Answer to Question #11793 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

I recently had a barium swallow with 16 images and 30 seconds of fluoroscopy time. I asked for the dose and was told that the absorbed dose was 5.31 milligray (mGy). Every reference I have seen lists typical doses in millisieverts (mSv) so I don't know how to interpret this dose. I have tried to read about mGy and mSv, and it seems they are sometimes equal and sometimes not. How can I figure out the effective dose in mSv? I am worried because 5.31 is higher than I expected if in fact mGy and mSv are the same here.

A

Your effective dose in mSv is definitely less than 5.31 mGy. Most fluoroscopic units report dose information in terms of a dose-area product (DAP) which is the dose to a point in air multiplied by the size of the x-ray field in square centimeters (cm2), or in terms of a reference dose which is the dose in air to a reference point. The units for DAP are mGy cm2; the units for the reference dose are mGy. The reference dose, in this case 5.31 mGy, represents a dose to a point in air in the fluoroscopy field a given distance away from the x-ray tube.

The reference dose most closely approximates your peak skin dose, although it generally overestimates that, too. Only if all of your body received 5.31 mGy would your effective dose be 5.31 mSv. When only part of your body received 5.31 mGy, the dose needs to be adjusted. When we adjust for partial-body exposures, we don't just adjust based on area or volume exposed, but we adjust on the body parts exposed and how likely detrimental health effects (e.g., cancer) are to occur. When we adjust the dose we call it effective dose and measure it in mSv.

Estimating effective dose is difficult to do for fluoroscopic procedures. A typical barium swallow study with 24 images and 106 seconds of fluoroscopy would result in an effective dose of around 1.5 mSv. Given fewer images and less fluoroscopy time, your effective dose is likely less than this.

The American Association of Physicists in Medicine has a position statement on radiation risk which states in part: "Risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent."

The Health Physics Society also has a position statement on radiation risk in perspective which states in part: "Substantial and convincing scientific data show evidence of health effects following high-dose exposures (many multiples of natural background). However, below levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero."

In other words, if you study 10,000 people who did not receive any radiation (except background radiation) and study 10,000 people who received 100 mSv above background, there is no difference in the number of cancers or genetic malformations in the two groups.

Your effective dose is more than 60 times less than the level at which we can start seeing an increase in the number of cancers in a population.

Kent Lambert, CHP, FHPS

Answer posted on 7 February 2017. 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.