Iodine kinetics and lesion dose per administered 131 I activity (LDpA) of differentiated thyroid cancer metastases were determined using 124 I PET. These data were analyzed to derive an optimized dosimetry protocol. Methods: We evaluated the timeactivity-concentration curves of 37 lesions in 17 patients who had undergone thyroidectomies. LDpA determination involved 124 I PET images acquired at 4, 24, 48, 72, and 96 h after intake of a capsule containing 20-40 MBq of 124 I. A combination of a linear and a monoexponential or a monoexponential function only parameterized the time-activity-concentration curves. The LDpAs, calculated using data from all 5 PET time points, served as reference. The lesions were classified into 3 groups, according to potential for cure with 131 I therapy: low (#5 Gy GBq 21 ; n 5 14), medium (between 5 and 10 Gy GBq 21 ; n 5 9), or high LDpAs (.10 Gy GBq 21 ; n 5 14). Using the reference approach, the differences in the empiric kinetic parameters within the LDpA groups were evaluated. The reference LDpAs were compared with those derived from only 2, 3, or 4 PET data points and from 1 adapted 2-point approach. Lin's concordance correlation coefficient (r c ) and the mean absolute percentage deviation in LDpAs were used to assess agreement between simplified and reference approaches. Results: The effective 124 I half-life, linear activity-concentration rate (a), and 24-h activity concentration (CpA) (the latter 2 per administered 124 I activity) differed significantly among the LDpA groups (P , 0.05). LDpAs correlated with 24-h CpAs (r 5 0.94, P , 0.001). Using the 4-, 24-, and 96-h measurements, a r c value of greater than or equal to 0.90 was found, and the mean absolute percentage deviation was less than or equal to 16%. Similar statistical values were obtained for the adapted approach, which was based on 24-and 96-h PET data points only. Conclusion: Lesion classification into LDpA groups was feasible using a single PET scan at ;24 h. Because of the highly variable kinetics, 1 additional measurement at ;96 h was needed to obtain a sufficiently reliable LDpA estimate. The adapted 24-96-h approach appears to be the optimal 124 I protocol and is a reliable simplification of the 5-point protocol.Key Words: PET; radionuclide therapy; dosimetry; iodine-124; differentiated thyroid cancer Several groups (1-5) have successfully used 124 I PET alone and with CT to guide postsurgical treatment and, in particular, radioiodine therapy in patients with differentiated thyroid carcinoma (DTC). Preradiotherapeutic 124 I PET is valuable in restaging disease; dosimetric analysis of the imaging data can identify the patients most likely to benefit from radioiodine therapy and can determine safe and effective individualized therapeutic activities of radioiodine.Our original ;20, and ;44 h after 124 I administration; sometimes a fourth scan was acquired at 96-144 h. However, to our knowledge, no group has yet published the results of any systematic investigation to determine the number and timing of PET sca...

The 124I PET(/CT) salivary gland dosimetry indicated that lemon juice stimulation shortly after 131I administration in radioiodine therapy increases the absorbed doses to the salivary glands.

The iodine uptake of salivary glands is significantly reduced, whereas the absorbed dose per administered (131)I activity was not significantly decreased during the course of therapy. Comparing the well-known dose-effect relationships in external radiation therapy, the absorbed dose per administered (131)I activity is too low to induce comparable radiation damage, suggesting an inhomogeneous distribution of (131)I in human salivary glands.