Scientific ArticleIs Thyroid Screening Necessary for Nuclear Medicine Personnel Receiving and Administering Capsules Containing Large Doses of 131I?
Introduction
Therapy licences issued by the Canadian Nuclear Safety Commission (CNSC) can authorize the administration of large quantities of iodine-131 (131I) to patients for the treatment of thyroid carcinoma. Such licences include conditions that require staff who work with volatile 131I to undergo thyroid screening, which is a self-administered estimation of 131I activity in the thyroid using an instrument that can detect at least 1 kBq of 131I in the gland. The screening measurement must occur within 5 days after the handling or administration of 131I. If more than 10 kBq is detected, the CNSC must be informed and the subject must undergo bioassay within 24 hours. A bioassay measurement is a formal estimation of the 131I activity in the thyroid gland performed by a certified dosimetry service using approved internal dosimetry measurement protocols.
The volatility of radioiodine depends on various physical and chemical properties of the solution and can be dramatically reduced by the addition of buffers, antioxidants, and stabilizers. Such changes reduced the number of positive thyroid burdens (>1 kBq) detected in health care personnel from 12% to 0 [1]. Further safety enhancements can be achieved by encapsulating radioiodine, although packaging materials containing radioiodine capsules can still be contaminated [2]. Direct measurements of the volatility of radioiodine from capsules have been made, and although release of 131I from therapy capsules can be detected, capsules are considered safe for the purpose of handling by nuclear medicine personnel [3].
Typically, in nuclear medicine facilities authorized to administer high-dose outpatient radioiodine therapy, a prescribed therapy dose is ordered as a capsule for each patient. At the McMaster University Medical Centre (MUMC) site of Hamilton Health Sciences, a capsule is received within the department of nuclear medicine by a technologist and is administered to a patient by one of three physicians. Radioiodine within a capsule for ingestion is considered by the CNSC to be volatile 131I, and screening requirements apply.
A CNSC inspection in December 2005 identified that one thyroid screening measurement required according to the regulations had not been performed. In response to this item of noncompliance, we modified our screening protocol to provide greater surveillance of screening records. In addition, we resolved to review the screening data collected during the subsequent 12 months to address the following questions: what is the incidence of positive thyroid radioiodine activities during routine screening, and what is the cost effectiveness of a thyroid screening program? The purpose of this article is to report the outcome of the prospective survey of thyroid screening results recorded over a 1-year period.
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Materials and Methods
The protocol in place for high-dose radioiodine therapy at Hamilton Health Sciences is based on that originally described by Caldwell and Ehrich [4] and approved by the CNSC. Briefly, patients suffering from thyroid carcinoma are given oral doses of radioiodine up to 7.4 GBq. Patients return home after therapy administration. This procedure has been shown to be safe with respect to radiation doses received by caregivers [5].
The MUMC site has a thyroid screening system based on a 5 × 5 cm sodium
Results
The count rate measured from the MUMC quality assessment (QA) phantom during the surveillance period is shown in Figure 1. The rate constant for the single exponential fitted to the data is 0.0001905 d−1, which yields a half-life of 9.96 years. The bias reported from the HML for the Hamilton screening system in January 2006, was +6% based on measurements of a low activity thyroid insert and +12% based on a moderate activity thyroid insert. The corresponding biases reported in January 2007, were
Discussion
The exponential fit to the measurements of the activity of the QA phantom shown in Figure 1 yields a half life (9.96 years), which is compatible with the known half-lives of the constituent radionuclides in the phantom. The predominant isotope present in mock iodine is 133Ba, with a half-life of 10.5 years and a gamma ray emission of 356 kev that is emitted with a probability of 0.61 [6]. The 133Ba simulates the 364 kev emission from 131I that has an emission probability of 0.81. There is a
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