Optimized 124I PET dosimetry protocol for radioiodine therapy of differentiated thyroid cancer

Iodine kinetics and lesion dose per administered I-131 activity (LDpA) of differentiated thyroid cancer metastases were determined using I-124 PET. These data were analyzed to derive an optimized dosimetry protocol. Methods: We evaluated the time-activity-concentration curves of 37 lesions in 17 patients who had undergone thyroidectomies. LDpA determination involved I-124 PET images acquired at 4, 24, 48, 72, and 96 h after intake of a capsule containing 20-40 MBq of I-124. 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 I-131 therapy: low (<= 5 Gy GBq(-1); n = 14), medium (between 5 and 10 Gy GBq(-1); n = 9), or high LDpAs (>10 Gy GBq(-1); n = 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 (rho(c)) and the mean absolute percentage deviation in LDpAs were used to assess agreement between simplified and reference approaches. Results: The effective I-124 half-life, linear activity-concentration rate (alpha), and 24-h activity concentration (CpA) (the latter 2 per administered I-124 activity) differed significantly among the LDpA groups (P < 0.05). LDpAs correlated with 24-h CpAs (r = 0.94, P < 0.001). Using the 4-, 24-, and 96-h measurements, a rho(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 similar to 24 h. Because of the highly variable kinetics, 1 additional measurement at similar to 96 h was needed to obtain a sufficiently reliable LDpA estimate. The adapted 24-96-h approach appears to be the optimal I-124 protocol and is a reliable simplification of the 5-point protocol.