Answer to Question #11211 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

About two months ago I had a chest computed tomography (CT) scan for a possible "fatty tumor" (or something similar) on my upper rib cage. The results all came back well as it was just swollen tissue from a rib injury. I asked about the risks of the scan before I had it, but the technician told me there is only a risk if I am pregnant. After some research, I am TERRIFIED of getting cancer from this scan. I have severe anxiety over it. I beat myself up every day for having it.

I am a 30-year-old female with two young babies, and I worry about this daily. I have had several x rays in my lifetime but only this one CT scan. I need to be at peace with this as I have already had this scan. Please help me to understand better my cancer risks.

Also, when I had the scan they scanned me twice. They said the first scan was just to set me up for the scan and had much less radiation than the scan itself. Is this normal protocol or did I unnecessarily receive double radiation? The CT technician informed me that my CT dose index (CTDI) was 4.63. Also, they mentioned that they used a newer machine that is equipped with "idose"; because I am smaller, they said this would result in about a 25% dose savings. What is this in mSv or rem?

I am trying to understand my cancer risks from this single chest CT and my individual dose. I am very concerned, because I am still so young and have time for radiation to do damage. Is this as much of a concern as I think it is?

A

First of all, we are so sorry to hear that you have been so worried and anxious about the CT scan. Diagnostic x rays, which include CT, are relatively low-dose procedures.

From your description, you were scanned two times, but the two scans are different. The first scan is called a "scout," and uses much less radiation to determine "landmarks" in your body to set the start and end of the CT scan. This scout scan is performed to minimize the dose from the actual CT scan by limiting the actual CT x-ray beams to the portion of the anatomy that the physician needs to see in order to perform his/her diagnosis. The second scan was the actual CT scan and was ordered by your physician because of your medical need.

Also, they correctly used terminology to indicate that this was indeed a new CT scanner, which is equipped with computer algorithms to further minimize your radiation dose by automatically adjusting the amount of x rays used based on body thickness. This means that the CT x-ray output, and hence your dose, is reduced when x rays are going through the thin portions of your body.

On average, a standard chest CT scan results in an effective dose to the patient of 7–10 mSv. The newer CT scanners with dose-saving features would result in even less dose.

We suggest that you check out the risk calculator at www.xrayrisk.com. Using the default values, the additional cancer risk from a chest CT to a 31-year-old female is 0.09%. Compare this additional risk to the baseline risk of 37.5% and you can see that the additional risk is negligible.

The cancer risks calculated on the website referred to above are population risks, not individual risks. The website gives a baseline cancer risk of 37.5%, i.e., the risk of getting cancer from all sources. You don’t have a 37.5% risk of cancer. Instead, in a group of 1,000 random people, 375 are expected to get cancer. It’s a subtle but important distinction. Think about it this way: your neighbor smokes tobacco products and works in a coal mine. You exercise and eat broccoli and blueberries every day. The two of you don't have the same cancer risk, but you both are in the group of 1,000 people from which there will be 375 cancers. The additional risk calculated is also a population risk—more than 1,000 people would need to get chest CT scans for there to be one additional cancer expected.

Another way of thinking about this is that so far in your lifetime, you have probably received 10 times this dose from naturally occurring radiation in the environment (background radiation).

The Health Physics Society states:

"There is substantial and convincing scientific evidence for health risks following high-dose exposures. However, below 50–100 mSv, risks of health effects are either too small to be observed or are nonexistent."

Similarly, the American Association of Physicists in Medicine states:

"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. Predictions of hypothetical cancer incidence and deaths in patient populations exposed to such low doses are highly speculative and should be discouraged."

Your dose is well below these levels.

We hope this helps to restore your peace of mind.

Jean Gresick-Shugsta, DABR
Kent Lambert, CHP

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 20 April 2015. 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.