Influence of dose rate on therapeutic success of radioiodine therapy of functional thyroid autonomy.

OBJECTIVE

To evaluate the effect of dose rate on the therapeutic success of radioiodine therapy (RIT) by retrospective analysis of two groups of patients with different dose rates during RIT.

METHODS

The average dose rate until deposition of 50% of the total radiation dose achieved (the DL50 value) was calculated. Ninety patients with autonomy of the thyroid receiving radiation doses between 300 and 400 Gy were separated in groups with high DL50 values (the DL50 H group) and low DL50 values (the DL50L group). Before and 4 months after RIT free triiodothyronine (FT3), free thyroxine (FT4), basal thyroid stimulating hormone (TSHB), stimulated TSH (TSHS), thyroidal technetium uptake (TcTU), and the existence of focal abnormalities in thyroid scintigraphy were evaluated.

RESULTS

The DL50 H and DL50L groups showed no differences before RIT in the parameters assessed as well as in total achieved dose and autonomous thyroid volume. After therapy a significant difference was found in TSHB with higher values in DL50 H (1.89+/-1.49 vs. 1.31+/-1.48 mU . l, P<0.05), FT4 showed a trend to lower values (13.8+/-4.0 vs. 16.1+/-6.2 pmol . l, P=0.09).

CONCLUSION

The DL50 value has an influence on therapeutic success of RIT. This influence is only discrete and not likely to produce clinically relevant effects in the practical use of RIT.