, this tool, which is available online, uses standard calculational techniques39Jones D.G. Shrimpton P.C. Survey of CT practice in the UK. National Radiological Protection Boar