Answer to Question #13013 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:


I had a computed tomography (CT) scan on both my knees. Four scans were required because the first set of scans was not clear enough. My scan report included the following numbers: total mAs 28369 and total DLP 1,148 mGy cm. My numbers tell me that I was exposed to 1,148 mGy or 114 rads. My knees seem fine, no burn, no redness. I have read that if you are exposed to 114 rad at one time it is considered acute radiation sickness. I am worried that I might get cancer or another radiation health effect later in life. Yes, I am aware I was exposed to radiation. Is the total DLP really equal to the amount of radiation I was exposed to? Should I continue to panic?


You can stop worrying. You will not see redness of the skin or burning sensations or any long-term health effects from the CT examination. You were not exposed to 1,148 mGy or 114 rad. Let me explain.

CT scanners report several parameters for each scan or exam. These include the product of tube current and time measured in milliamperes seconds (mAs); the tube voltage given in thousands of volts (kV); the volume weighted CT dose index, CTDIVOL, in milligray (mGy); and the dose length product, DLP, in milligray centimeters (mGy cm).

The CTDIVOL is a weighted average of exposure measurements made at various points in an acrylic cylinder. These measurements are made at the various CT scanner settings and entered into its computer. This way, it "knows" the CTDIVOL for all the scan types performed on the machine. Let me be clear, the CTDIVOL is a machine parameter, not your radiation dose. The DLP is the CTDIVOL multiplied by the length of the scan in centimeters.

So, DLP in no way represents your radiation dose. It can, however, be used to estimate effective dose from an exam. For the exam of your knees, an article in Radiology indicates that the effective dose is about 0.5 millisieverts (mSv). As noted in the Health Physics Society position statement "Radiation Risk in Perspective," at effective doses below about 100 mSv, the risks of health effects, if they exist, are too small to be observed.

Kent Lambert, CHP, FHPS

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 30 October 2019. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, 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. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.