Estimation of effective half-life and lesion-specific kinetics in I-131 therapy for differentiated thyroid cancer...

Accurate estimation of lesion-specific effective half-life (T_eff) is essential for patient-specific dosimetry in radioiodine (I-131) therapy for differentiated thyroid cancer (DTC). Although conventional multi-timepoint imaging provides detailed kinetic information, it is often impractical for routine clinical implementation. This study prospectively evaluated a simplified dual-timepoint planar imaging protocol aimed at balancing quantitative reliability with clinical feasibility.

Fifty patients with DTC undergoing I-131 therapy were prospectively analyzed. Whole-body planar images were acquired twice after administration: an early scan at 1.98-3.25 days (mean, 2.71 days) and a delayed scan at 8.97-17.16 days (interval between the two scans ranged from 6.18 to 14.28 days [mean, 12.35 days]). Lesion-specific T_eff values were calculated assuming mono-exponential clearance between the two scans using a log-linear approximation of measured activity counts. Quantitative reproducibility was assessed by comparing activity-count calibration slopes derived from an internal reference I-131 capsule between independent imaging sessions.

The mean T_eff was 2.16 ± 0.56 days and 3.11 ± 0.74 days for thyroid bed (n = 35) and metastatic (n = 19; p < 0.001) lesions, respectively. Among metastatic sites, bone and lung metastases showed numerically similar T_eff values (3.22 ± 0.76 and 3.36 ± 0.80 days, respectively); however, this subgroup comparison was limited by the small sample size. Calibration slopes derived from the internal reference capsule demonstrated only 5.0% inter-session variability, indicating high quantitative reproducibility.

Dual-timepoint planar imaging incorporating internal capsule calibration enables reproducible estimation of lesion-specific T_eff in patients undergoing I-131 therapy for DTC. This approach achieves a practical balance between quantitative accuracy and clinical feasibility, enabling reliable characterization of iodine kinetics with minimal imaging burden and supporting individualized dosimetry in routine clinical practice.