Answer to Question #3679 Submitted to "Ask the Experts"

I’m a medical researcher interested in using serial computerized tomography (CT) scans, for example, 3-4 CT scans of the whole body and trunk, to assess body composition. How much radiation exposure will an adult study subject, average of 50 years old or more, get from a whole-body CT scan? Is there an annual maximum dose recommended for a research-study subject? Do you know of any literature on the subject?

Thank you for the opportunity to discuss radiation exposures and doses delivered during research clinical studies. Please permit me to preface my response by qualifying that radiation dose estimates for external exposures that are machine-generated are dependent upon the machine used and the operating parameters selected. That is, doses will depend upon the specific scanner model as well as factors such as machine voltage, current, number and thickness of slices, and patient size.

My review of the literature indicates that the doses associated with CT scanning have been decreasing steadily in the last few years (Huda et al. 2000; vander Bruggen-Bogaarts et al. 1995). Currently, the typical dose associated with a chest CT is less than 1 rem (Wall and Hart 1997). Again, because there is a wide variation in the dose due to machine and techniques used, it is prudent to be familiar with the specific exposure and dose values for the machines used in your institution.

At our institution, a CT scan of the chest, abdomen, and pelvis to an adult will give an effective dose to the patient of 1.4 rem. Effective dose best describes the exposure, since it takes into consideration the relative radiosensitivity of the different organs exposed. Thus, if you were to perform a series of 4 CT scans on a single research subject at our institution, the total effective dose would be 5.6 rem.

Regarding the question of radiation exposure limits for research subjects, there are no limits to exposures unless the radiation is delivered as part of a study that involves radioactive research drugs. For these studies, the United States Food and Drug Administration (FDA) has specific limits of 3 rem per single administration and 5 rem per year to the lens of the eye, blood-forming organs, gonads, and whole body, and 5 rem per administration and 15 rem per year to all other tissues and organs (US Code of Federal Regulations Title 21, Part 361.1).

For all other research studies in which the radiation exposure is not part of a radioactive research drug study, there are no limits to the exposure. The exposure is evaluated as a risk-benefit assessment. Most institutions have established guidelines that are based upon the regulations and definitions in 45 CFR 46 (US Code of Federal Regulations Title 45, Part 46). This set of regulations must be followed when the research is conducted using federal funds. In this code, "minimal risk" is defined as "the probability and magnitude of harm or discomfort anticipated in the research are not greater in and of themselves than those ordinarily encountered in daily life. . . ." Extending this definition and applying it to radiation exposure, our institution has decided to use the average annual background dose of 0.3 rem per year as the quantity that equates to minimal risk. All doses above this 0.3 rem level are deemed "slightly greater than minimal risk."

Our radiation safety committee has also established guideline values for research doses that are helpful to our institutional review committees in evaluating the risk versus benefit of the studies. These guidelines are 5.0 rem per year, that is, 12 consecutive months for adults, and 0.5 rem for subjects younger than 18 years of age.

Since you mentioned that you wish to perform CT imaging to assess body composition, I suggest you consider the use of DEXA (dual energy x-ray absorption) scans. DEXA scans are best used for evaluating percent body fat, visceral fat, subcutaneous fat, bone mineral density, or muscle mass. The dose associated with DEXA scan is significantly lower than that for CT scanning. Typical DEXA doses are in the range of a few millirem.

Dr. Jamey West
National Institutes of Health

References

• Huda W, Scalzetti EM, Roskopf M. Effective doses to patients undergoing thoracic computed tomography examinations. Medical Physics 27 (5) 838-44; May 2000.
   
• van der Bruggen-Bogaarts BA, Broerse JJ, Lammers JW, van Waes PF, Geleijns J. Radiation exposure in standard and high resolution CT scans. Chest 107(1) 113-5; Jan. 1995.
   
• Wall BF, Hart D. Revised radiation doses for typical x-ray examinations. The British Medical Journal of Radiology pages 437-39; May 1997.