Pretherapeutic quantitative I-131 SPECT/CT for blood and tumour dosimetry in thyroid cancer patients with bone metastases: a pilot feasibility study

I-131 radioactive iodine (RAI) is widely used in thyroid cancer patients with bone metastases (BM). While blood-based dosimetry has been standard for predicting marrow toxicity, tumour dosimetry remains an unmet need. We evaluated the feasibility of pretherapeutic quantitative I-131 SPECT/CT for patient-tailored dosimetry in poorly differentiated thyroid carcinoma (PDTC) patients with BM. Five PDTC patients (18 BM lesions) underwent serial pre-therapeutic I-131 SPECT/CT (111 MBq, eight time points up to 120 h) with concurrent blood sampling. I-131 concentrations (MBq/g) were quantified using quantitative SPECT/CT technology and validated against gamma counting. Absorbed dose rates (Gy/h) for blood and tumours were estimated using Monte Carlo simulations (customized Geant4, cross-validated by TOPAS). Tumour absorbed doses were calculated via trapezoidal integration, and clinical outcomes were assessed after subsequent high-dose RAI therapy. Blood I-131 concentrations measured by SPECT/CT showed excellent agreement with gamma counting ( p < 0.001). Blood absorbed doses per administered activity were comparable between SPECT/CT and conventional methods ( p = 0.6250). Tumour I-131 concentration strongly correlated with absorbed dose rate, resulting in a regression equation [tumour absorbed dose rate (Gy/h) = 0.1256 × tumour RAI concentration (MBq/g) (R 2 = 0.9918, p < 0.0001)]. Reducing imaging time points from 8 to 4 maintained the accuracy of absorbed dose estimation. Although exploratory, tumours with expected absorbed doses > 50 Gy after high-dose RAI showed no progression, whereas lower-dose lesions progressed more frequently (87.5%, p = 0.0161). In this pilot study, quantitative I-131 SPECT/CT demonstrated the reliability of pretherapeutic blood and tumour dosimetry in PDTC patients with BM. Although preliminary, these findings support its potential as a practical tool for personalized RAI therapy planning in thyroid cancer. Larger, prospective studies are warranted to validate clinical dose–response thresholds.