Feasibility of pre-treatment dosimetry using Tc-99m in radioiodine therapy for Graves’ disease and multinodular goitre: a comparison with the fixed-activity approach

Background and purpose In radioiodine therapy for hyperthyroidism, patient-specific activity determination is recommended. This study investigated whether a 99m Tc thyroid uptake measurement at 20 min ( 99m Tc RIU(20 min)) could be correlated to 131 I uptake at 24 h ( 131 I RIU(24 h)) and used to estimate the activity required to deliver the prescribed absorbed dose. The absorbed dose delivered with 99m Tc-based activity was compared to that delivered with the fixed activity recommended in absence of prior evaluation at our center. Materials and methods A logarithmic conversion formula relating 99m Tc RIU(20 min) to 131 I RIU(24 h) was derived from 66 patients with Graves' disease and multinodular goitre who underwent both 99m Tc and 131 I pre-treatment dosimetry at our center. 99m Tc RIU(20 min) was assessed via scintigraphy, while 131 I RIU(24 h) with a dedicated probe. Recommended activities were estimated using EANM formulas. In the same group of patients, absorbed doses delivered with the tailored approach were compared to those delivered with the fixed-activity method. Results The relationship between 99m Tc RIU(20 min) and 131 I RIU(24 h) was described by the equation 131 I RIU(24 h) = 12.07ln( 99m Tc RIU(20 min)) + 31.93. The administration of a fixed activity resulted in 50% of patients receiving absorbed doses higher than 400 Gy, whereas 99m Tc-based dosimetry reduced this proportion to 10%. The median fixed-to-tailored absorbed dose discrepancy was 26% (range: −78% to 555%), highlighting the dosimetric variability introduced by the standard approach. Conclusion 99m Tc pre-treatment dosimetry proved to be a feasible alternative to 131 I. This approach enables tailoring activity estimation and reducing the risk of excessive absorbed dose administration compared to a fixed-activity approach.