Answer to Question #10518 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues

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

Q

My husband was implanted with 23 125I sealed seeds in his brain.

The initial maximum exposure rate at 1 meter without a lead cap is estimated to be 3 µSv h-1.
 

The initial maximum exposure rate at 1 meter with a lead cap is estimated to be 0 µSv h-1.

It has been a little over a month since his implant. Would it be safe for me to be with him every day, 24 hours a day, if he stops wearing the lead cap? For example, can I sleep next to him every day and be safe?

A

Do you want the good news first, or the bad news? Let me give you the good news—it is safe to live and sleep with your husband. The bad news . . . my explanation involves physics and mathematics.

The half-life of 125I is 60 days which means that after 60 days the amount of radioactivity is half of the original amount. After 120 days, there would be a quarter (1/2 times 1/2) of the original amount. Since it has only been about 30 days, around 70 percent remains. Since the exposure rate is proportional to the amount of radioactive material, the exposure rate now (at one meter without the lead cap) would be about 2 µSv h-1.

Let's assume for a moment that you will be one meter from your husband without the lead cap, 24 hours a day. Your total exposure (until all the radioactive material decays away) would be about 4.15 mSv. 

Of course, it is not realistic to assume that you would be that close all the time, so this is significantly higher than you will actually receive. Usually it is assumed that one-third to one-fourth of the time is spent one meter apart and that the rest of the time is spent two or more meters away. Taking this into consideration, your exposure would be between 1 mSv–1.4 mSv.   

Because the radiation emitted from 125I is not very penetrating, the above exposures would be to your skin; underlying organs would receive lower doses. This means that your actual exposure (we use the term whole-body exposure) would be significantly less than 4.15 mSv. I don't know how much difference this makes, but it is easily a factor of 2 and a factor of 10 would not surprise me. But, since I don't know it off the top of my head, let's not use it other than to recognize that the above is an overestimate of your exposure.

In the United States, the exposure limit for a person exposed to a radiation medicine patient released from the hospital is 5 mSv. Even the worst-case number calculated is below this. In other words, the restrictions are no longer needed to keep your exposure below the limits. 

Kent Lambert, 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.
Answer posted on 5 December 2012. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.