D. Huic et al., RADIOIODINE UPTAKE IN THYROID-CANCER PATIENTS AFTER DIAGNOSTIC APPLICATION OF LOW-DOSE I-131, Nuclear medicine communications, 17(10), 1996, pp. 839-842
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.