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

Iodine kinetics and lesion dose per administered 131I activity (LDpA) of differentiated thyroid cancer metastases were determined using 124I PET. These data were analyzed to derive an optimized dosimetry protocol.

We evaluated the time-activity-concentration curves of 37 lesions in 17 patients who had undergone thyroidectomies. LDpA determination involved 124I PET images acquired at 4, 24, 48, 72, and 96 h after intake of a capsule containing 20-40 MBq of 124I. 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 131I therapy: low (< or =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.

The effective 124I half-life, linear activity-concentration rate (alpha), and 24-h activity concentration (CpA) (the latter 2 per administered 124I 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.

Lesion classification into LDpA groups was feasible using a single PET scan at approximately 24 h. Because of the highly variable kinetics, 1 additional measurement at approximately 96 h was needed to obtain a sufficiently reliable LDpA estimate. The adapted 24-96-h approach appears to be the optimal 124I protocol and is a reliable simplification of the 5-point protocol.