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08 July 2008

Answer to Question #1724 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:

Is it safe for me to have a bone mineral density test during the month of March 2002? I had two CT scans in January 2001. They were Contrast Enhanced Abdominal and Pelvic CT scans and they were done at the same time. The technique was a spiral acquisition with axial images. How much radiation does a person get from these CT scans?

In any use of diagnostic x rays, the benefit to the patient or subject should outweigh the risk for the x-ray exposure. In your case, the benefits would be an evaluation of calcium content in your bones and determination of the effectiveness of future treatments that may be required. The risk to you is based on the radiation dose used to produce the data used to make the measurements. For information I have, for one type of bone scanning unit, the effective dose equivalent values are 0.0115 mSv for a total-body scan, 0.0013 mSv for an AP spin (L1-L4) scan, and 0.0035 mSv for a proximal femur scan. As a comparison, the average exposure from all sources of radiation in the United States is 3.60 mSv (360 mrem). Thus, the exposure from the highest dose scan I listed would be less than 0.2% of the typical exposure a person receives in a year. For a "plain" spiral abdomen or pelvic CT scan, the effective dose would be about 6 mSv each. I do not have information on a contrast-enhanced scan, but I would expect that the values would be about the same. Thus, the total for your two CT scans would be about 12 mSv. The risk from low doses of ionizing radiation is generally considered to be the development of cancer after a latent period. Current estimates are that the risk of cancer is 0.00005/mSv. As a comparison, the risk of cancer in the population is approximately 25%. While it is generally inappropriate to make estimates of individual risks, one can say that for large populations receiving two CT scans, the increased cancer risk could be calculated to be approximately 0.0006%, so that the total cancer risk might be 25.0006%. John Jacobus, MS Certified Health Physicist

Answer posted on 15 March 2002. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.