Determining thyroid I-131 effective half-life for the treatment planning of Graves' disease

Abstract

Purpose: Thyroid I-131 effective half-life (T-eff) is an essential parameter in patient therapy when accurate radiation dose is desirable for producing an intended therapeutic outcome. Multiple I-131 uptake measurements and resources from patients themselves and from nuclear medicine facilities are requisites for determining T-eff, these being limiting factors when implementing the treatment planning of Graves' disease (GD) in radionuclide therapy. With the aim of optimizing this process, this study presents a practical, propitious, and accurate method of determining T-eff for dosimetric purposes. Methods: A total of 50 patients with GD were included in this prospective study. Thyroidal I-131 uptake was measured at 2-h, 6-h, 24-h, 48-h, 96-h, and 220-h postradioiodine administration. T-eff was calculated by considering sets of two measured points (24-48-h, 24-96-h, and 24-220-h), sets of three (24-48-96-h, 24-48-220-h, and 24-96-220-h), and sets of four (24-48-96-220-h). Results: When considering all the measured points, the representative T-eff for all the patients was 6.95 (+/- 0.81) days, whereas when using such sets of points as (24-220-h), (24-96-220-h), and (24-48-220-h), this was 6.85 (+/- 0.81), 6.90 (+/- 0.81), and 6.95 (+/- 0.81) days, respectively. According to the mean deviations 2.2 (+/- 2.4)%, 2.1 (+/- 2.0)%, and 0.04 (+/- 0.09)% found in T-eff, calculated based on all the measured points in time, and with methods using the (24-220-h), (24-48-220-h), and (24-96-220-h) sets, respectively, no meaningful statistical difference was noted among the three methods (p > 0.500, t test). Conclusions: T-eff obtained from only two thyroid I-131 uptakes measured at 24-h and 220-h, besides proving to be sufficient, accurate enough, and easily applicable, attributes additional major cost-benefits for patients, and facilitates the application of the method for dosimetric purposes in the treatment planning of Graves' disease. (c) 2013 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4788660]