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CT and radiation - a never ending story


I am 51. During the last 6 years different spesialist had sent me to several CT & xrays as part of finding the reason for my medical problems. So far nothing has been found sometging that is very positive. However with all the symptoms & health issues i have been having it never went through my mind about the radiation risk. When i now through some referral i see the dr asked for MR or xray but i ended up getting CT instead without being aware. Most of the test where at the same facility so i am also surprise the place did not noticed i had some CT & xray done at that place. I had 2CT this year, 1 in 2018, 3 in 2019, bone scan and some x rays: knee, elbow & hands. How can i find out i have been expose to too much radiation in a short period of time and if so what can be done now & in the future? Thanks

Kvinne 51


This is probably the most asked question I get at this site. The short answer is as always "CT /X-Ray is safe whenever done for a good medical reason".

We know for a fact that radiation can do harm to our body, including the radiation from medical imaging. From a range of accidents including Chernobyl and Fukushima and the work done at the nuclear program of the US and Russia we have learned quite a lot about how harmful radiation is in large doses. CT has been done for 50 years, and X-Ray for more than 100 years, so we do have quite some data also for lower doses.

Radiation used for medical imaging is measured in Sievert - Sv. 1 Sv is a very high dose, so for medical imaging we use the term millisievert - mSv (1/1000 Sievert). Radiation may also be measured in Gray or G, but for imaging we can say that 1 G = 1 Sv. Some other units exist (Rad, Rem, Roentgen) but these are only used in physics and theoretical use. A regular CT scan of the body is about 1-7 mSv. An X-ray is about 0.1 mSv. The same for mammography, dental x-ray is much lower at about 0.01 mSv.

What we have learned over the years has been put together in recommendations from the Radiation Protection Authorities. For the moment we can say as follows:

Background radiation in Europe pr year (from air, ground, cosmic radiation and nuclear contamination) - 3.5 mSv

Yearly allowed dose for radiological personell - 20 mSv

Max. allowed dose for pregnant women directly at the womb - 100 mSv

Possible sterility in males (dose aimed at testicles) - 500 mSv

Radiation sickness - 1000 mSv (1 Sv).

Acute radiation sickness and 50 % deaths - 5-6 Sv

Almost certainly lethal - over 8 Sv.

Highest recorded dose - 32 Sv (Slovin accident in US during the Alamo project / atomic bomb). Some firefighters in Chernobyl may have gotten higher doses but this was never confirmed.

From what I can see you have had a total of 6 CT-scans. You do not mention what kind of CT, with or without contrast, or what part of the body. This will of course alter the amount of radiation. But as this is over 3 years the dose will still be low. If we say that each CT is max at 7 mSv this totals 42 mSv, half of what the Radiation Protection Agency allows at one time in a pregnant woman. 

To get up to dangerous doses with regular x-ray would take several thousand pictures. So this is not worrisome. Bone scan is a little higher but not much more.

Some sources cites that out of every 100 000 CT scans 1 pasient will get cancer. This may be true, but what they do not say is this: Each of us have a 30 % lifetime risk for cancer - we must die of something. If 1 out of every 100 000 get a cancer this will increase our cancer risk of 1/100000, meaning that the risk increases from 30 % to 30.00001 %. In comparison, smoking increase your risk of cancer by 30-40 %.

Also cancer is rarely caused by 1 factor alone, and it takes 30-40 years from radiaton exposure until cancer appears (we know this from multiple accidents in the atomic bomb programs in the 40s and 50s). So we simply do not know if a CT scan done 30 years ago is the reason why someone now have a cancer.

I also want to stress the fact that medical imaging is done for a reason. And the benefit always outweighs the risks. An example: We scan your lungs for a lung tumor. You get a certain dose, and you MAY have a 1/100 000 increased risk of later cancer. But if we do find a tumor, we can spot it so early that it can be surgically removed, and you`ll have a 98 % chance of surviving lung cancer. If we on the other hand do NOT find the tumor out of fear of radiation, it will grow and metastasize. Then you do not get at cancer from radiation dose, but your survival chance from lung cancer is at 2-3 %.

You ask why we do not do MRI (no radiation). This is because not all medical conditions can be solved with MRI. We are certified specialists in imaging - our task is to choose the correct examination and method to solve the problem put forward by your doctor. He or she may want MRI, but we are by law required to choose e.g. a CT if this is better for whatever condition is suspected in you. A surgeon must decide to operate or not, a gastroenterologist must decide to do a scopy or not. Our job is to choose the correct radiological modality (CT, MR, X-Ray).

So in total I would say that this is not dangerous. The doses you have received are well within the normal range. You should not worry, and if there is a medical reason for doing a CT scan later on, you should trust your doctor and have it done.

Have a nice day!

Best regards

Bjørn Løndalen



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