Answer to Question #11469 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Diagnostic Nuclear Medicine

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

Q

I received a nuclear stress test, which was a technetium-99m (99mTc) stress test. They told me that most of the nuclear medicine tracer would be excreted in my urine. The test was done at 10 a.m. That afternoon I went deer hunting and urinated in a plastic bottle around 2 p.m., then I put the bottle in my backpack to throw away. I forgot to throw the bottle away until the next morning around 10 a.m.

The bottle was probably only about 0.6 meters (m) away from me for most of the time before I remembered the backpack beside my bed. The bottle was sealed with the cap on it. It was right beside me in the deer stand for about three hours and then within 0.6 m of me for the other 15 hours before I flushed it down the toilet. How much radiation was I exposed to being around the bottle of urine for about 18 hours?

I was told I would receive a dose of about 12 millisieverts (mSv) from the nuclear stress test. I wonder if being in close proximity to the urine for 18 hours will increase my dose much over that 12 mSv. Would my backpack also be contaminated? I know this is a silly question but it is on my mind and I figured you would have an answer.

A

The increased risk to you from the urine was minimal, and there is no need to be concerned. Let me explain why.

As you know, nuclear medicine uses radioactive tracers to help diagnose and determine, in a nonsurgical way, the status or severity of many diseases (including heart disease) through the use of cameras that can image the organ of interest. Small amounts of radioactive tracer are usually administered by injection. Part of the reason these tracers are considered safe is due to their very short physical half-life.

The physical half-life is the amount of time it takes for the tracer to decay, or disappear, by half. Short half-life tracers minimize the radiation exposure to the patient, and 99mTc has a short half-life of six hours. A nuclear medicine tracer, when inside a patient, both decays by its physical half-life and is removed biologically. Urination is the main way the tracer is biologically removed from the body.

Patients are provided instructions after administration of a radioactive tracer that includes information on how long the tracer will be in their bodies and ways to minimize contamination. It is appropriate and permissible (in a regulatory sense) to use toilets. When urine with a small amount of radioactive tracer in it is flushed to a sewer, the urine will be contained by the sewer and the tracer will decay by its very short physical half-life. If urine is collected in a bottle, as in your case, then the rules based on physical half-life apply, and the bottle containing the urine emits radiation. Also, there is a risk of contamination based on spillage.

Based on the information you provided and using the very conservative assumption that the bottle was always within 0.6 m of you for 24 hours, you probably increased the total dose of your procedure by about 4%. Any contamination in the pack was removed by half-life decay a little over 24 hours later. The risk from a 4% increase in exposure from a necessary procedure is minimal, and the dose is well below what we all receive annually due to natural radiation in our environment. There is no need to be concerned.

Dawn Banghart, 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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