Answer to Question #14517 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment and Shielding — Equipment

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

Q

I work in a nuclear medicine facility as an operator (preparing the radiopharmaceuticals and doing the imaging). I have a dosimeter called the QuartaRad Radex RD ONE, which can measure the dose rate as well as being a cumulative personal monitoring device. You can view a video of it here.

I have a lead apron and a lead thyroid shield and I work with technetium-99m (Tc-99m) only. Should I wear the dosimeter outside the apron or under the apron while I am working and why there? At what level should I wear it so it can count the really true effective dose I am getting every day? There is no regulatory institution to tell me that, so I am improvising. How could I measure my true dose by this dosimeter?

When the dosimeter shows that I got 20 microsieverts (uSv) in the last five days of work, does that mean that I got a 20 uSv effective dose to the whole body? Or is that only the dose that the dosimeter measured in that small area not counting for my whole-body dose (local dose or whole-body dose)

Please provide sources on how to wear dosimeters so I can share them with my colleagues. Thanks!

A

Briefly, wearing a dosimeter allows you to monitor your dose. The dosimeter you are using gives you an instant readout of both the dose rate and accumulated dose. The goal of monitoring is to ensure that the cumulative dose you receive does not exceed the allowable limits set by the regulatory agency that authorizes your facility to possess and use Tc-99m. It is best practice to wear the dosimeter on the outside of the apron at the level of the collar. This allows for an estimate of the whole-body dose as well as dose to the eyes. For the purposes of complying with doses limits specified in regulations, the dose recorded on the dosimeter is assumed to be representative of the whole-body dose. This dose however is not an effective dose. That cannot be recorded on any dosimeter.

Why is it necessary to wear a dosimeter?
The main goal of a radiation protection program is to make sure that radiation doses are As Low As Reasonably Achievable (ALARA). Implementation of this policy requires that doses be monitored and recorded so that appropriate measures can be taken if doses exceed either the limits specified in regulations or the institution's policies. Therefore, individuals who handle radioactive materials, like Tc-99m, wear a dosimeter when handling these materials. 

The International Atomic Energy Agency (IAEA) publishes the International Basic Safety Standards for radiation protection. This document provides guidance to governments on basic provisions of radiation protection regulations, such as limits on the annual radiation dose workers can receive during their employment (Section 2.13). It also includes guidance to institutions on implementation of these radiation protection standards. Chief among them is a requirement to monitor the radiation dose of workers to ensure that the regulatory limits are not exceeded (Requirement 9, Page 33).

Where to wear the dosimeter
Dosimeters monitor and record the radiation exposure at the position where they are worn. Therefore, the reading represents a local dose. However, for purposes of compliance with radiation dose limits, it is assumed to be representative of a whole-body dose.

IAEA radiation safety guide No. RS-G-1.3, Assessment of Occupational Exposure Due to External Sources of Radiation provides guidance on how to wear a dosimeter. This document advises (Sections 3.11-3.13) that in most cases, wearing a single dosimeter on the trunk (between the collar and waist) is adequate to establish an estimate of the radiation dose absorbed at 1 cm depth in tissue. If there is concern about dose to the eye, the dosimeter should be worn at the collar.

The dosimeter should always be worn outside of the lead apron. The apron can absorb much of the radiation and this therefore reduces the recorded dose both to the whole body and to the eyes, which are not covered by the apron.

Effective dose
The dose recorded on the dosimeter is not the effective dose. Effective dose is a value calculated by estimating the dose absorbed in all organs, multiplied by a correction factor for each organ that considers the sensitivity of the organ to radiation exposure (see Section 2. Dosimetric Quantities in RS-G-1.3). The effective dose is always substantially less than the dose recorded on the dosimeter, perhaps as much as 100 to 1,000-fold lower, depending on the how the radiation exposure is distributed (e.g., uniformly [rare] or variable [common]) and the sensitivity of individual organs.

In summary, it is important to wear a device that monitors and records your radiation dose to ensure that your dose does not exceed any regulatory limit. The dosimeter should be worn outside the lead apron, preferably at the collar level. The dosimeter reading is considered representative of your whole-body and eye doses and is thus used to demonstrate compliance with regulatory limits. That being said, effective dose cannot be measured directly by a dosimeter.

If you have further questions, we recommend you consult the designated radiation protection officer at your institution.

Thomas L. Morgan, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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