Answer to Question #1042 Submitted to "Ask the Experts"

Q

At what dose level do you recommend using a thyroid shield? How effective are thyroid shields in protecting the radiation worker from unnecessary exposure?

A

Thyroid shields are mostly used with lead aprons during medical fluoroscopy studies. In the United States most regulations regarding x-ray fluoroscopy beam output and scatter measurements are still specified and measured in roentgens (R), thus, assume 1 R = 1 rem for radiation protection purposes and in this response. In the SI system, 1 Sv = 100 rem. For fluoroscopy equipment, the Food and Drug Administration's regulations allow skin entrance exposure rates up to 5 roentgens per minute on old systems without automatic brightness control, 10 R min^-1 for systems without a High-Level option, and 20 R min^-1 for the High-Level mode. Though much shorter in duration than the fluoroscopy time, cine imaging can increase radiation levels by a factor of 10. X-ray beam energy (kVp) and diameter, patient size, and tube/image receptor angle all affect the scatter radiation levels to the operator.

Various studies have shown scattered radiation in a fluoroscopy procedure room will range from 0.03 to 0.5 R per hour of actual x-ray beam time. Current regulatory radiation protection dose limits for radiation workers are 5 rem y^-1 for the whole body, 15 rem y^-1 to the lens of the eye, 50 rem y^-1 to the skin or extremities, with a 50 rem y^-1 limit to any organ or tissue for internal (or in this case when an apron is worn, nonuniform exposure). These limits are to prevent nonstochastic biological effects (for example, cataract of the lens) and limit the probability of stochastic effects (for example, thyroid cancer).

In a scenario of nonuniform external personnel exposure during fluoroscopy, that is, an operator wearing a lead apron and perhaps glasses or leaded glasses to reduce the lens dose, the limitation of 50 rem y^-1 to the skin of the neck would also limit the thyroid dose to less than 50 rem y^-1. The sensitivity of an adult thyroid for cancer induction appears to be higher than that of red bone marrow for the development of leukemia, but mortality of thyroid cancer is lower because of slow progression and success in thyroid cancer treatment. Current federal and state regulations in the United States are based on the International Commission on Radiological Protection Report No. 26, which notes several significant tissues for dose summation when calculating whole body total effective dose equivalent (TEDE). For comparison, when calculating TEDE the tissue weighting factors for the red bone marrow and thyroid are 0.12 and 0.03 respectively. Typically, when a lead apron is worn during a fluoroscopy procedure, where close-in work is needed, thyroid shields are used by physicians, and perhaps other support staff, to keep the thyroid dose as low as reasonably achievable (ALARA). As noted above, often lead glasses are used too during fluoroscopy x-ray procedures to reduce the dose to the lens of the eye.

Often in some fluoroscopy rooms (for example, heart cath labs), whole-body exposures approach regulatory limits when monitored by personnel dosimetry worn on the outside of the lead apron. In these cases, some states allow the modification of dosimeter readings to more accurately reflect the TEDE when aprons are always worn, there is no higher-energy gamma radiation exposure, and there is an obvious nonuniform dose distribution due to the presence of the apron.

To answer the questions, if the personnel dosimetry shallow- and deep-dose readings demonstrate that the regulatory limits for the skin and whole-body TEDE can be met with a dosimeter worn on the outside collar of a lead apron, then a thyroid shield would technically not be needed. Nonetheless, there should always be an administrative dose limit set, such that the regulatory limits are not exceeded. However, wearing lead glasses and/or a thyroid shield would be a technique to keep exposure ALARA.

It is this writer's opinion, when personnel dosimetry measurements are modified for fluoroscopy exposure scenarios where the operator is always wearing a lead apron, all exposure-reduction techniques should be utilized if possible—that is, thyroid shields, glasses, lead drapes, leaded gloves, leaded glass, or plastic head shadow shields, etc.

Lastly, a typical 0.5-mm lead-equivalent apron or thyroid shield will provide 85% to 95% attenuation of scattered x rays. Lastly, there is one other radiation-exposure scenario where a lead apron, thyroid shield, and other radiological personnel protective equipment might be utilized. That is, hands-on work with plutonium, where there are low-energy characteristic x rays, and in-growth of ²⁴¹Am with emitted 60 keV photons. The above statements would also be applicable to this scenario. And in either case, medical fluoroscopy or special nuclear material work, at least one dosimeter measurement must be made outside the lead apron (preferably near the head) to provide an initial "entrance" (preattenuator) deep, shallow, and lens of the eye dose value.

David J. Allard, CHP