Purpose
The residence time of 131I in the blood is likely to be a measure of the amount of 131I that is available for uptake by thyroid remnant tissue and thus the radiation absorbed dose to the target tissue in 131I ablation of patients with differentiated thyroid cancer (DTC). This hypothesis was tested in an investigation on the dependence of the success rate of radioiodine remnant ablation on the radiation absorbed dose to the blood (BD) as a surrogate for the amount of 131I available for iodine-avid tissue uptake.
Methods
This retrospective study included 449 DTC patients who received post-operative 131I ablation in our centre in the period from 1993 to 2007 and who returned to us for diagnostic whole-body scintigraphy. The BD was calculated based on external dose rate measurements using gamma probes positioned in the ceiling. Success of ablation was defined as a negative diagnostic 131I whole-body scan and undetectable thyroglobulin levels at 6 months follow-up.
Results
Ablation was successful in 56.6% of the patients. The rate of successful ablation correlated significantly with BD but not with the administered activity. Patients with blood doses exceeding 350 mGy (n = 144) had a significantly higher probability for successful ablation (63.9%) than the others (n = 305, ablation rate 53.1%, p = 0.03). In contrast, no significant dependence of the ablation rate on the administered activity was observed.
Conclusion
The BD is a more powerful predictor of ablation success than the administered activity.