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OBJECTIVE The aim of this study was to evaluate the relative error (RE) in the thyroid absorbed dose (TD) of iodine-131 (131I) in patients with Graves' disease comparing the simplified Quimby-Marinelli-Hine formula method (sQMHF) and the Standard Operational Procedures for dosimetry (SOPD) recommended by the European Association of Nuclear Medicine. PATIENTS AND METHODS This study included 45 patients with Graves' disease 12 men and 33 women; age 44.1±12.8 years. Thyroid mass (TM) was measured using ultrasound. Uptake of 131I (RAIU) was tested at 2, 4-6, 24, 48-72, and 96-168h after its administration and the half-life (T1/2eff) and resident time (RT) of 131I were computed. According to the sQMHF, a prescribed TD of 75Gy required 3.7MBq/g of 131I, correction based on the RAIU24h and T1/2eff. Subsequently, the therapeutic TD was computed according to the SOPD and the RE was recorded. The data were analyzed using t-tests. RESULTS The TM, RAIU24h, therapeutic TD, and RE were 36.5±23.9g, 0.54±0.14, 89.4±9.4Gy, and -0.01±0.02, respectively. There was a significant difference (t-value 9.84, P<0.01) between the prescribed and therapeutic TD because the sQMHF ignores the absorbed dose deposited in the thyroid during the first 24h, which is included in the SOPD. In addition, the RE was significantly smaller than the variable coefficient (VC) of the therapeutic TD (t=-39.6, P<0.01). CONCLUSION When the activity of 131I was calculated using the simplified Q-M-H formula, the therapeutic absorbed thyroid dose was significantly higher than what was expected for the prescribed dose. Precision of the individualized therapeutic absorbed dose could be improved by computing the activity of 131I using the standard operational procedures for dosimetry of the EANM.