Answer to Question #12318 Submitted to "Ask the Experts"

Does radiation get scattered to other organs such as thyroid and breast tissues when you get two head computerized tomography (CT) scans and if so how much and how dangerous is it?

Thanks for asking. This question is certainly on the minds of many people who receive CT scans. The simple answer is that other organs including the thyroid and breast do receive radiation doses from head CT scans, but, the radiation dose is so low that there has never been any demonstrable evidence of any long-term harm from such exposures. That may sound contrary to what you have heard or read in the popular media. The key word in the previous statement is "demonstrable." Let's explore this a bit more.

The actual radiation dose to various organs from CT scans depends on a large number of factors—the type of CT scanner, how the scanner is set up, and even the anatomy of the person being scanned. Due to those factors, we can estimate by calculation the approximate radiation dose to those organs for a typical individual. One can search the internet and find many such estimates from both scientific and nonscientific publications. Note that it is wise to only rely on scientific publications for that type of information. There are also various computer programs that allow this calculation as well.

One such computer program that I have used to calculate organ doses from CT scans is called ImPACT CTDosimetry version 1.04. Using that program along with a standard setup for performing a CT scan from a CT scanner manufacturer, I calculated an equivalent dose to the thyroid of 4 millisieverts (mSv) and an equivalent dose to the breasts of 0.042 mSv. For two head CT scans, one would simply double those values (i.e., 8 mSv to the thyroid and 0.084 mSv to the breast). Most of the radiation dose comes from the CT x rays scattered inside the body, so it makes sense that the breasts would receive lower radiation doses than the thyroid simply because they are farther from the head. Don't forget these are estimates to a typical person using typical setup parameters for a specific CT scanner—the true equivalent doses could be a little more or less.

There is another radiation dose calculation that we sometimes make to compare radiation doses from different sources called the effective dose. This calculation assigns a tissue weighting factor to each exposed organ, multiplies that organ dose by that tissue weighting factor, and then adds all of those products up to arrive at the effective dose—think of it as a risk-based number that allows us to make comparisons to other types of radiation exposures. The tissue weighting factors for the thyroid and breasts are 0.04 and 0.12 respectively, which simply means that, in terms of overall risk, the breasts are considered a little more sensitive to radiation exposure than the thyroid. When one calculates the contributions from the thyroid, breasts, and all other organs that receive radiation doses from the head CT scans, the total effective dose from a single head CT scan as calculated using the ImPACT program is 2.2 mSv. Thus, the effective dose from two CT scans would be 4.4 mSv.

Let's put these doses in context. According to the National Council on Radiation Protection and Measurements (NCRP), the average member of the U.S. population receives about 3.11 mSv each year from background radiation (e.g., radiation from outer space, radioactive material in soil and rocks, and radon gas) (NCRP 2009). That number can vary considerably depending upon where one lives, but since we're dealing with averages here, let's use it for comparison to our calculated CT scan doses. Using those comparisons, this means that a single head CT scan results in an effective dose that is about 71% of what the average member of the US population receives every year and two head CT scans would be 71% of what an average member of the US population receives every two years.

Another important comparison which addresses your question about these CT scans being "dangerous" relates to my statement about "demonstrable" evidence of long-term effects. The Health Physics Society states that "below levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero." In other words, the risk, if it exists, is too small to be seen. What this means is that scientific data has shown that above 100 mSv, there appears to be a "statistically significant" increase in health effects (i.e., cancer). Since cancer occurs spontaneously from all types of causes, in the context of the above statement, this means that if a large group of individuals all received 100 mSv, the number of cancers that would occur could be shown to be high enough above the average cancer incidence, that one could conclude the radiation caused those additional cancers. So while we can assume there is some risk from radiation doses of less than 100 mSv, the data doesn't demonstrate that a risk actually exists—that's what the popular media fails to understand or publish. They treat the assumption of risk as reality. The same could be said of some individuals with scientific backgrounds who should know better.

So, considering that the calculated effective dose from two head CT scans (4.4 mSv) is only about 4.4% of the radiation dose needed to cause a demonstrable effect, we can certainly say that these CT scans are not dangerous. The other thing to consider is that there was actually a benefit to you from the CT scans because those scans likely provided your physician valuable information regarding your medical condition and he/she used the information provided by those scans to provide you appropriate medical care.

Mack L. Richard, MS, CHP

Reference

Health Physics Society. Radiation risk in perspective. Health Physics Society Position Statement. Available at http://hps.org/documents/radiationrisk.pdf.

National Council on Radiation Protection and Measurements. Ionizing radiation exposure of the population of the United States. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No 160; 2009. Available at https://ncrponline.org/publications/reports/ncrp-report-160-2/. Accessed 3 April 2018.