Answer to Question #9524 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Equipment

The following question was answered by an expert in the appropriate field:

The unit of workload is "mA-min" for x-ray units while it is "Gy" for cobalt-60 units, linear accelerators, and brachytherapy units. Why aren't the units the same? What is the difference?

The unit milliamp-minute "mA-min" is a measure of the electrical current flow integral over time, usually in an x-ray tube filament when used for determination of weekly workloads for diagnostic shielding design.  It was originally taken in the National Council on Radiation Protection's (NCRP), Report No. 49, Structural Shielding Design and Evaluation for Medical Use of X-Rays and Gamma Rays of Energies Up to 10 MeV (1976), to be equivalent to 0.01 Gy at 1 meter, as a very conservative assumption to simplify workload calculations, due to the fact that the radiation output of an x-ray tube varies considerably as the kilovolt-potential "kVp" is increased.  Since very few x-ray exams are performed at maximum kVp, it was unlikely that mA-min workloads would exceed rads at 1 m.

Since this report was produced in 1976, it was a reasonably conservative assumption because at that time the unrestricted maximum permissible dose was 0.1 mGy wk-1. When this limit was reduced to 0.02 mGy wk-1 in both International Council on Radiation protection (ICRP) and NCRP recommendations, this assumption proved to be too conservative and sometimes lead to overshielding medical x-ray facilities. This was primarily due to the fact that while shielding requirements increase linearly with workload, they increase exponentially with kVp at diagnostic energies. By assuming all workload was at the highest kVp, the shielding requirements were increased to much more than necessary.

The NCRP revised its recommendations for shielding diagnostic x-ray facilities, in their publication of NCRP No. 147, Structural Shielding Design for Medical X-Ray Imaging Facilities (2004). In this revision, workload is weighted by its kVp distribution and mA-min are no longer used directly as a measure of workload. In Table 4.5 of this publication, the weighted kVp mA-min/patient is converted to air kerma in units of mGy per patient normalized to a primary beam distance of 1 meter.

In teletherapy, accelerator and brachytherapy shielding design, the weekly dose rates are much more easily determined and usually are specified in units of air kerma at 1 m (Gy at 1m).

Mike Bohan
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