Answer to Question #5616 Submitted to "Ask the Experts"

I recently had a pelvic and abdominal CT scan performed with and without contrast. I have been concerned about the amount of radiation I was exposed to. Upon calling the hospital I was given the following values pertaining to my exam:

DLP Output: 639 milligray per centimeter
Dose: 21.39 milligray

How does this translate into risk in millisievert? Is this a normal exposure for this kind of procedure?

The DLP refers to Dose Length Product and is used to determine the scanner output for various procedures. As they also supplied the dose for the types of scans you received, I used this value in my calculations.

A dose of 21.39 milligray does sound normal for a pelvic and abdominal CT scan; however, I think that they told you the dose for only one scan. The dose should be multiplied by two to determine the dose for both the scan with contrast and the scan without contrast, so the total dose is 42.8 milligray.

To convert the tissue dose to an effective dose in millisievert (mSv), we can assume that 1 gray is approximately 1 sievert, so that 42.8 milligray is approximately 42.8 millisievert. To convert it to an effective dose we have to take into account the radiation risk factors. I came up with an estimate of approximately 30 millisievert. I used a radiation organ weighting factor of 0.7 after adding up the fractions of all the organs that I thought would be exposed during the scans.

At exposures below 100 millisievert (100 mSv) adverse effects of radiation exposures have not been demonstrated, as noted in the Health Physics Society Position Paper "Radiation Risk in Perspective." The position statement says in part, "Below 10 rem (100 mSv) . . . risks of health effects are either too small to be observed or are non-existent." To put this dose in perspective, I would also recommend you read our Web page "Risk/Benefit of Medical Radiation Exposures."

Marcia Hartman, MS
University of California Davis Health System

Editor's Note: As a comparison to the values listed on our Web page "Doses from Medical Radiation Sources," a typical CT pelvic scan or CT abdominal scan is 10 mSv. A combined pelvic and abdominal CT scan would be 20 mSv. Repeating the test with contrast results in a total of 40 mSv. As the values provided by the hospital were for the machine and settings used to make your scans, the value of 30 mSv is probably more accurate.