RADIOIODINE UPTAKE IN THYROID-CANCER PATIENTS AFTER DIAGNOSTIC APPLICATION OF LOW-DOSE I-131

The aim of this study was to investigate the influence of the diagnost ic administration of 74 MBq I-131 on subsequent uptake of therapeutic radioiodine in thyroid cancer patients. Retention measurements were pe rformed using a whole-body counter in 24 patients 6 weeks after total thyroidectomy. Profile scans were performed 2, 24, 48 and 72 h after t he administration of the diagnostic dose and 72 h after the administra tion of the ablation-therapeutic dose (4.4 GBq). The mean (+/-S.D.) ef fective half-life of the diagnostic dose in thyroid remnants was 40.3 +/- 23.0 h. The uptake in the thyroid remnants of the subsequent ablat ion dose 72 h after administration was 30.4 +/- 19.8% of that predicte d from the diagnostic study. The greater reduction in uptake was assoc iated with the longer half-life of iodine and higher uptake in the thy roid remnants at 24 h, with a longer interval between surgery and admi nistration of the diagnostic dose and a shorter period between adminis tration of the diagnostic and ablation doses. Our results show that a diagnostic dose of 74 MBq I-131 markedly reduces thyroid uptake of an ablation dose of I-131. This should be taken into account during radia tion dose planning whenever a quantitative dosimetric study is to be p erformed.