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

Abstract

The present study was conducted to investigate salivary iodine kinetics and dosimetry during repeated courses of radioiodine ((131)I) therapy for differentiated thyroid cancer (DTC). Such data could provide a better understanding of the mechanisms of (131)I induced salivary toxicity and help to develop appropriate methods to reduce this injury. Seventy-eight consecutive DTC patients (mean age 45 ± 17 years, 60%, female) undergoing (131)I 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 (131)I to assess kinetics in the salivary glands of patients. Salivary absorbed doses were calculated based on the schema of Medical Internal Radiation Dosimetry. The maximum uptakes in percentage of administered (131)I 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 (131)I 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. 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 (131)I 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 (124)I voxel-based dosimetry, are warranted to further investigate the (131)I-induced salivary gland toxicity.