Answer to Question #5306 Submitted to "Ask the Experts"

Q

I am a 33-year-old female. In September I had a cinedefecography performed because of pelvic pain. At the time I was told that the radiation dose was "like a chest x ray." I have since found out that the dose was quite different. I called the physician who then contacted the university radiologist and determined that for this procedure using their machine, the dose was just over 17 mSv for the average 2.5-minute procedure. I was told that my procedure lasted about 90 seconds. Can I simply divide to determine my approximate dose? Also, the organs exposed on the fluoroscopy for this exam included the bladder, ovaries, and rectum. What is the absorbed dose to these organs and how does that relate to the total dose and the associated risk of future cancers? The test determined some problems with some of the pelvic muscles, but I still remain concerned about the elevated radiation dose. I also had an abdominal CT done four years prior to this procedure. You have covered the risks associated with CT very well, but what is the risk when multiple exams have been performed to the same area of the body over a period of time?

A

It was really nice that you went back to the organization to obtain the approximate radiation dose from your exam—that is very helpful for you and for us.
The answer to your first question about simply converting the 17 mSv in 2.5 minutes to what you might have received in 90 seconds isn't very straightforward. It depends on whether it was all fluoroscopy, whether it was all cineradiography (where they actually film the fluoroscopy procedure), and whether plain films were taken. I think it is better for us to simply apply the 17 mSv as the dose you received.

As for the dose to organs in or near the area, all I would be able to do is assume they also received the 17 mSv. For a cinedefacography, it is likely that at some point one or both of your ovaries was in the beam but we don't know for how long. It would be the same with the bladder and rectum, although I suspect those were in the beam throughout the entire procedure. The individuals who conducted the exam would know if the organs were in the primary x-ray beam or not and, if not, how close they were to the beam plus how long the organs were in the beam. That is the only way to know the actual dose received.

When we discuss radiation risks, the effects are probabilistic in nature. We say that the increase in the cancer incidence rate is about 0.005 % per mSv of radiation dose based on projections from high doses. However, it is impossible to say that any single person will get a radiation-induced cancer. Second, it may be impossible to even demonstrate that additional cancers have occurred as the normal incidence of cancer is about 33 percent plus or minus some natural variation in this rate.

So, for your exposure—the cinedefacography at 17 mSv and an abdominal CT scan at 10 mSv (taken from Doses from Radiation Sources)—your incidence would change from a base rate of 33 percent to 33.1 percent. This is an increase of less than 1 percent which is probably within the normal variations of the cancer rate. Also, it has not been shown that there is an increase in cancers below radiation doses of 100 mSv (HPS 2004).

Kelly Classic
Certified Medical Health Physicist