Answer to Question #11976 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 have just had a lymphoscintigram. The letter describing my appointment stated: "The amount of radiation is equivalent to that which you receive from background radiation in about nine months. This adds very slightly to your risk of, for example, developing cancer. However, as one in three people will develop a cancer at some stage in our lives, the added risk is very small. The consultant believes that the benefit of having the scan is likely to be greater for you than any of the potential risks involved. If possible avoid close contact or prolonged contact with pregnant mothers, babies, and children for six hours after the procedure. This helps to avoid exposing them to unnecessary radiation. Although the possible radiation dose to a child is very small, it is sensible to reduce this to a minimum." The letter also mentioned that if I were going to fly soon, I should let them know.

I found the letter a little daunting. I rang the nuclear medicine department before the appointment, and a technologist stated that a back x ray is 1–2 millisieverts (mSv) and this test is 0.09 mSv, which is a lot less than 1 mSv. A bone scan is 3–5 mSv and this is 0.09 mSv, which is an awful lot less—really a very small amount. If you drink or smoke or take drugs, that's worse. It's a good thing to have done, and the radiation dose is tiny—extremely low—for this investigation.

At my appointment, they said I had a very, very low dose equivalent—lower than a computed tomography (CT) scan. My son has just had five x rays for a broken leg, and they said my dose is lower than his. After the treatment, I was allowed to go home to my children (although the letter said to stay away from young children for six hours) as the dose was very low (miniscule). They also said that when you receive larger doses you can't fly for a week later, but mine was small so it's no problem.

Please can you reassure me that what they told me is true?

A

The letter you received is correct in that your dose and the dose to those around you are very low, and virtually no precautions are necessary after the procedure.

Patient doses from lymphoscintigraphy vary significantly for a variety of reasons, but the radiation doses to the patient, clinical caregivers, members of the public, and members of the patient's household are low. In an article in the British Journal of Radiology, lymphoscintigraphy doses were calculated to range from 0.024 to 0.204 mSv (Law et al. 2003). People around the patient receive much less than this, since the patient receives a higher dose than people nearby.

It is standard practice at hospitals to suggest means to reduce unnecessary radiation dose, even if it is small. The radioactive material used in this procedure is technetium-99m (99mTc). It has a six-hour half-life, which means that after six hours, a container of 99mTc will contain half as much 99mTc as there was originally. Measured at 12 hours, the amount of 99mTc will be a quarter of the original amount. Of course, since the material is placed into your body, your body excretes a certain amount of the 99mTc through your urine at the same time that the 99mTc also decays away naturally. The result is that the 99mTc in your body is very quickly reduced. The biggest concerns would be the dose to an embryo/fetus (if the patient were pregnant) and to a breast-feeding child.

There are two issues with flying after a nuclear medicine procedure. One is the dose to the person sitting next to the patient, and the other is that many airports employ radiation detection as a part of security screening. For a diagnostic procedure such as lymphoscintigraphy, the dose to the people in neighboring seats is minimal, even for a long flight. (On the other hand, treatment of thyroid disease with radioactive iodine and other radiopharmaceutical therapies can result in higher doses. In these cases, restricting air travel may be necessary to keep doses below dose limits for members of the public.)

Radiation detection equipment used in airport security systems can detect very low levels of radiation. Therefore, even though the doses to the patient and people around the patient are low, airport security systems may detect the radioactivity, and the patient may be detained while it is confirmed that the radiation was due to a legitimate medical procedure. Hospitals may issue letters to patients who travel to help confirm this.

Kent Lambert, CHP, FHPS

Reference

Law M, Cheng KC, Wu PM, Ho WY, Chow LWC. Patient effective dose from sentinel lymph node lymphoscintigraphy in breast cancer: a study using a female humanoid phantom and thermoluminescent dosemeters. British Journal of Radiology 76(911):818–823; 2003.

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