Answer to Question #14413 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'm worried about two recent head CT scans. The CTDIvol for each scan was 68 mGy. My understanding is that I received an absorbed dose to the brain of about 136 mGy. My question is not about cancer as that risk seems to be well covered by many existing sources. My question is more about the risks of the concentrated dose specifically to the brain. What gives medical professionals comfort that a CT scan provides a safe brain dose? I'm thinking about risks such as cognitive impairment, personality changes, neurological risks, Alzheimer's, and dementia. It seems well established that the extremely high doses associated with brain radiotherapy (30–60 Gy) are certainly associated with potential negative noncancer effects. Obviously, there's a big difference between 30+ Gy and 136 mGy, but where is the line, and how do we know?
For CT scans of the brain, the dose to the brain is about 70% to 80% of the reported CTDIvol. For the scans you recently had, the CTDIvol of 136 mGy corresponds to a brain dose of about 100 mGy.
There are still a lot of unknowns about the exact effects from exposure to these lower levels of radiation, although it is generally thought that there are no negative effects from acute doses less than 500 mGy to the brain. (It is important here to make a distinction between acute and cumulative doses. Acute doses—delivered over a short time period of a day or two—have a very different effect than if that same dose is spread out over time.)
People have studied how radiation affects animal brains, but there are not many studies that have been able to look at these effects in humans. The results from those studies are mixed. Some studies suggest that low amounts of radiation can damage cells, although it is unclear what effect this has on brain function or disease. Other studies show that low amounts of radiation provide a protective benefit. At higher doses (around 2,000 mGy) there is stronger evidence that radiation to the brain can—over time—change the blood vessels in the brain or cause mild changes in brain function.
There is still a lot to learn about the exact effects that radiation has on the human brain. But based on the information that is available, there are unlikely to be negative effects at acute brain doses below 500 mGy. Medical professionals use this information to evaluate the potential risks from a CT scan with the potential benefits (for example, accurately diagnosing a disease). As further research provides more information, medical professionals will incorporate this into that decision-making process.
Rebecca Milman, PhD