Answer to Question #11883 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:

Q

I recently had three computed tomography (CT) scans and one positron emission tomography (PET)/CT scan. I asked the hospital what my doses from the CT scans were, and they said 375.81 milligrays centimeters (mGy cm) + 100.10 mGy cm + 147.27 mGy cm = 623.18 mGy cm total. I had the PET/CT scan at another hospital, and the information they gave me is as follows: injected 260 megabecquerels (MBq); Ct-ctd (after "ctd," there is either a 7, i, or z) = 1.83 mGy; dlp = 197 mGy cm.

I went on Google (I know I should not have done it) and tried to work out what total dose of radiation I've had. I was looking at a conversion site, and I calculated 623.18 mGy cm = 623.2 millisieverts (mSv), just for the three CT scans. They say a normal scan ranges from 1.5 mSv to 7 mSv, so I'm hoping I have worked this out incorrectly as this seems to be a large dose.

Is there anyone that could give me the correct answer, or do you know where I could obtain the correct information? If you could work out the dose from four scans (three CTs = 623.18 mGy cm and one PET/CT = 197 mGy cm) in mSv and rem, I think I could understand that a bit better. I'm sorry if this email is a bit complicated; I hope it makes some sense. PLEASE, could you help as I'm very worried.

A

I have good news, your calculation is wrong. (It's not often that being told you were wrong is good to hear!) The radiation dose parameter you were given for the three CT scans is called the dose-length product, or DLP. DLP can be used to estimate the effective dose. Based on a DLP of 623 mGy cm, the effective dose is somewhere around 9 mSv. The effective dose for the PET part of the PET/CT scan is around 5 mSv. And for the CT portion of the PET/CT, the effective dose is around 3 mSv. The total effective dose is on the order of 17 mSv.

For perspective, the Health Physics Society's position statement, Radiation Risk in Perspective, states, "[B]elow levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero."

Another scientific organization dedicated to assuring that radiation is used safely in medicine is the American Association of Physicists in Medicine (AAPM). AAPM's position is, "Risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent."

I would be remiss if I did not point out that you received a medical benefit from these procedures. The benefits from properly performed, clinically indicated, diagnostic imaging procedures, including CT and PET/CT scans, far outweigh any hypothetical cancer risk.

Kent Lambert, CHP, FHPS

Editor's Note: This letter points out that patients should not rely on the internet to calculate a complicated dose such as that received from a CT scan. Insist that the physician and hospital that performed the exam contact their physicist to have a proper dose calculated and to provide an explanation of the extent of the dose and any risks that the patient incurred.

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 31 March 2017. 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.