Analysis of activity uptake, effective half-life and time-integrated activity for low- and high-risk papillary thyroid cancer patients treated with 1.11 GBq and 3.7 GBq of 131I-NaI respectively

Purpose: To analyse the activity uptakes, effective half-lives and time-integrated activities, of relevance for remnant dosimetry, for patients treated for papillary thyroid cancer (PTC) with a different amount of activity of I-131-NaI. Methods: Fifty patients were included. Of those, 18 patients had low-risk PTC and were treated with 1.11 GBq of I-131-NaI (Group 1), and 32 patients had high-risk PTC and were treated with 3.7 GBq (Group 2). Radioiodine was administered after total thyroidectomy and rhTSH stimulation. Two SPECT/CT scans were performed for each patient to determine the remnant activities and effective half-lives. Results: Significantly higher values (p < 0.05) were obtained for Group 1 for the remnant activity at 7 d (medians 1.4 MBq vs 0.27 MBq), the remnant activity per administered activity at 2 d (0.35% vs 0.09%) and at 7 d (0.13% vs 0.007%), and the effective half-life (93 h vs 40 h). Likewise, the time-integrated activity coefficient was significantly higher for Group 1. The time-integrated activity did not differ significantly between the two groups (p > 0.05). Conclusions: We found a significant difference in the remnant activity per administered activity, the rate of washout from thyroid remnants, and the time-integrated activity coefficient between low-risk PTC patients treated with 1.11 GBq and high-risk PTC patients treated with 3.7 GBq. On the contrary, there was no such difference in the time-integrated activity. If remnant masses were also not statistically different (reasonable assumption for this monocentric study) no difference in time-integrated activity would imply no difference in remnant absorbed dose, of relevance for treatment efficacy and the risks of stochastic effects.