Answer to Question #8224 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Therapeutic Nuclear Medicine

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

Q

Can a patient be given 3,700 MBq of 131I and released (with written instructions) if the patient did NOT have a total thyroidectomy? (This patient, because of health reasons, cannot have the surgery and our facility just doesn't have the room to isolate the patient. This patient lives alone.) Do I just need to recompute the patient-specific calculations and modify the release criteria? I can assume the thyroidal fraction is about 20% and can give instructions to increase the time the patient needs to stay away from others from two to maybe seven days.

A

It's not that uncommon for patients with intact thyroids to be treated with 131I when they are not good candidates for standard surgery. However, sometimes they are given a small initial dose to radiologically reduce the gland and then they are given a second definitive dose to complete the process. In these cases, you should be aware that the initial dose may cause significant swelling in the neck due to the large thyroidal mass present and that may be a complication from the treatment.

As for the patient-specific calculations, since you say that the patient lives alone, this should not be a problem. Using the method in the Nuclear Regulatory Commission's (NRC) NUREG-1492, Regulatory Analysis on Criteria for the Release of Patients Administered Radioactive Material, February 1997, you could assess the exposure to others in this manner:

Assume an initial uptake of 740 MBq in the thyroid and 2,960 MBq as the extra-thyroidal fraction. From Table 4.6 of the referenced document, and using the factor for a 30% uptake, the thyroidal component would have a biological half-life of 65 days and the extra-thyroidal component would have a biological half-life of 0.33 days. This would yield an effective half-life of 7.31 days and 0.32 days respectively for the thyroidal and extra-thyroidal fractions, assuming that the patient has normal renal function and is instructed to stay well hydrated.

Consequently, the exposure at 1 meter from the thyroidal component until total decay/excretion would be:

1.44 x 24 hr/day x 4.8 x 10-5 mSv-m2/MBq-hr x 740 MBq x 7.31 day/(1 m2) = 10.4 mSv.

The exposure at 1 meter from the extra-thyroidal component until total decay/excretion would be:

1.44 x 24 hr/day x 4.8 x 10-5 mSv-m2/MBq-hr x 2960 MBq x 0.32 day/(1 m2) = 1.8 mSv

Thus, if a person was at 1 meter away for 24 hours/day, until all of the 131I was gone, his total exposure would be 12.2 mSv. NRC limits exposure to family, friends, and the public to 5 mSv/yr. So, if you limit contact (at 1 meter) to no more than 25% per day (six hours), the estimated exposure would be 3.1 mSv. Since, the patient lives alone, this should be easily accomplished without any other special requirements.  

If the patient were to return to work where he would be in reasonably close contact with others, you might recommend that he get the treatment on a Friday so he could stay at home over the weekend. By that time, most of the extra-thyroidal component will have been eliminated through excretion and contamination issues would be minimized. Assuming 60 hours (2.5 days) have elapsed between the administration time and the patient's return to work, the thyroidal component would have been reduced to 585 MBq at the start of the week and down to 363 MBq by the end of the week. If we calculate the exposure at 1 meter for both of these values and subtract the Friday value from the Monday value we can estimate the weekly exposure for the first week:

8.2 mSv - 5.1 mSv = 3.1 mSv

Now, if we simplify this a bit and assume a thyroidal effective half-life rounded to seven days, the total cumulative exposure at 1 meter from the work week would approximate a series of 8.2 mSv + 1.6 mSv + 0.8 mSv + 0.4 mSv ..., for a total of approximately 6.3 mSv. Using an occupancy factor of 1/3 (eight hours/day), a maximally exposed coworker at 1 meter during the entire workday would be estimated to receive an exposure of 2 mSv.  

In reality, the exposures are smaller due to the NRC's conservatism of using a point-source model, which doesn't account for self-attenuation by the patient's anatomy. The only special instructions you would need to give the patient is to limit his/her time close to others for the first three or four weeks due to the larger than normal uptake in the thyroidal tissues, for this class of patients.

Mike Bohan, Radiation Safety Officer

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