Answer to Question #5425 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:
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:
- What is the risk (if any measurable) to me for the 6 rad exposure?
- Is the total risk lessened by the fact that the two scans will done on separate parts of the body?
- Is the risk lessened by the fact that the two scans are going to be done about three months apart?
- 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."
- 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.
- 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
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.
John P. Jacobus, MS, CHP
Answer posted on 11 May 2006. The information and material posted on this website is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of 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 specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Website. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.