Answer to Question #5425 Submitted to "Ask the Experts"

Two months ago I had two CT scans done of my neck—one with contrast and one without—and I was told that the total for the scan was about 3.5 rad. Now I am scheduled to have a chest CT scan and the total for that I was told was about 2.5 rad, which would bring me to a total of about 6 rad.

I have three questions:

1. What is the risk (if any measurable) to me for the 6 rad exposure?
2. Is the total risk lessened by the fact that the two scans will done on separate parts of the body?
3. Is the risk lessened by the fact that the two scans are going to be done about three months apart?

1. The calculation of risk from diagnostic radiation exposures are calculated based on three factors:
   
   
   • the radiation delivered to the body area, e.g., 3.5 rad for your neck scans,
   • the dose to the organs irradiated, and
   • to the relative risk of cancer in those organs following the exposure. The computed value is termed the effective dose, ED. The calculation allows a comparison to be make of the risks to patients for various radiation studies. Examples of effective dose values can be found on our Web page "Doses from Medical Radiation Sources." These are representative quantities as an exact determination would have to be made on the same CT scanner used for your studies.
   
   The website listed above shows that a chest CT gives 800 mrem. While a CT scan of the C-spine is not listed, it is probably equal to the dose for a CT of the head, or 200 mrem. For the two CT scans, that would be 400 mrem for a total effective dose of 1,200 mrem. To put this dose in perspective, I would recommend our Web page "Risk/Benefit of Medical Radiation Exposures" bearing in mind that this exposure is below the 10,000 mrem (10 rem) level where the adverse effects of radiation exposures have not been demonstrated, as noted in our Position Paper "Radiation Risk in Perspective."
   
   
2. As noted above, the effective dose allows for a determination of the risk to the individual and not each organ. Obviously, the heart would get higher doses in the CT chest scan than the CT neck scans. Nevertheless, it is the total risk to the individual that is of concern.
   
   
3. This question is difficult to answer because of our current knowledge about the effects of ionizing radiation at the doses that you were exposed to. In radiation teletherapy when a high dose is prescribed for a tumor, the dose is delivered at small increments over several weeks. This allows for the recovery of normal tissues that are also being irradiated. If the same total dose were delivered in a few minutes, the result would lead to an acute response and maybe death. At smaller doses cells in normal tissue repair themselves, or there is a repopulation of healthy cells into the area where the cells have been killed. In the situation when sub-lethal diagnostic doses are delivered, it is logical to assume that such repair mechanisms are also involved. However, it would be next to impossible to quantify cellular recovery as you would have to examine changes occurring at the cellular level. Generally, each individual exposure is added to get a total, conservative risk estimate. This probably results in an over estimation of the stochastic risk and that is why it cannot be demonstrated that there is an increased risk below 100 mSv or 10 rem.