Answer to Question #14389 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

I have a question regarding dose rate, or time between radiological exams and its effect on stochastic risks. I have read conflicting information. Would two head computed tomography (CT) scans performed within a four-day time span pose a greater stochastic risk than two head CT scans performed within a four-year time span?

A

The quick answer is that there would be no measurable difference in the probability of a stochastic effect from the exposure to radiation from two CT examinations spaced four days or four years apart.   

Stochastic effects from ionizing radiation are "chance events," with the probability of the effect increasing with dose, but the severity of the effect not changing with increased dose.   

The dose rates that are typically encountered in diagnostic imaging do not have a significant impact on the probability of stochastic effects. Generally, for diagnostic radiology exams, the stochastic risk would be calculated to increase when examinations are performed serially (one after another). However, when there is a time lag between events, biological repair occurs. For diagnostic exams taken a couple of days apart, the exams can be viewed as independent events, similar to what occurs when you flip a coin, i.e., the coin flip prior does not influence the likelihood of the next flip being heads or tails.   

Given a four-day interval between your two head CTs, each head CT would have the same stochastic risk and would be no different even if exams were to have occurred four years apart. 

Something to keep in mind is that stochastic risks, or probability values, are only applicable to large population groups. When applied to an individual the ability to predict future outcomes tends to break down. That is because stochastic effects are independent events of "chance." For example, if I had a bag with 100 balls, 99 white and 1 red, and I had 100 people draw out a ball from the bag, I know that one person will definitely draw the red ball. But if there was only one person drawing out a ball and after each draw the ball is placed back into the bag, then even after the 100th draw the chance of drawing a red ball is still 1 in 100. This is similar to what has occurred with your CT examinations, the "bag" was reset after each exam so the "risk" from each exam did not change. 

In conclusion, I would not expect the two CT examinations to have any measurable impact on your long-term health. The stochastic risks from a CT exam would not be impacted by the time between exams.    

John Gough, MS, CHP, DABMP 

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.
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