Answer to Question #11153 Submitted to "Ask the Experts"

I was advised to have a CT (computerized tomography) scan of my orbit (eye socket) due to various issues. I would like information about my dose, such as if it was too high or a typical amount. I asked questions at the facility, was not given much information. I was told by one person that a calculation was performed by a physicist and my dose was 22 mGy and that the American College of Radiology guidelines were under 75 mGy (from online information, it looks like this is for an entire head, not the orbit). The radiologist report mentions that there was an accumulated dose rate of 186 mGy. That sounds large. Both of the numbers sound large compared to much of the information online about CT scans, which is in mSv.

Also, I am concerned because I keep reading about how sensitive the lens is to radiation. I had no idea going in to this procedure about that and am very concerned about the risk of cancer now. I have not been able to find much information on orbit scans. A few papers I read listed them as having low effective doses in mSv, but also warn of their high absorption rate (indicating that the lens could absorb over 100 mGy). If the eye absorbs so much radiation, how is it possible that there could be a low effective dose? How could my dose be 22 mGy and not higher?

I would first mention that a CT scan of the orbit is not that common of a study. Information on doses from CT scans of the orbit is very limited compared to more typical scans such as head scans. Still, this scan is probably performed on hundreds of people a year. If there were a significant risk, we would know about it.

To get to your questions about dose and cancer risks, there are many types of radiation doses, both measured and calculated. Two common terms used specifically with CT scans are the volume adjusted CT Dose Index, CTDI(vol) or CTDI_VOL, and the Dose-Length Product (DLP). The CTDI(vol) is an average dose in milligray (mGy) measured in a solid acrylic cylinder. (Acrylic is used because it has similar radiation-absorbing properties as tissue.) The CTDI(vol) can be used to compare the hospital's CT scanner's output to, for example, the American College of Radiology (ACR) reference doses, such as 75 mGy for head CT scans. A hospital might want to adjust its machine's settings if the CTDI(vol) was much higher (or much lower) than the ACR reference dose.

The DLP is the CTDI(vol) multiplied by the scan length. The scan length will vary, of course, on the area of interest and the patient's anatomy. A chest+abdomen+pelvis scan of Kareem Abdul-Jabbar will have a higher DLP than the same scan on Vern Troyer. Since a CT scan of the orbit would be shorter than a CT scan of the entire head, the DLP would be less.

There is one other dose term to discuss: effective dose. Because in diagnostic radiology only a part of the body receives radiation, we need to take into account that the whole body is not exposed. Since some organs/tissues are more sensitive to radiation than others (e.g., the brain is not very sensitive to radiation, but the lungs are) we have to consider this as well. Effective dose does this. Each organ/tissue has been assigned a tissue weighting factor based on the risk (mainly of cancer) from radiation dose. The radiation dose to each organ is multiplied by its tissue weighting factor and then all these doses are added to get effective dose. See Question 10540 for an explanation of effective dose. Effective dose is given in millisieverts (mSv).

I found a study that looked at doses from orbital CT scans. Using conventional techniques, the CTD1(vol) was 62.5 mGy, the DLP was 375 mGy-cm, and the effective dose was 0.86 mSv. The thrust of that paper was that the authors could adjust the scan parameters to reduce the dose and still get acceptable images. By doing that, their results were a CTDI(vol) of 20.7 mGy, a DLP of 125 mGy-cm, and an effective dose of 0.26 mSv (Wang et al. 2012). I also found a case study that reported a DLP of 142 mGy-cm and an effective dose of 0.45 mSv to a 14-year-old patient (Montgomery 2011).

Now to your numbers: you were given 22 mGy and 186 mGy, but it is not clear what these represent. The 22 mGy could be the CTDI(vol) and the 186 mGy could actually be the DLP at 186 mGy-cm. If this is correct, your effective dose would be roughly 0.6 mSv. Knowing for sure what the DLP was for your scan would be the best way to estimate your effective dose in mSv. And knowing for sure what the CTDI(vol) was would be the best way to compare their CT output dose to the ACR reference dose.

Regarding your second question about radiations doses to the lens of the eye, the potential effect is the creation of a radiation-induced cataract. For a one-time exposure to the lenses of the eyes of 22 mGy, the risk of cataracts is insignificant.

The Health Physics Society Position Statement on Radiation Risk states that "cancer and other health effects have not been observed consistently at low doses (that is, less than 100 mSv) because the existence of a risk is so low as to not be detectable by current epidemiological data and methods." So you should rest easy that you have not been exposed to any significant radiation levels that would cause you long-term harm. And you should also understand that you received a medical benefit from these exams. The benefits from clinically indicated and properly performed imaging procedures far outweigh any theoretical risks.

John Jacobus, CHP
Kent Lambert, CHP

References
Montgomery P. Radiation CT and beyond: Measurements, risks, what should be done. Grand rounds presentation. Women and Children's Hospital of Buffalo; 17 June 2011. 

Wang JW, Tang C, Pan BR. Data analysis of low dose multislice helical CT scan in orbital trauma. Int J Ophthalmol 5(3):366–369; 2012.