[Is a dosage calculation for radio-iodine therapy possible?]

When trying a single-dose 131I therapy in thyrotoxic patients a dose calculation must be done to avoid an overtreatment raising the risk of posttherapeutic hypothyroidism. The calculation is based on the diagnostic 131I uptake, the thyroid weight, and the effective intrathyroidal halflife of the diagnostic 131I dose. In 46 thyrotoxic patients the radiation dose was calculated with 7000 R for small and 10 000 R for large (50--100 g) glands using the formula of Billion. Iodine-131 was given in a single dose. Therapeutic dose uptake was measured every day and the effective T 1/2 calculated using a least squares computer fitting to an monoexponential decrease. The radiation dose was then recalculated using the same formula. Small glands received 10 700 +/- 4300 R (SEM 1008 R) instead of calculated 7000 R; large glands 8600 +/- 4100 R (SEM 860) instead of 10 000 R. There was a significant correlation: the smaller the gland the higher the radiation dose. The smallest glands received 12 000 to 16 000 R. There might be such a great difference between calculated and received radiation dose than an individual prediction will be impossible. In our patients a careful calculation for the single-dose treatment was done; nevertheless in some cases an overtreatment of more than 100% was seen. A comparison of responding or overresponding to therapy with the radiation dose is only reliable using individual dosimetry with the therapeutic 131I dose.