Is Thyroid Screening Necessary for Nuclear Medicine Personnel Receiving and Administering Capsules Containing Large Doses of ¹³¹I?

Abstract

The cost-effectiveness of a thyroid screening procedure for the detection of internal contamination of personnel working with large doses of encapsulated iodine-131 (¹³¹I) was examined by reviewing the results of screening measurements performed during 1 year. Thyroid burdens were measured by self-assessment using a sodium iodide (NaI) probe. Of 113 screening measurements, 101 indicated a negative thyroid content of ¹³¹I. The largest burden observed was statistically not different from zero. The count rate corresponding to the largest observed burden would represent an ¹³¹I content of 23 Bq, which is approximately 0.2% of the level at which results have to be reported to the Canadian Nuclear Safety Commission. Negative count rates arose because of a small overestimation of the background contribution that was measured with a neck phantom positioned in front of the thyroid probe. The cost of a thyroid screening program for personnel who handle encapsulated ¹³¹I is considerable, especially when the need for such measurements is questionable.

Résumé

Les chercheurs ont examiné le rapport coût-efficacité du dépistage thyroïdien pour la détection de la contamination interne du personnel travaillant avec des doses élevées deI en gélules en étudiant les résultats de mesures de dépistage prises sur une période d'un an. Les charges thyroïdiennes ont été mesurées par autoévaluation au moyen d'une sonde NaI. Des 113 mesures de dépistage, 101 ont indiqué un contenu thyroïdien négatif de I. La charge observée la plus importante n'était pas significativement différente de zéro. Le taux de comptage de la charge la plus élevée observée représenterait un contenu I de 23 Bq, ce qui correspond à environ 0,2 % du seuil à partir duquel les résultats doivent être signalés à la Commission canadienne de sûreté nucléaire (CCSN). Les taux de comptage négatifs ont augmenté en raison d'une légère surestimation de la contribution de l'arrière-plan, mesurée avec un fantôme de cou placé devant la sonde thyroïdienne. Le coût d'un programme de dépistage thyroïdien pour le personnel qui manipule du I en gélules est considérable, notamment lorsque la nécessité de telles mesures est sujette à caution.

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.