Radioiodine Therapy for Graves Disease Thyroid Absorbed Dose of 300 Gy-Tuning the Target for Therapy Planning

Purposes: Based on the committed thyroid absorbed dose, the aim was to compare the efficiency of I-131 therapy against Graves disease (GD) within 1 year after treatment and, by exploring the dose-response relationship, indicate an absorbed dose to be targeted into patient therapeutic planning. Methods: Thyroid-absorbed doses were calculated to 196 patients with GD by applying Medical Internal Radiation Dose formalism and taking into account administered I-131 activity, thyroid radioiodine uptake, effective half-life, and gland tissue mass. Statistical analysis was applied to assess the relationship between absorbed doses and the patient's clinical response. Results: Overall, successful therapy was achieved in 167 patients, whereas in 29 the disease persisted, even though 64.8% and 89.3% of all the treated patients had received, respectively, thyroid absorbed dose and activity superior to 300 Gy and 11.1 MBq/g (300 mu Ci/g) of thyroid tissue. Among those in whom the disease persisted, 24 (83%) had a 6- to 24-hour I-131 uptake ratio equal or superior to 0.9, whereas only 5 (17%) presented a lower ratio. According to statistical analysis, there was no difference in cure rate between the groups that received 300 Gy or less and that which received more (84.1% vs 85.8%, P = 0.8336). Conclusions: A thyroid absorbed dose of 300 Gy is plausible as a targeted therapeutic dose in GD therapy planning, because statistical analysis has proven there to be no advantage in treating patients with doses above this level. On the other hand, numerous efforts should be made to develop an optimized and easily applicable protocol of patient-specific dosimetry and to provide data that show its clinical impact on patient management.