What does this research mean for the field?

A simplified dual-timepoint planar imaging protocol with internal capsule calibration enables reproducible and clinically feasible estimation of lesion-specific effective half-life in patients undergoing I-131 therapy for differentiated thyroid cancer. Novelty: ClaimNovelty.METHODOLOGICAL. Consensus alignment: ConsensusAlignment.NEUTRAL.

What question did this study set out to answer?

The study aims to estimate lesion-specific effective half-life of iodine-131 and assess imaging reliability for DTC dosimetry.

July 9, 2026EJNMMI Physics0 citationsOpen Access

Estimation of effective half-life and lesion-specific kinetics in I-131 therapy for differentiated thyroid cancer using dual-timepoint imaging

YIYusuke IizukaRIKEN Center for Integrative Medical Sciences RNRyota NakashimaKyoto University RARihito AizawaKyoto University

Key Points

The study aims to estimate lesion-specific effective half-life of iodine-131 and assess imaging reliability for DTC dosimetry.
Fifty patients with differentiated thyroid cancer were analyzed through dual-timepoint planar imaging after I-131 administration.
Early and delayed scans were performed at mean intervals of 2.71 days and 12.35 days, respectively.
Lesion-specific effective half-life values were calculated using log-linear approximation of measured activity counts.
Mean effective half-life was 2.16 ± 0.56 days for thyroid bed lesions and 3.11 ± 0.74 days for metastatic lesions (p < 0.001).
Bone and lung metastatic lesions showed numerically similar effective half-life values of 3.22 ± 0.76 and 3.36 ± 0.80 days.
Calibration demonstrated only 5.0% inter-session variability, indicating high quantitative reproducibility.

Abstract

Abstract Background 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. Methods 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. Results 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. Conclusions 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.

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