Objective:The present study was to investigate salivary iodine kinetics and dosimetry in repeated courses of radioiodine (131I) therapy for diffcrentiated thyroid cancer (DTC) for better understanding the mechanisms of 131I induced salivary toxicity and helping to find the appropriate ways of reducing this injury.Methods: Seventy-eight consecutive DTC patients (mean age±SD, 45±17; 60%, women)undergoing 131I therapy for remnant ablation or metastatic tumors were prospectively recruited.Planar quantitative scintigraphy of head-neck images was serially acquired after administration of 2.9-7.4 GBq of 131I to assess kinetics in the salivary glands of patients.Salivary absorbed doses were calculated based on the schema of the Medical Internal Radiation Dosimetry.Results: The maximum uptakes in percentage of administered 131I activity per kilogram of gland tissue (%/kg), were 12.9±6.5 %/ks (range, 0.43-37.3 %/kg) and 12.3±6.2 %/kg (range,0.4-35.1%/kg) for the parotid and submandibular glands, respectively.Statistically significant correlations of maximum uptake versus cumulative activity (r =-0.74, P < 0.01, for the parotid glands; r =-0.71, P < 0.01, for the submandibular glands), and treatment cycle (P < 0.001, for both gland types) were found.The effective half-lives of 131I in the parotid and submandibular glands were 9.3±3.5 h (range, 1.5-19.8 h), and 8.6±3.2 h (range, 0.8-18.0 h), respectively.Only for the parotid glands, a statistically significant correlation was observed between effective half-life with cumulative activity (r =0.37, P < 0.01), and treatment cycle (P=0.03).The calculated absorbed doses were 0.18±0.06 mGy/MBq (range, 0.01-0.86 mGy/MBq), and 0.23±0.08 mGy/MBq (range, 0.01-1.44 mGy/MBq) for the parotid and submandibular glands,respectively.Conclusions: The iodine uptake of salivary glands is continuously reduced in the courses of therapy.The short period of time of iodine uptake suggests that a short-lasting, reversible,blockade of the uptake process may likely reduce salivary absorbed doses.The phenomenon of hyper-radiosensitivity may to some extent account for the occurrence of salivary gland hypofunction at very low radiation doses with low dose rates in 131I therapy.124I voxel-based dosimetry is warranted to further investigate the 131I induced salivary gland toxicity.