Assessment of Simplified Blood Dose Protocols for the Estimation of the Maximum Tolerable Activity in Thyroid Cancer Patients Undergoing Radioiodine Therapy Using ¹²⁴I

Abstract

In high-activity radioiodine therapies for differentiated thyroid cancer, blood dosimetry has been developed to estimate the maximum tolerable activity (MTA) of 131 I that can be safely administered without leading to toxic effects. The reference protocol involves a series of both blood sampling (BS) and whole-body counting (WC) over a period of several days. The aim of this retrospective study was to identify simplified protocols without an appreciable loss of accuracy. Methods: Data from 211 thyroid cancer patients who received 124 I blood dosimetries were retrospectively analyzed. BS and WC acquired at approximately 1-2, 4, 24, 48, and 96 h or longer after 124 I administration were included. This dataset was used to determine the reference MTA and estimations based on a reduced number of combined data from BS and respective WC. MTA estimates were also determined on the basis of either BS or WC alone using some simplifying assumptions in the dosimetry approach. A simplified protocol was considered equivalent to the reference protocol if the estimates of 95% of the MTAs were within the ±20% range and the absolute maximum percentage deviation did not exceed the limit of 30% in a few cases. Lin's concordance correlation analysis was applied to assess the protocol's agreements. Results: Two equivalent protocols were identified that included both BS and respective WC acquired at only 3 time points (1-2, 24 or 48, and $96 h). Further equivalent protocols with only 3 blood samples drawn at similar time points were discovered for patients, who had undergone at least 1 radioiodine therapy. For all equivalent protocols, deviations of the mean absolute percentage MTA were below 9% and Lin's concordance correlation coefficients of 0.95 or greater were found, indicating almost excellent agreement (according to Partik's criteria). Conclusion: The pretherapy blood dosimetry protocol can be substantially shortened and may be beneficial to patients and patient management while reducing the radiation exposure to medical staff.

Fixed 3.7-GBq 131I Activity for Metastatic Thyroid Cancer Therapy Ignores Science and History

“…The individual calculated MTAs at MSKCC were not reported (1), but the AAs were lower than in our experience (4). This raises concern that AAs differ from calculated MTAs.…”

124I PET Assessment of Response of Bone Metastases to Initial Radioiodine Treatment of Differentiated Thyroid Cancer

“…In contrast, we perform an individual pretherapy dosimetry for high-risk patients using the positronemitting tracer 124 I-NaI ( 124 I). In this dosimetry approach, the lesion-absorbed radiation dose (AD) per unit of 131 I activity is estimated using 124 I PET/CT and the maximum tolerable activity is derived from blood sample and whole-body retention data (3,4). The maximum tolerable activity is defined as the maximum therapeutic activity that can be administered without producing severe bone marrow or lung toxicities ($grade 3 according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 4.0).…”

The role of iodine-124 positron emission tomography in molecular imaging

“…It helps to measure (average) absorbed doses of 131 I to organs of interest, enabling quantitative dose-response studies, dose planning before therapy, dose evaluation during therapy and further investigations of the ''stunning'' phenomenon [27][28][29][30][31]. On the basis of a theoretical dosimetry based model using 124 I, Jentzen et al, showed that the lowest effective therapy activity required for thyroid remnant ablation in low risk differentiated thyroid cancer appears to be approximately 2.2 GBq [32].…”