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

Thyroid (131)I 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 (131)I 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.

A total of 50 patients with GD were included in this prospective study. Thyroidal (131)I 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).

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).

T(eff) obtained from only two thyroid (131)I 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.