Answer to Question #15165 Submitted to "Ask the Experts"

Recently I had three computed tomography (CT) scans. The first one was to the head and neck (head CT Dose Index [CTDI] 32.62 milligray [mGy], head/neck CTDI mGy 14.59). The other two were about two weeks apart and were of the pelvis and abdomen, one CT with contrast (CTDI 6.78 mGy) and one without contrast (8.00 mGy). I am extremely worried about the radiation damage from these. Please help me to understand what damage may have occurred or is it likely in the future that the CT exposures will result in me getting a fatal cancer?

I want to begin by reassuring you that the risk of health effects from the CT scans is extremely low and may not exist. It is important to note that radiation exposure to a single organ is not additive, that different areas of the body which are exposed don't contribute to an overall effect to any specific organ. In some regards, this is similar to exposure to the sun. If you wear long pants to the beach one day, and a long-sleeved shirt the next day, the doses to your arms and legs aren't added. In this case, your exposure to your head, neck, and pelvis/abdomen are independent of each other and shouldn't be added together. In a similar manner to exposure from the sun, the same body parts exposed at different times are not cumulative. If you lie on a beach for one hour this year, it doesn't matter how much you were exposed last year or when you were a child. Previous exposures have no effect on whether you get sunburn. This is because the body repairs itself. So, your two pelvis/abdomen scans, received two weeks apart are independent events. It is probable that any effect caused by the first scan was repaired by your body by the time you got the second scan.

Effective dose in mSv, is a concept that is used to compare the risk of cancer from different exams or events. The following is from the American Association of Physicists in Medicine (AAPM) Position Statement on Radiation Risks from Medical Imaging Procedures: "The ….AAPM supports the position that medical imaging should be appropriate and should use the radiation dose necessary to accomplish the clinical task. At the present time, epidemiological evidence supporting increased cancer incidence or mortality from radiation doses below 100 mSv is inconclusive. As diagnostic imaging doses are typically much lower than 100 mSv, when such exposures are medically appropriate, the anticipated benefits to the patient are highly likely to outweigh any small potential risks. Given the lack of scientific consensus about potential risks from low doses of radiation, predictions of hypothetical cancer incidence and mortality from the use of diagnostic imaging are highly speculative. The AAPM, and other radiation protection organizations, specifically discourages these predictions of hypothetical harm. Such predictions can lead to sensationalistic stories in the public media. This may lead some patients to fear or refuse safe and appropriate medical imaging, to the detriment of the patient. Medical physicists continuously strive to improve medical imaging by optimizing radiation doses while ensuring that the needed level of image quality is obtained, thereby contributing to the widely recognized benefits of medical imaging."

The effective dose for a head and neck CT is extremely low, on the order of 1 or 2 millisievert (mSv). In your case, the relevant value is for the abdomen, which is as high as 8 mSv. Even if we took the two pelvis/abdomen scans together, we come up with less than 15 mSv, much less than the 100 noted in the above position paper.

The facility where you had the imaging should be able to answer your questions about the radiation dose and risk.

Joseph G. Och, MS, DABMP, MRSE