Answer to Question #10639 Submitted to "Ask the Experts"

I am a nuclear medicine technologist and I have a question about patients wearing a dosimeter after treatments.

We are wondering what the regulation or protocol is if patients receive a ^99mTc injection or even the higher-dose ¹³¹I treatment and they work in a medical field where they have to wear a dosimeter.

Should they wear their dosimeter after the treatment and, if not, for how long do they keep it off?

Thank you for the interesting and thought-provoking question. Unfortunately, the answer to this question is best handled on a case-by-case basis. The short answer is that regulators would rely on the facility's radiation safety officer (RSO) to use professional judgment in each case as it comes up. Therefore, only the site-specific RSO or policies would be able to answer for your facility: "Should this patient wear/not wear a dosimeter for a certain time-frame post radiotherapy/diagnostics for his or her occupational exposures?"

But maybe we can shed some light on the issue. Let's start with a regulatory review and look at some of the details that might help.

According to state and federal regulations, employers are required to monitor workers who are likely to exceed 10 percent of a legal limit for occupational exposure (10 CFR 20.1502, WAC 246-221-090). Occupational exposure is defined such that it excludes exposures from medical and background radiation (10 CFR 20.1003, WAC 246-220-010). For more information on the external monitoring requirements, see the Nuclear Regulatory Commission's (NRC) Regulatory Guide 8.34.

As you pointed out, if someone were administered ^99mTc or ¹³¹I, he or she would emit small amounts of radiation. If he or she were wearing his or her dosimeter during this time, the dosimeter would collect a radiation dose that is the sum of the person's occupational and medical exposures. So the dose reported for that individual would not be representative of the person's occupational dose. In addition, there is a regulatory requirement that states: "Personnel monitoring devices assigned to an individual . . . shall not intentionally be exposed to give a false or erroneous reading" (10 CFR 20.1502, WAC 246-221-090). As such, it is not appropriate for the worker to wear a dosimeter while he or she is still emitting radiation from medical sources. Therefore the question evolves to: "When is it appropriate to begin wearing a dosimeter post a nuclear medicine procedure?"  

Some facilities will set a policy that the worker can get his or her dosimeter back "when the worker returns to background levels" or "when they are indistinguishable from background." The simple procedure is to use a handheld radiation detector and measure the individual at time intervals and return the dosimeter when he or she can no longer measure radiation above background levels. This is acceptable for many places, but could be too restrictive as you will be able to measure the residual activity in the body beyond the time that it would cause any significant dose to the worker's dosimeter.

The ^99m Tc has a six-hour physical half-life. Based on that, after two days following a ^99m Tc administration, there will be less than 0.4% of the original activity left in the person's body. This reduction is just based on the physical decay and neglects any biological removal, which will also reduce the residual amount of activity. So for ^99mTc, 48 hours after the administration should be a sufficient time period to wait before returning the worker's dosimeter.

The use of ¹³¹I in thyroid disease diagnostic and therapy and cancer therapy is a slightly different case. The half-life of ¹³¹I is 8.1 days, so in comparison to the ^99mTc, it will remain longer in the body. For large doses of ¹³¹I used as a cancer therapy, the worker probably will not return to work for several weeks and so will not require a dosimeter. But for thyroid function tests or minor iodine administrations, some of the radioactive iodine will become bound in the thyroid, which is a small endocrine gland at the base of the neck. What isn't bound in the thyroid will be eliminated within a few days from the body; however, retention can vary widely among patients based on individual thyroid function. Once the radioiodine is accumulated in the thyroid, the activity will decrease based on the half-life with only a small amount of biological removal. So for workers who have received ¹³¹I, the RSO would likely have to make the call. It's very possible that the workers would not be able to wear their dosimeters for one to two months, based on a review of the initial activity used, thyroid up-take, etc.

The next question is: "Does this mean that the workers cannot work with radioactive material or in radiation fields?" To answer that question, you need to look to see if the workers have to be monitored at all. Basically, do they fall under the requirement of "likely to exceed 10% of a legal limit"? This should be based on an evaluation of the workers' duties, prior exposures, exposures to coworkers performing the same work, published literature, etc. If a worker does not need to be monitored, then according to the regulations, he or she could work without a dosimeter. It should be noted that besides the regulations, facility policies or license conditions may require monitoring even beyond what the regulations require. Therefore, checking all of the requirements that pertain to each patient facility would be an important aspect in the determination of patient occupational dosimetry.

If the worker is required to be monitored, then it would be hard to allow him or her to receive occupational radiation exposure without some monitoring. If required, the RSO could consult with the state and professionals to determine a manner of monitoring that would be acceptable.

Another issue that would need to be addressed is: "Does the worker need to perform a self-survey, such as a nuclear medicine technician would have to do before leaving the hot lab?" If the worker cannot actually check himself or herself for contamination due to his or her own radiation levels, then he or she should not be working with unsealed radioactive materials until his or her "background" is low enough to allow detection of personnel contamination. This would not be the case for x-ray technicians, linear accelerator operators, HDR operators, or other workers who are not working with unsealed radioactive materials.

In closing, in regards to advising patients about appropriate dosimeter wear post treatment, it would be best to direct a patient to discuss options with the facility's RSO where that patient works.

Bruce Busby, RSO
Fred Hutchinson Cancer Research Center

Emily Brown
Senior Radiation Safety Technician