The default method of measuring radiation exposure is to be as conservative as possible. What this means is that we make assumptions that error on the side of overestimating the dose. Dosimeters are used in a way that measures the highest dose point on an individual and assign that dose to the whole person. This methodology works for most wearers of radiation badges who typically receive relatively low doses. The conservative approach almost always overestimates the actual dose to an individual. It is an easy way to operate and lets you pay attention to other aspects of your work without having to consider any variables or adopt special procedures. Overestimation can cause a regulatory issue for personnel doing fluoroscopy or other dose-intensive procedures with X-rays. Fortunately, for heavy users of X-rays there are easy ways to improve accuracy and reduce the reported dose. This post discusses the use of “dose weighting” to reduce your reported dose from x-ray exposure.

If you get a high dose, a health physicist is typically called in to look at the numbers. If the reported dose is more than a regulatory limit, you would report this to your State or Federal regulatory Agency, but you would also perform an investigation to assess the actual dose a person received. We at CHP Dosimetry receive a “high call notice” any time a deep dose of 400 mrem or greater is recorded for anyone monitored by us.. Some of you may have received one of these notices from me along with recommendations for future action.

Using Dose Weighting

Thanks to VETgirl for permission to use their veterinary CE photo.

I get concerned anytime a quarterly dose approaches 1250 mrem (¼ of the annual limit for deep dose of 5000 mrem), or a monthly badge exceeds 400 mrem.

Should this happen to you, there will be communication from me about what we can do.

We at CHP have no idea what happens at your facility. Whether or not the dosimeter is worn properly, or if it is somehow exposed while not in use (such as leaving it in the X-ray room on an apron). CHP Dosimetry has a short video you and your employees can review that discusses how to wear radiation badges that you can review at the end of this Post. Ultimately you, the license holder, are in control and are responsible for what happens at your facility.  It is you who are required to notify your State or Federal regulator. There are 50 States, and several federal agencies that regulate radiation exposure. As a licensee, it is up to you to keep up with the regulations that apply at your facility. This blog discusses the use of dose weighting, sometimes called the “Webster Method” of calculating radiation dose from X-rays. Some States allow this routinely, other States don’t.  Some will let you recalculate dose in communication with them and/or using outside experts. Some will let you use the Webster Method, but require you notify them if an annual limit is exceeded without it. Dose reports from CHP Dosimetry include both doses. The original measurement of “Deep”, “Eye”, and “Shallow” dose, and on the line underneath, the “EDE (effective dose equivalent)”.  The EDE is the recalculated deep dose.  You can find your State regulator with a google search such as “radiation protection regulations (insert your State)”, or on your license paperwork. To get a clear understanding of dose weighting methods, we suggest the following reference: Nuclear Regulatory Commission Reg Guide 8.40 Section 3.a.1.

IIf radiation doses from X-ray exposures approach regulatory limits, the first thing you should consider is whether or not you can adjust your practices in a way to reduce the dose.  Minimize beam time, maximize distance from the source (or the scattered source), and utilize shielding. Many X-ray rooms have separate booths to stay inside while the X-ray beam is on. They are there to provide distance and shielding from the source. Many dental offices have a button outside the room for the same reason. There are some procedures such as fluoroscopy where the distance from the source cannot be effectively altered and the beam time (X-ray Energized time) is much higher.  You can always optimize beam time, but if you do a lot of procedures, the dose will add up. If permitted by your regulator, we can change your dose assessment method and significantly reduce future dose.

There are two methods of “dose weighting” generally used to recalculate and improve dose assessment from X-ray exposures. One uses a single badge, and the other uses two badges. We always recommend using a single badge method first to see if it will solve the problem. The reason is that there are procedures that must be followed, and it is easier to follow the single badge than the double badge method. If you are performing dose weighting, it is essential that you always follow the requirements. Having procedures and training to always follow requirements is essential to gain the trust of your regulators that you are in control of the radiation dose at your facility.

Single Badge Dose Weighting

The single badge method provides a radiation badge that is fixed to your collar and is not obscured by any clothing or shielding you may be wearing.  The photo showing the Doctor wearing shielding with a radiation badge clearly visible on the upper body is how it is done. The radiation badge should always be completely unobscured by personal shielding like aprons and face shields to the source of radiation. This is because in addition to measuring your “deep dose”, it is measuring your skin and eye dose. The single badge method requires that you ALWAYS wear a shielding apron over your upper body. These aprons typically don’t cover your face, nor your upper arms. All areas that are considered part of the “whole body” and are regulated for radiation exposure. Items like face shields, shielding glasses, thyroid guards, and the aprons themselves are VERY EFFECTIVE at shielding you from exposure to X-rays. Some of these can hinder your performance, but for a single badge dose weighting method, the shield apron MUST always be worn, and the badge MUST BE fully exposed, even if wearing the other items. This requirement allows you to use other portable shields such as a moveable transparent wall, and to stay behind them.

In summary, the single badge dose weighting (Webster) method requires:

  1. A single radiation badge is ALWAYS worn at the collar level, uncovered by any type of personal shielding (such as an apron or face shield).
  2. The individual is exposed to X-rays.
  3. The individual ALWAYS wears the shielding apron while being exposed to X-rays.

The single badge method reduces the EDE to 30% of the original Deep Dose.

Double Badge Dose Weighting

The Double Badge Dose Weighting (Webster) method requires one badge worn on the outside of the shield apron (like the single badge method) and a second badge worn under the shield apron. If we use that method, the dose is calculated by reducing the external badge deep dose by 97% and summing it with the 150% of the deep dose measured under the apron. To get the full benefit of dose weighting, you need to make sure the badge worn under the apron is not switched with the external badge.  If they are reversed at any time, the dose will be amplified in the final dose report. The two-badge method is the most effective way of accurately measuring dose and is employed in cases where very high doses from X-rays prohibit us from using ultra-conservative dose estimation methods.

The double badge dose weighting is a more accurate assessment of radiation dose under the following circumstances:

  1. A single radiation badge is ALWAYS worn at the collar level, uncovered by any type of personal shielding (such as an apron or face shield).
  2. The individual is exposed to X-rays.
  3. The individual ALWAYSwears the shielding apron while being exposed to X-rays.
  4. The individual ALWAYSwears the second badge UNDER the apron.

The reason we don’t always use the double badge method of dose weighting is because it requires you keep track of the location of the two badges, and provide effective training so personnel wear their dosimeters properly.  The philosophy of radiation monitoring is to make very conservative assumptions, and if those assumptions don’t cause a problem, keep using them. This way we typically overestimate a person’s actual radiation exposure. When using the more conservative method causes problems, we can utilize dose weighting.

To set up dose weighting, we require express permission from the facility.  It is up to you to ensure that personnel are trained to comply with these requirements, and that your regulator allows this practice. The Collar (CL) badges are clearly marked. The CL badge is to be worn outside at collar level.  The other badge is placed under the shield apron on the torso.

For more information about dose weighting, see the attached reference NRC Reg Guide 8.40 section 3.a.1.

Feel free to contact or call us at 888-766-4833 with any questions.