XaHP Document 112, September 2001
- CT doses below are merely "ballpark" values. Entrance doses during CT scans are almost never measured. Actual doses --- even from the same equipment for the same patient --- can vary many-fold according to the settings selected for kVp, mAs, pitch, filtration, slice-width, and some other variables.
- Real doses in centi-Gray units (cGy) are distinctly different entities from "effective" doses in centi-Sievert units (cSv). Real doses quantify energy per gram of tissue delivered by an x-ray exam to the irradiated sections of the body, whereas "effective" doses are artificial values based on assumptions about risk ("detriment"). Please see reverse side.
- The centi-Gray (cGy) and the rad are identical units. There are 10 milli-Gray (mGy) per centi-Gray (cGy) or rad, because milli means one-thousandth and centi means one-hundredth. The centi-Sievert (cSv) and rem are interchangeable units.
- This table begins with the typical extra "effective" radiation dose from commercial flying in the USA, because medical patients are so often told that their x-ray dose is about the same as the extra radiation from one trip. For CT exams, the table shows that the claim would be very mistaken. The righthand column divides CT "effective" doses by the "effective" dose from a ten-hour airplane flight in the US (0.003 cSv). The lowest ratio is 50, for just one scan. The ratio for one CT "study" involving 2 or 3 scans would be 2 or 3 times higher than the ratios in the righthand column. Please see additional notes below the tabulation.
|TOPIC||TYPE OF DOSE||ESTIMATED DOSE||SOURCE||Eff.Dose: CT/flying|
|Extra radiation during commercial airplane flights within USA.||
"Effective" dose/hr. And
per 10 hours.
0.0003 cSv per hour. And
0.003 cSv per 10 hours.
|UNSCEAR 1993, p.38.|
|CT scans, general.||Tissue dose per scan.||1-3 cSv.||Mettler 2000, p.352.|
|CT head scan, adult.||Surface dose.||3-7 cGy (rads).||Nickoloff 2001, p.285.|
|CT head scan, adult.||"Effective" dose.||0.15 cSv.||Mettler 2000, p.352.||50 to 1.|
|CT chest, typical. *||Surface dose.||2-5 cGy (rads).||Nickoloff 2001, p.286.|
|CT chest, typical.||Internal dose.||Below 2 cGy (rads).||Ravenel 2001, p.283.|
|CT chest, typical.||"Effective" dose.||0.54 cSv.||Huda 2000, p.843.||180 to 1.|
|CT chest, unspecified. **||Breast: Mean glandular dose.||Up to 5 cGy (rads).||Gray 1998-a, p.63.|
|CT multi-slice of heart for calcium score.||Surface dose.||Up to 10-20 cGy.||Nickoloff 2001, p.286.|
|CT chest angiograph.||Surface dose.||2-4 cGy (rads).||Nickoloff 2001, p.286.|
|CT chest, cancer screening.||Surface dose.||0.2 - 0.4 cGy (rad).||Nickoloff 2001, p.286.|
|Electron Beam CT chest angiography or cardiac calcium score. "EBCT."||X-ray beam travels from back to front.||Reduced dose to breasts and front chest wall.||Nickoloff 2001, p.286.|
|CT abdominal "series."||Surface dose.||2-5 cGy (rads).||Nickoloff 2001, p.285.|
|CT abdominal, adult.||"Effective" dose.||0.39 cSv.||Ware 1999, p.64.||130 to 1.|
|Young adult.||"Effective" dose.||0.44 cSv.||Ware 1999, p.64.||147 to 1.|
|Child.||"Effective" dose.||0.61 cSv.||Ware 1999, p.64.||203 to 1.|
|CT-fluoroscopy, for imaging in biopsies, etc.||Range of typical dose-rates.||20-60 cGy (rads) per minute.||Nickoloff 2001, p.285.|
- The "effective" doses above, for medical procedures, have very probably not been properly adjusted upward yet for the greater mutagenic power per cGy (rad) of 90-120 kVp x-rays, compared with 250 kVp x-rays and a-bomb gamma rays (details in Gofman 1999, pp.46-48).
- The "effective" doses above do not yet incorporate the risk of x-ray-induced coronary artery disease (Gofman 1999).
- A handy approximation is that, during helical CT scans, the real dose (cGy or rads) at the body's center is approximately half of the surface dose (Nickoloff 2001, p.285). Except for Electron Beam CT (EBCT), the CT procedures above irradiate the body by revolving the x-ray beams fully around the head or torso.
- One CT "study" may involve 2 or 3 repeats ("phases") on the same day. Over 90% of abdominal/pelvic CT studies use 2 or more CT scans, and the dose from such studies is the sum of single per-scan doses above (Mettler 2000, p.355, p.357).
Page 2 of XaHP Document #112, September 2001:
Estimated Doses to Patients from CT X-Ray Examinations.
Real Dose vs. "Effective" Dose
A real dose (dose-unit = cGy or rad) reflects something objective: The energy deposited by x-rays per gram of irradiated body-tissue. By conrast, an "effective" dose is a calculation which estimates what dose, if given to the entire body, might produce approximately the same amount of risk as would the real dose actually received by the irradiated sections. "Effective" doses (dose-unit = cSv or rem) incorporate a crude adjustment for the different types of ionizing radiation, plus "tissue weighting factors" which attempt (despite woefully inadequate evidence) to assess the attributable probability of fatal cancer in different organs, the additional detriment from non-fatal cancer and hereditary disorders, and the different latency periods for cancers in various tissues.
Footnote * Dose-Comparison.
The typical chest CT exam must not be confused with the typical chest film. The back-to-front chest film delivers a surface entrance dose of about 0.02 cGy (rad), which is at least 100 times lower than the reported dose-range of 2-5 cGy (rads) for the typical chest CT exam.
Footnote ** Dose-Comparison
The breasts receive a mean glandular dose of x-rays "up to 5 cGy (rads)" from chest CT exams. By comparison, the mean glandular breast-dose from a complete 2-view mammographic screening exam is about 0.2 cGy (rad). Mammographic doses vary with the thickness of the compressed breasts. Thicker breasts require higher doses.
Full-Body CT Screening Exam
References for Pages 1 and 2
- Gofman 1999. John W. Gofman, Radiation from Medical Procedures in the Pathogenesis of Cancer and Ischemic Heart Disease; Dose-Response Studies with Physicians per 100,000 Population. 699 pages. ISBN 0-932682-97-9. CNR Books, San Francisco, CA 94142. November 1999.
- Gray 1998-a. Joel E. Gray, "Lower Radiation Exposure Improves Patient Safety, "Diagnostic Imaging Vol.20, No.9: 61-64. September 1998.
- Huda 2000. Walter Huda et al, "Effective Doses to Patients Undergoing Thoracic Computed Tomography Examinations," Medical Physics Vol.27, No.5: 838-844. May 2000.
- Mettler 2000. Fred A. Mettler et al, "CT Scanning: Patterns of Use and Dose," Journal of Radiation Protection Vol.20: 353-359.
- Nickoloff 2001. Edward L. Nickoloff + Philip O. Alderson, "Radiation Exposures to Patients from CT: Reality, Public Perception, and Policy," (Commentary), American J. Roentgenology Vol.177: 285-287. August 2001.
- Ravenel 2001. James G. Ravenel et al, "Radiation Exposure and Image Quality in Chest CT Examinations," American J. Roentgenology Vol.177: 279-284. August 2001.
- UNSCEAR 1993. United Nations Scientific Committee on the Effects of Atomic Radiation, Sources and Effects of Ionizing Radiation, with Scientific Annexes. 922 pages. ISBN 92-1-142200-0. United Nations 1993.
- Ware 1999. Dan E. Ware et al, "Radiation Effective Doses to Patients Undergoing Abdominal CT Examinations," Radiology Vol.210, No.3: 645-650. March 1999.
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