Answer to Question #4468 Submitted to "Ask the Experts"

I am a 37-year-old female in good health who recently had an abdominal CT scan with contrast. Before the CT scan, I was trying to weigh the relative risk/benefit of having it and found this extremely difficult. I believe my 1 in 2,000 increased risk of having cancer from the CT scan itself was actually higher than my risk of some abdominal cancer at my age (probably 1 in 100,000). Nevertheless, my inquiries regarding the radiation at the hospital was met with numerous deer-in-the-headlight looks and everyone thought they were safe.

Anyway, the CT scan took about 30 seconds or less. The machine was less than 1.5 years old, so I assume it is a modern one. What I still don't understand is how radiation dose is related to time. When people say that my CT scan is the equivalent of working in an Australian uranium mine for a year or 500 chest x rays, I'm wondering whether the comparison is valid. Isn't it different to receive 20 mSv over the period of 5 years to the entire body (such as hanging out at the Chernobyl site) versus having 20 mSv beamed right into your body for 35 seconds? It seems the shorter duration would mean an increased intensity. After all, I understand that the dose I received is equivalent to the dose a cancer patient receives as a first dose—so 20 mSv targeted to a specific area seems to produce more damaging affects (to the cancer) than 20 mSv diffused over five years?

Is it really reasonable to compare the radiation from a mammogram to the risk from several hikes at high altitude? Assuming the same effective dose of radiation, wouldn't you expect a higher incidence of cancer coming from a mammogram versus 30 hikes over 10 years or something?

Another question, my ovaries have now been exposed. Your experts say after a period of time, any damaged eggs would have been shed and I understand from your site that three months to a year is reasonable for holding off getting pregnant (although at my age, waiting to conceive would increase other risks). I thought all of the eggs in a woman were present at birth, so why would only some eggs have received radiation if the entire ovaries were exposed?

I appreciate the information, as your site is the only one that provides any information beyond the standard one-liner that the benefits usually outweigh the risks in terms of diagnostic tests. I think in my case the risk outweighed the benefit, but fortunately, I hope, the risk is still small.

As I read the first part of your question, there are really two issues that need to be considered. The first involves the question of partial-body irradiation, and the second issue is about protracted exposures versus acute exposures.

For the first issue, an understanding of how radiation risks are determined must be examined. In order to make some sense of the risk associated with various types of exposure situations, the effective dose (ED) is used. Basically, the ED is computed by first determining the radiation dose delivered to a specific critical organ or group of organs. Then the organ doses are multiplied by various tissue weighting factors that are based on the risk of developing cancer if the whole body was uniformly irradiated. For example, consider that the tissue weighting factor for the breast is 0.05. If the breast alone received a dose of 10 mSv, the risk of developing cancer would be equal to an ED of 0.5 mSv, as 10 mSv (organ dose) X 0.05 (tissue weighting factor) = 0.5 mSv. However, if the whole body and all the critical organs of the individual are uniformly irradiated to a dose of 10 mSv, the ED 10 mSv, as the sum of all the tissue weighting factors is 1.0. This allows the risk from mammogram to be compared to a whole-body exposure or some other radiation exposure scenario.

Your comment questioning the effects of a dose if delivered over a long period versus a short period of time is a valid one. If a total dose is delivered over a long time, as with background radiation exposures encountered in the certain places where people live, there is a reduction in observed risks when compared to the dose being in a short period of time, as in a radiation accident or medical x-ray examination. Generally, it is believed that the dose rate effect factor (DREF) may reduce radiation effects by a factor of 2 or more. For more information, see "Health Effects of Exposure to Low Levels of Ionizing Radiation: BEIR V."

With regard to your final question about the risk to the eggs in the ovaries, I suspect that not all of the oocytes are equally damaged. That is, some cells are completely inactivated by the radiation and others are not, based on statistical probabilities. Those that are no longer viable will be "cast off" in a short period of time following the irradiation. These cells cannot be fertilized, or, if they are fertilized, the resulting embryo does not develop properly. Thus, their presence interferes with the normal chance of conception and fetus development. This is why attempts to conceive following some radiation exposures should be delayed.

John Jacobus, MS
Certified Health Physicist