Answer to Question #10494 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:

Q

First of all, thank you for taking the time to consider my two-part question. I am hoping you can help put my mind at ease. My question is regarding clarification on one of your risk position statements and x-ray/CT (computerized tomography) imaging I have had done in the recent past. I'm a 32-year-old female and have had the following imaging studies from 2005 to 2012:

  • Two brain CTs
  • One cervical spine x-ray series (four views)
  • One lumbar spine x-ray series (three views)
  • Two bilateral digital modified mammograms (one view of each breast at two different points in time—so four images total)
  • One dental CT

The Health Physics Society (HPS) position statement I'm referring to is Risk- PS010: "1. HPS has concluded that estimates of health risk should be limited to individuals receiving a dose of 50 mSv in one year or a lifetime dose of 100 mSv above that received from natural sources."

Based on research I have done from your organization's resources and others (I'm a special education teacher and by no means am an expert on radiation dosage information), I believe I have received approximately 6 mSv (effective dose to the whole body) from these procedures, which I believe is similar to natural background radiation levels. I know you don't have my films and machine information in front of you, but, do you feel that is a correct estimate/statement?

The second part to my question is: in your organization's position statement mentioned above, does "50 mSv" refer to specific absorbed organ doses of the parts of the body (like the parts of the body imaged in a diagnostic study) or does it refer to effective doses to the whole body (I'm assuming the later)? I'm just wondering if I need to get a risk estimate for the medical imaging I have had done—in other words, have I crossed over that 50 mSv threshold from those imaging procedures? I do not know the absorbed organ dose estimates from my imaging procedures and I'm sure they are higher than the whole-body dose estimates. I'm hoping I don't need to worry about the absorbed doses because the effective dose takes into consideration the effect on the whole body and allows us to compare to natural background radiation—right?

Essentially, are we talking apples to apples here? Can I compare my additional "6 mSv" from my medical imaging to the "50 mSv" and say, "My 6 mSv happened over a longer period of time and are nowhere near the 50 mSv, so I do not need to worry and am not a candidate for a risk estimate." I am hoping (and thinking) that is the case. I would just like some confirmation from an expert.

I would really appreciate your clarification and advice. Thank you very much!

A
You did your homework well. Your dose calculation is close to my "back of the envelope" estimate and your interpretation of the HPS position statement is correct.

Your first question is whether your dose estimate is correct. Using the medical dose calculator, typical radiation exposures from the procedures you received over the last seven years are:

Head CT (two): 4.4 mSv

Cervical Spine: 0.3 mSv

Lumbar Spine: 1.8 mSv

Mammograms (two): 0.3 mSv

Dental CT: 0.9 mSv*

Total: 7.7 mSv

Considering the variations in machines, technique, etc., your estimate and the above are reasonably close, so I have not attempted to refine the estimate further. This is about two to three years of the U.S. average natural background radiation exposure. Considering that these procedures were performed over a seven-year period, your radiation exposure to background radiation during the same period exceeds these medical procedures.

Your second question is regarding HPS Position Statement PS010-2 and whether the doses refer to effective doses or to organ doses. Your assumption is correct that it refers to effective dose. Effective dose takes into account nonuniform sensitivity of specific organs to radiation. For example, if your whole body was exposed uniformly to a radiation dose of 0.3 mSv, your risk of a fatal cancer would be greater than if only a single organ was exposed to 0.3 mSv. To account for this, risk factors have been assigned to specific organs depending on their sensitivity to radiation and are used to determine effective dose.    

The 50 mSv dose referred to in the HPS position statement is effective dose and it corresponds to the 6 mSv medical dose you calculated (or the 7.7 mSv medical dose I calculated). You are indeed comparing apples to apples.

I would be remiss to not mention that these medical exposures were not without benefit to you. These procedures provided you and your physician(s) with information they could use in the diagnosis and treatment of your medical condition.

Kent Lambert, CHP

* The website does not have a dose estimate for dental CT so I used the sinus CT scan dose estimate.
Answer posted on 30 January 2013. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.