Answer to Question #9237 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
The simple answer to this question is “yes” there would be an advantage. At the same time, one must ask if it is “reasonable” for a cardiologist to wear two lead aprons. Allow me to explain that comment. In radiation protection, we operate under the philosophy that occupational radiation exposures should be maintained “as low as reasonably achievable” or “ALARA.” Thus, one must consider several factors when determining reasonable methods to reduce radiation exposures.
Consider that a 0.5 mm equivalent lead apron will allow only about 3.2 percent of 100 kVp x rays from the primary beam to penetrate the lead apron (see “Ask the Experts” question Q2827 for more information on the penetrating ability of x rays through lead aprons). It is important to note that the 3.2 percent value is for the “primary” x-ray beam. Lead aprons are actually designed to shield the wearer from “scattered” radiation. The importance of that relates to the relative penetrating ability of the primary beam vs the scattered radiation. Very simply, the energy of the scattered radiation will be lower than the primary beam. From the practical standpoint, that means the penetrating ability of the scattered radiation would be less as well, resulting in an even lower percentage of the scattered x rays actually penetrating the lead apron and exposing the cardiologist.
Most state regulations allow occupationally exposed individuals to receive up to 50 mSv per year. For simplicity, let’s ignore the primary beam vs scattered radiation attenuation issue. Thus, to exceed the regulatory limit of 50 mSv in one year, the unshielded exposure would have to be 50 mSv/0.032 or 1,563 mSv per year. Many regulatory agencies and institutions conservatively consider any occupational exposures that are =10 percent of the occupational limit (or 5 mSv) to meet the ALARA philosophy. Thus, one would need to take 10 percent of the aforementioned numerical values to meet this somewhat conservative interpretation of the ALARA philosophy.
To determine if the cardiologist’s annual exposure meets the 10 percent ALARA criteria, one would need to have an idea of the shielded and/or unshielded dose that the cardiologist receives. That could be determined by the results from the personnel monitoring badge the cardiologist wears (assuming the cardiologist is wearing such a device). In other words, if the cardiologist wears his/her badge under the lead apron, if the measured value is less than 5 mSv per year, most regulatory agencies would consider that to meet the ALARA philosophy. Alternatively, if the cardiologist wears his/her badge on the outside of the lead apron, it would appear that the ALARA philosophy is met if the measured radiation level on the outside of the lead apron is less than 156 mSv per year.
Now, one could argue that wearing an additional lead apron could further reduce the annual exposure to 3.2 percent (or less when one considers that scattered energy issue) of the previous values. For example, it could reduce the 5 mSv per year value to 0.16 mSv per year—that would seem to be a good thing, right? But one must also consider the downside of wearing two lead aprons. One of the primary disadvantages to lead aprons is their weight. Even the newer “light lead” aprons can be tiring to wear for long periods of time due to the weight placed on the shoulders and/or the hips of the wearer. Most would agree that fatigue can make one more prone to mistakes or errors. Thus, while wearing an additional lead apron might reduce the exposure to the cardiologist, he/she might be more likely to tire during the procedures and more prone to mistakes—that’s not something most of us would want to happen if we were the patient.
Sometimes it is helpful to make analogies when discussing what is “reasonable.” Take driving a car as an example. I think we would all agree that the number of car accident fatalities would be reduced significantly if we all drove our cars no more than 48 kilometers per hour (kph) on an interstate highway. While we would like to see highway fatalities reduced, most of us would not consider driving 48 kph on an interstate “reasonable.”
In summary, the best way to determine whether or not a cardiologist should wear two lead aprons would be to review his/her personnel monitoring results with one lead apron. From those results, one can consider if an additional lead apron is reasonable. If the cardiologist has not been badged in the past, I would suggest badging him/her both outside and under the lead apron for a period of time (e.g. three to six months) and then evaluating the badge results from that monitoring period. It should be understood that even if the badge results indicate the cardiologist’s dose equivalent exceeds 10 percent of the occupational limit, that doesn’t necessarily mean that his/her exposure isn’t ALARA. That’s something that should be evaluated by the cardiologist, the radiation safety officer or medical physicist, and the departmental manager. I think you’ll find that a single lead apron will be adequate to protect the cardiologist and meet the ALARA philosophy.
Mack L. Richard, MS, CHP
Radiation Safety Officer – Indiana University Medical Center