Answer to Question #10613 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:


I have a kidney disease called medullary sponge kidney and I pass an abnormal amount of stones. As a result I have to go to the hospital emergency room one or two times a month. I have had just under 100 CT (computerized tomography) scans over the last 10 years. I have been told to watch my exposure but I do not know how, other than to tell them not to do the test. Now I am worried about my exposure and what I can do to reduce the effects.


Let me first say I am sorry to hear of your medical condition. I am sure it is further frustrating to keep going to the emergency room when you know what the diagnosis will be. I will try to give you some perspective of risk associated with your CT scans.
The common concern from repetitive diagnostic x-ray and CT scanning is an increased risk of cancer. It must, however, be first considered that cancer is a common disease, as 42 percent of humans in a population will develop a cancer sometime in their lives. For women and men, the lifetime cancer risk without any radiation exposures is about 37.5 percent and 44.9 percent, respectively. And there is also an association based on the age at which the person is exposed to the radiation as very young children are more sensitive to radiation, while cancers are more common in elderly people so radiation-induced risks are lower.
From current studies, it is estimated that an effective dose of 100 mSv to every member in a population will increase the rate of cancer by 1 percent to an estimated 43 percent. See Figure PS-4 at the bottom of this report. It should be clear that risks are not for individuals, but are statistical risks to individuals in a population. It is impossible to determine which of the cancers were due to the radiation and which were not in a population where each person received a dose of 100 mSv or more of radiation.
To provide some estimate of risk, I will assume that all of your CT scans were limited to the kidney. You did not indicate if the kidney exams were two-phase tests, involving two CT scans in which contrast was used during one phase, or a single-phase exam involving one CT scan. Typically, one half of all kidney exams are two-phase exams, but I am not sure if that is common practice in most emergency rooms so I only considered a single phase CT kidney exam. From one reference, the effective dose for a single axial CT scan is 6.6 mSv, while it is 5.9 mSv for a helical CT scanner (CRCPD 2007).
Taking the average effective dose of 6.0 mSv and assuming that you have had 100 CT scans, your accumulative effective dose of 600 mSv suggests that you may be at a significantly higher risk than an average person in the population. It is important, however, to consider that the increased risk is to the members of the population, not to individuals. That is, for 1,000 males of the same age, 449 will develop cancer in their lifetime. If each person received 600 mSv, then 509 would develop cancer, and 491 would not, so the odds of developing cancer is about 50-50. 
Due to the potential increased risk of cancer, it is recommended that patients take an active part in medical decisions involving radiation exams. The patient should be told why the test is being ordered, and the consequences of NOT having the test. It is recommended that patients should also ask if alternative tests that would supply the same information are available, such as ultrasound or MRI (magnetic resonance imaging) which do not involve any radiation, or plain x-ray film which results in significantly lower radiation doses than CT scans. If the physician believes that CT scanning, for example, is the most appropriate test, the patient should ask if appropriate dose-sparing imaging techniques, i.e., those for a small adult rather than a standard, can be used, and if the facility is properly accredited. I recommend the Image Wisely website for further information.
I hope this information is helpful.
John Jacobus, MS
Certified Health Physicist

Conference of Radiation Control Program Directors, Inc. Nationwide evaluation of x-ray trends (NEXT), tabulation and graphical summary of 2000 survey of computed tomography. CRCPD Publication E-07-2. Conference of Radiation Control Program Directors, Inc.: Frankfort, KY; August 2007.

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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