Is the time of TSH normalization after radioiodine treatment for thyroid autonomies related to the maximum dose rate to the target?

Abstract

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Objectives: The response to radioiodine treatment for uni or multifocal autonomy (UA, MA) is unpredictable and factors influencing the response to therapy are unknown. Aim of this study is to explore the correlation between the maximum dose rate to the target nodules and the time of normalization of TSH.

Methods: We audited clinical outcome in 125 pts.(f:90 m:35; mean age 66±11 yrs) treated with 131I over the last six years (mean activity 372±134 MBq). UA (41%) and MA (59%) were defined by the existence of one or more hot nodules at scintigraphy in presence of overt or subclinical hyperthyroidism. The maximum dose rate (Gy/h) and the total delivered dose to the nodules were calculated through multiple uptake determinations on γ-camera images; in multifocal autonomies only the dominant nodule was considered.

Results: On the basis of the maximum dose rate values the patients were divided into two groups, superior (G1) and inferior (G2) to the median value which was 0.66 Gy/h. In G1 (mean maximum rate 0.90 Gy/h) the time of normalization of TSH was 2.5 months, while in G2 (mean maximum rate 0.50 Gy/h) it was 7.8 months (p<0.01). No correlation (r=0.60) was found between maximum dose rate and the total delivered dose (mean value 165±48 Gy).

Conclusions: Our data suggest that the maximum dose rate to the target nodule is strictly related to the time of normalization of TSH value. The maximum dose rate could be a new parameter to consider in therapy planning.