Pre-therapeutic ¹²⁴I PET(/CT) dosimetry confirms low average absorbed doses per administered ¹³¹I activity to the salivary glands in radioiodine therapy of differentiated thyroid cancer

Purpose: Salivary gland impairment following high activity radioiodine therapy of differentiated thyroid cancer (DTC) is a severe side effect. Dosimetric calculations using planar gamma camera scintigraphy (GCS) with ¹³¹I and ultrasonography (US) provided evidence that the average organ dose per administered ¹³¹I activity (ODpA) is too low to account for observed radiation damages to the salivary glands. The objective of this work was to re-estimate the ODpA using ¹²⁴I PET(/CT) as a more reliable approach than ¹³¹I GCS/US. Methods: Ten DTC patients underwent a series of six (or seven) PET scans and one PET/CT scan after administration of ∼23 MBq ¹²⁴I-iodide. Volumes of interest (VOIs) drawn on the CT and serial PET images were used to determine the glandular volumes and the imaged ¹²⁴I activities. To enable identical VOIs to be drawn on serial PET images, each PET was co-registered with the CT image. To correct for partial volume effect and for the artificial bias in the activity concentration due to cascading gamma coincidences occurring in ¹²⁴I decay, the imaged activity was effectively corrected using isovolume recovery coefficients (RCs) based on recovery phantom measurements. A head-neck phantom, which contained ¹²⁴I-filled spheres, was manufactured to validate the isovolume recovery correction method with a realistic patient-based phantom geometry and for a range of activity concentration regimes. The mean±standard deviation (range) ODpA projected for ¹³¹I was calculated using the absorbed dose fraction method. Results: The ODpAs (in Gy/GBq) for the submandibular and parotid glands were 0.32±0.13 (0.18-0.55) and 0.31±0.10 (0.13-0.46), respectively. No significant differences (p>0.2) in the mean ODpA between ¹²⁴I PET(/CT) and ¹³¹I GCS/US dosimetry was found. The validation experiment showed that the percentage deviations between RC-corrected and true activity concentrations were <10%. Conclusion: ¹²⁴I PET(/CT) dosimetry also corroborates the low ODpAs to the salivary glands. A voxel-based calculation taking into account the nonuniform activity distributions in the glands is necessary to possibly explain the radiation-induced salivary gland damage.