Answer to Question #14100 Submitted to "Ask the Experts"

I am a 30-year-old female. I received a head computed tomography (CT) scan when I was 12 years old. I recently received a neck CT scan with contrast. I understand the effective dose from my most recent scan is about 2.2 millisievert (mSv). I am hoping that you can help me with a few questions and put my mind at ease.

1. Should I be worried about thyroid cancer due to my neck CT and possible scatter radiation from my head CT? I have read that the thyroid is sensitive to radiation, especially in young women.
2. I have read that organ dose differs from effective dose. Is it possible that my thyroid received a large dose of radiation? Does this put me at increased risk for thyroid cancer?
3. Between the two scans, I am assuming that I have received a cumulative dose of around 4 msv.

Should I be concerned?

First, let me reassure you that any cancer risk associated with a CT (or other radiographic examination) is largely a hypothetical, conservative ("worst case") estimate, but still would represent a very low risk compared to the benefits gained. I hope I can provide some perspective for you to ease your concerns.

Some explanation of the concept of "effective dose" will be helpful. "Effective dose" is used to represent an overall risk of biological effects (primarily cancer induction) from non-uniform irradiation of the body, where different organs and tissues receive different radiation doses as opposed to them all getting the same dose. That is certainly the situation with your CT exams, where the head and neck received the majority of the radiation dose. The dose to each organ or tissue (including scatter to organs or tissues not exposed to the primary x-ray beam) is multiplied by the tissue weighting factor for each organ or tissue, and each of those multiplications is added together to get the "effective dose," which is typically given in mSv. This means that an effective dose reported for (for example) a neck CT represents a calculated risk for all cancers, including thyroid cancer. You can think of the risk for the "most exposed" organ or tissue as being "built in" to the total effective dose that results from such calculations.  

What you have read about the thyroid being radiosensitive is true in the sense that it is more sensitive to radiation effects compared to most other organs or tissues in the body but (I need to emphasize) the level of that risk is still extremely small for low dose radiation. And yes, in general, the organ or tissue that received the most dose would be the one most at risk compared to organs and tissues that received less dose, but again, low radiation dose means negligible risk.

So where do your doses stand from a risk point-of-view? I am presuming that the 2.2 mSv effective dose that you noted for your head CT with contrast was provided by the facility that performed the study. This is certainly around the average effective dose from such procedures. As to your head CT when you were 12 years old, data reported from that time period indicates an average effective dose of about 1.4 mSv for a pediatric (less than 18 years old) head CT. Summing those gives a cumulative effective dose of 3.6 mSv. Your 4 mSv estimate is—practically speaking—correct. Additional information about radiation doses from common radiographic procedures can be found in the Health Physics Society (HPS) medical exposures fact sheet and at radiologyinfo.org.

Let's put that 3.6 mSv into perspective. It is only slightly more radiation than the average person living in the United States gets every single year from natural background radiation (about 3.2 mSv). And people in many areas, such as around Denver, Colorado, get significantly more than the average. All of the human groups in which an extra number of cancer cases have been measured were subjected to high doses of radiation above background (above 100 mSv, and most of the data is from above 200 mSv), typically delivered in short periods of time. Below these levels, any excess cancer cases that may be caused by the radiation have not been measurable against the number of cancer cases from all other causes. So, any estimate of risk from low dose radiation that you may read somewhere is based on taking the high dose risk value, applying it to low doses, and simply doing a math calculation. Such calculations are based on a number of assumptions and many of those are unproven. The reality is that no one has measured an actual increase in cancer cases in actual patients (or anyone else) who have received radiation doses below 100 mSv (above background radiation levels) from CT or any other radiation source. Compared to the risk of developing cancer from all causes that each of us faces over a lifetime, any added radiation risk is negligible. Here is something to consider: if radiation was a major cause of cancer from low doses, it would be easy for scientists to observe and measure any increase in cases. The HPS has published a position paper titled "Radiation Risks in Perspective" which discusses the above and which points out many of the possible problems associated with such low dose risk estimates. It 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.” Among other things, the position paper also points out: "Scientists evaluate and estimate radiation risk using several assumptions that, taken together, may lead to a range of hypothetical health risk estimates for any given exposure scenario."

You should also keep in mind that a CT (or other exam using radiation), ordered by a physician for sound medical reasons, provides enormous benefits in properly diagnosing a medical condition, guiding treatment planning, and evaluating treatment progress. And CT imaging (like any x-ray imaging) does not carry any of the risks associated with invasive procedures, such as risk of infection, bleeding, or physical damage to healthy organs or tissues. Taken together, those benefits far outweigh any small, largely hypothetical, cancer risks.

So the real takeaway message for you here is that your CT exams have not added to your risk in any way that warrants real concern or worry.

John C. Keklak, MS Hyg., CHP