Answer to Question #11944 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

Our son is a twin, born prematurely at 32 weeks. At his three-month checkup, it was suggested he have a computed tomography (CT) scan of his head to rule out ventriculomegaly. This CT at 3 months led to two more, one at 7 months and another at 13 months. He never did develop hydrocephalus; he just had a big head.

The estimated dose-length product (DLP) for each of the three scans was 130 milligray centimeters (mGy cm), and the resulting effective dose was 2.5 millisieverts (mSv) per scan. His total DLP was therefore 390 mGy cm, and his total cumulative effective dose was 7.5 mSv. We have read several articles that recommend no more than 5 mSv effective dose to children over the course of one year. These include an article coauthored by David J. Brenner (Brenner et al. 2001) and another coauthored by Paul C. Shrimpton (Shrimpton et al. 2005), and both are very scary.

Our doctor based his calculation of our son's risk for future cancers on Brenner's article, and it came to 1 in 370. We thought a lifetime risk of 1 cancer in 1,000 children was excessive, but this estimate was much worse!

Is there any reassurance you can give our family? Our son is now 16 years old, perfectly healthy, and just learning to drive. It bothers us greatly to have this risk hanging over him. We wish he had never had the CT scans, but that's in hindsight.

Another thing that is especially frightening to us is that the Brenner et al. article holds to the linear no-threshold hypothesis. We're not sure whether Shrimpton and colleagues do, but it seems that the majority of articles use the linear no-threshold hypothesis. Assuming the linear no-threshold hypothesis is correct, could this account for our son's risk being so high, at 1 in 370?

The cancers related to head CT that we've read about were brain, leukemia, and thyroid, with an increased risk for all other cancers later in life. We were hoping it might be fair to assume that associated cancers would manifest within a decade or so, but Brenner states that effects may not be seen until 20–40 years later. Statistically however, how would we know that a cancer was caused by CT radiation versus natural background radiation? Again, these statistics and corresponding risks assume a direct linear relationship between radiation dose and cancer, without any threshold where risk is minimal to nonexistent.

We were also curious about terminology in the Brenner article, which refers to risks in Hiroshima survivors after doses of 5–20 mSv. We assume they mean effective dose, although the article doesn't explain this. The other problem with some articles and corresponding tables is that they seem to jump from effective dose to tissue-weighted dose without specifically stating which is meant.

Yet another problem we've found is that some authors calculate risk associated with total absorbed dose as opposed to effective dose. Are we correct in assuming that with a total absorbed dose of 390 mGy cm (130 mGy cm from each of three CT scans of the head), that amount gets absorbed into the various tissues (brain, red marrow, and thyroid); then when you multiply the tissue-weighting factors, an effective dose is obtained? In one article, the author quoted an infant head CT as producing 25 mGy of effective tissue absorbed dose for combined brain, red marrow, and thyroid. This confused us further because if the conversion factor for mGy to mSv is 1:1 for x rays, would the tissue effective dose be 25 mSv? But every other article clearly states that an infant head CT with total DLP of 390 mGy cm was about 7.5 mSv total effective dose.

Maybe we're not understanding properly. It is quite confusing trying to understand all the information (and misinformation) out there. The thought of giving our son a lifelong worry is very distressing.

A

Let's see if I can put your minds at ease.

First, both Brenner and Shrimpton assume radiation effects in the low-dose range, that is, at effective doses below 100 mSv. The problem with this assumption is that scientific evidence does not support it. The Health Physics Society, in its position statement Radiation Risk in Perspective, states that "substantial and convincing scientific data show evidence of health effects following high-dose exposures (many multiples of natural background). However, below levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero." Similarly, the American Association of Physicists in Medicine (AAPM) has a position statement which says that "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."

Using methods described in AAPM Report 96 (AAPM 2007), I estimate your son's effective dose per scan to be 1.4 mSv, for a total of 4.3 mSv. Regardless of which is closer to correct, my estimate of 4.3 mSv or your estimate of 7.5 mSv, the dose your son received is well below doses for which increased incidence of cancer has been seen. Also, at 16 years of age, your son has probably received an effective dose from natural background radiation of close to 50 mSv (based on the U.S. average of approximately 3 mSv per year).

Also, let's assume for the moment that the linear no-threshold hypothesis is correct. The normal risk that your son will develop cancer over the rest of his life without having the CT scans is 1 in 2.5 or 40% or 148 in 370. The additional 1 in 370 risk calculated by your doctor changes this to 149 in 370 or 40.27%. That 1 in 370 risk can also be looked at from a different angle. The chance that the CT scans will result in no effect is 99.7%.  

But again, these are hypothetical cancer risks that have not been seen in epidemiological studies. Your son received a real benefit from the scans. Even if the result of the imaging exam was negative, the physicians were provided information they could use to determine the next course of action. Refusing medical imaging procedures may result in real and substantial risk to the patient who will not receive the clinical benefits of the procedures.

The risks of health effects from radiation doses received during diagnostic imaging procedures are either too small to be observed or are nonexistent. The benefits from properly performed, clinically indicated, diagnostic imaging procedures (including CT scans) far outweigh any hypothetical cancer risk. Diagnostic medical imaging procedures provide a medical benefit to you even if they do not appear to reveal anything, and they are of less risk than their alternatives, such as exploratory surgery. So you did the right thing 16 years ago.

With regard to your questions about the linear no-threshold hypothesis, I would suggest that you check out a website by the organization Scientists for Accurate Radiation Information (SARI) at http://radiationeffects.org/. The SARI organization provides arguments against the linear no-threshold hypothesis. You might also want to search for and read articles by Jeffrey A. Siegel, PhD, Nuclear Physics Enterprises. John Boice, PhD, reviewed several well-publicized studies of the risk of CT and found that they were scientifically flawed in a way that rendered their conclusions unsupportable (Boice 2015). The International Dose-Response Society may also be a source of information at http://dose-response.org/ (check out articles written by Mohan Doss, PhD, among others).

Now let's tackle your questions about dose terminology. Radiation "dose" is a general term; there are several types of dose. Let's concentrate on three.

To start, we have radiation absorbed dose, or simply absorbed dose. This is the amount of radiation energy absorbed by a material per unit mass of the material. (The material could be a steel capsule or living tissue.) A joule (J) of energy absorbed in a kilogram (kg) of material is called a gray (Gy). A milligray (mGy) is one-thousandth of a Gy.

Certain types of radiation can cause more biological damage than others for the same absorbed dose. We use a multiplier to take this into the account. Alpha radiation is a good example; the multiplier is 20 for alpha radiation. We apply the multiplier to the absorbed dose. The name for this new dose is equivalent dose (or dose equivalent), and the unit for equivalent dose is the sievert (Sv). A millisievert (mSv) is one-thousandth of a Sv. For x rays and gamma rays, the multiplier is 1. So when talking about absorbed dose from x rays in mGy and equivalent dose in mSv, one gets the equation 1 mGy = 1 mSv.

If each organ and tissue always received the same equivalent dose or if each organ and tissue in the body were equally sensitive to radiation, then we could stop there. But neither is the case. Certain organs and tissues are more sensitive than others. We assign risk factors to each organ or tissue. We take the equivalent dose to each organ or tissue, multiply it by the organ or tissue risk factor, then add these all together. This gives us effective dose. As with equivalent dose, effective dose is measured in Sv or mSv. (Why we use the same units for two different quantities, I have no good answer.)

Since some organs and tissues get less radiation when we do diagnostic imaging, the effective dose will always be less than the equivalent dose and the absorbed dose. We would have to expose the whole body uniformly for the effective dose to be equal to or greater than the equivalent dose and the absorbed dose, and we never do that in medicine.

So those are three types of dose: absorbed dose, equivalent dose, and effective dose. Then there are some terms that are specific to CT procedures.

When looking at the dose information from a CT scan there are typically two values given, the CTDI(vol) and the DLP. CTDI stands for computed tomography dose index. It is determined by measuring the output of a CT scanner under different imaging parameters in an acrylic phantom and entering the resulting radiation output in the CT scanner's computer system. When a CT scan is done on a person, the CT scanner gives the CTDI(vol) based on the machine settings in use. CTDI(vol) is measured in mGy. The second value, DLP, stands for dose-length product. It is the CTDI(vol) multiplied by the scan length in cm. The units for DLP are mGy cm.

Again, I will remind you that a risk of 1 in 370 is not a high risk when compared to the background risk of 148 in 370. Even David J. Brenner states that the individual risk from CT scans is low and that the benefit from a clinically indicated CT scan far outweighs the risk.

Kent Lambert, CHP, FHPS

References

American Association of Physicists in Medicine. The measurement, reporting, and management of radiation dose in CT. College Park, Maryland: American Association of Physicists in Medicine. AAPM Report 96; January 2008.

Boice JD Jr. Radiation epidemiology and recent paediatric computed tomography studies. London, England: Sage Publishing Co. Annals of the International Commission on Radiological Protection 44 (Supplement 1):236–248; 2015.

Brenner DJ, Elliston CD, Hall EJ, Berdon WE. Estimated risks of radiation-induced fatal cancer from pediatric CT. American Journal of Roentgenology 176:289–296; 2001.

Shrimpton PC, Hiller MC, Lewis MA, Dunn M. Doses from computed tomography (CT) examinations in the UK—2003 review. Chilton, England: National Radiological Protection Board; NRPB-W67; March 2005.

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 July 2017. 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.