Iodine kinetics and dosimetry in the salivary glands during repeated courses of radioiodine therapy for thyroid cancer

Purpose: The present study was conducted to investigate salivary iodine kinetics and dosimetry during repeated courses of radioiodine (I-131) therapy for differentiated thyroid cancer (DTC). Such data could provide a better understanding of the mechanisms of I-131 induced salivary toxicity and help to develop appropriate methods to reduce this injury. Methods: Seventy-eight consecutive DTC patients (mean age 45 +/- 17 years, 60%, female) undergoing I-131 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 I-131 to assess kinetics in the salivary glands of patients. Salivary absorbed doses were calculated based on the schema of Medical Internal Radiation Dosimetry. Results: The maximum uptakes in percentage of administered I-131 activity per kilogram of gland tissue (%/kg) were 12.9%+/- 6.5%/kg (range, 0.4%-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 I-131 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. 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) only for the parotid glands. The calculated absorbed doses were 0.20 +/- 0.10 mGy/MBq (range, 0.01-0.92 mGy/MBq) and 0.25 +/- 0.09 mGy/MBq (range, 0.01-1.52 mGy/MBq) for the parotid and submandibular glands, respectively. The photon contribution to the salivary absorbed dose was minimal in relation to the beta dose contribution. Photon-absorbed dose fractions of total absorbed dose were 4.9%+/- 1.3% (range, 1.1%-8.7%) and 3.7%+/- 2.5% (range, 0.8%-7.9%) for the parotid and submandibular glands, respectively. Conclusions: The iodine uptake of salivary glands is continuously reduced during the courses of therapy. 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 I-131 therapy. On the other hand, failure to incorporate a nonuniform and preferential uptake by salivary gland ductal cells may result in underestimating the actual dose for the critical tissue. Other methods, including I-124 voxel-based dosimetry, are warranted to further investigate the I-131-induced salivary gland toxicity. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3602459]