The Result of Multiple I-131 Treatments on the Effective Half-Life of Retained Radioactivity in Patients Ablated for Differentiated Thyroid Cancer: Possible Evidence for Thyroid Remnant Function Impairment.

The ablation of differentiated thyroid cancer by ingested I-131 depends on the activity absorbed by the remnant. This depends on the function of the thyroid cells and on the rate that radioactivity is excreted from the blood. The reduction of radioiodine is described by the effective half-life (EHL), which is the time taken to half the retained radioactivity. If the tumor recurs, more treatments are prescribed, often with escalating activities. Patients may receive several treatments during the evolution of the disease, and the total radioactivity administered (TRA) is the sum of all such activities. The patients' archived information permitted the calculation of EHL and TRA. The patient cohort processed here comprised 274 females and 101 males treated during 1997 to 2015. The TRA to the patients ranged between 1.1 and 129.5 GBq (average = 7.93 ± 9.9 GBq) and the EHL varied between 5.06 and 43.87 hours (average = 14.13 ± 5.7 hours). The data were processed as follows: (a) the EHL corresponding to the last treatment of each patient was plotted against TRA to patients who were treated once and to those treated several times for comparison and (b) using a small subgroup of 16 patients who were treated at least 5 times, the EHL and TRA corresponding to each treatment of each patient were plotted. A function of the form y = p-k·ln(x) was fitted on the data in all graphs and k was calculated. For patients treated once, EHL was independent of TRA. A decrease was seen in (a) multitreated patients, with the gradient (k) ranging between -0.541 and -13.880 and (b) 13 out of 16 patients, with the gradient (k) ranging between -5.55 and -31.17, both indicating an impairment of the remnant function, perhaps identified as "stunning." Since this is not avoidable, the uptake may be boosted by splitting the prescribed activity into low radioactivity fractions, which will also reduce patient hospitalization.