Calculation of the radioiodine dose for the treatment of Graves' hyperthyroidism: Is more than seven-thousand rad target dose necessary?

Some authors recently suggested a significant increase in the target dose o f radioiodine treatment in Graves' disease. The aim of the present study wa s to investigate the impact of thyroid gland mass on the success rate of ra dioiodine treatment. For this purpose, the thyroid function of 105 consecut ive Graves' patients was assessed 6 and 12 months after a I-131 treatment a nd correlated to the gland mass. The patients were categorized according to the gland mass into small (less than or equal to 30 g; 19 patients), mediu m size (31-50 g; 40 patients), and large size (> 50 g; 46 patients) groups (S, M, L groups, respectively). None of the patients received more than a 1 0,000-rad (100-Gy) target dose. During the calculation of administered I-13 1 activity, late uptake measurement has also been routinely used, in additi on to the usual maximal uptake parameter. The established effective half-li fe of I-131 was highly variable (5 +/- 1.2 days; range: 2-7.6 days) and cou ld not be predicted based on other clinical data without measuring an exten ded radioiodine uptake curve of the given patient. However, the correlation between the administered activity calculated from the complete set of upta ke values and that of only a single late one was excellent (r = 0.99). Six months after the I-131 treatment, hyperthyroidism was cured in 81% of patie nts with small and medium size thyroid glands, with 62% euthyroid and 19% h ypothyroid ratios respectively. In the early phase of study for large goite rs, the same linear mass activity function was used during calculation as i n smaller glands. In these 17 patients the nonhyperthyroid result was compa rable to the results of treatment of the small and medium size gland groups only after 1 year (77%), but the 6-month success rate was significantly lo wer (53%; p < 0.05). After obtaining these results, the usual 7000-rad targ et dose was increased to 8000-10,000 rad (depending on the gland mass) in a nother group of 29 patients with large thyroid glands that result in an acc eptable 6-month success rate of 72%. In conclusion, instead of the "mCi I-1 31/g gland mass/maximal uptake" dose calculation, we suggest a method in wh ich (1) the late I-131 uptake measurement is taken into account and (2) for large goiters there is an additional dose adjustment, ie, increase is need ed over the usual linear, size driven calculation. No overall increase of t arget dose over 10,000 rad is necessary if no antithyroid medication is giv en shortly before I-131 treatment.