engleski

Hold the Threshold

Some existing data may provide indications for a revaluation of dosimetric approach to successful ablation of residual thyroid tissue in postsurgical patients. The aim of this study was to investigate the success rate of thyroid remnant ablation in relation to the radiation absorbed dose and dose-rate of therapeutic radioiodine-131 (I-131). Forty patients after total thyroidectomy for thyroid cancer were included in the study. A diagnostic and therapeutic activity of 70-200 MBq I-131 and 1.8-4.5 GBq I-131, respectively, was given orally after about 4 and 5 postsurgical weeks of waiting period. The initial absorbed dose-rate and absorbed dose of I-131 were determined then at the basis of serial measurements of I-131 uptake in remnant thyroid during the first week post-administration. Successful ablation after the first administration of therapeutic I-131 was achieved in 35 out of 40 patients. The initial absorbed dose-rate of therapeutic I-131 appeared, at least, as important as its absorbed dose since all patients with thyroid remnants exposed to >3.5 Gy/h were successfully ablated (p<0.01) as well as all patients who received >420 Gy (p=n.s.). However, ablation success rate was significantly higher (p<0.05) in patients whose thyroid remnants received the absorbed dose of >100 Gy delivered at the initial dose-rate of >1.6 Gy/h. We were not able to confirm the most widely accepted threshold of 300 Gy needed for successful ablation of residual thyroid tissue. Instead of this, we recommend following double criterion for successful thyroid remnant ablation, i.e. to achieve a certain value of the initial absorbed dose-rate together with certain absorbed dose of therapeutic I-131. For the sake of redundancy and future confirmation the latter values may be set to 2 Gy/h and 200 Gy/h, i.e. two thirds of the current thresholds.

thyroid cancer; radioiodine therapy; dosimetry

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