Whole-Body Radioiodine Effective Half-Life in Patients with Differentiated Thyroid Cancer

Background: Radioactive I-131 (RAI) therapy is used in patients with differentiated thyroid cancer (DTC) after total thyroidectomy for remnant ablation, adjuvant treatment or treatment of persistent disease. I-131 retention data, which are used to indicate the time at which a I-131 treated DTC patient can be released from the hospital, may bring some insights regarding clinical factors that prolong the length of hospitalization. The aim of this study was to investigate the I-131 whole-body retention in DTC patients during I-131 therapy. Methods: We monitored 166 DTC patients to follow the I-131 whole-body retention during I-131 therapy with a radioactivity detector fixed on the ceiling of each protected room. A linear regression fit permitted us to estimate the whole-body I-131 effective half-life in each patient, and a relationship was sought between patients' clinical characteristics and whole-body effective I-131 half-life. Results: The effective I-131 half-life ranged from 4.08 to 56.4 h. At multivariable analysis, longer effective I-131 half-life was related to older age and extensive extra-thyroid disease. Conclusions: I-131 effective half-life during I-131 treatment in DTC patients is highly variable among patients and is significantly longer in older and in patients with RAI uptake in large thyroid remnants or in extrathyroidal disease that significantly prolongs the whole-body retention of I-131.