Should we reconsider blocking the thyroid for 123I-Ioflupane studies in elderly patients: quantifying radiation dose to the unblocked thyroid and implications for clinical practice

Abstract

Objectives To measure the absorbed dose to the thyroid in patients injected with 123I-Ioflupane where the thyroid was not blocked with prophylaxis to investigate whether thyroid blocking should be limited to younger patients. This risk from the additional absorbed dose to the thyroid was then compared to the risk from iodine overdose through ingestion of the iodide prophylaxis, resulting in iodine-induced hyper/hypothyroidism (IIH). Methods A cohort of patients (n = 30) who did not receive thyroid prophylaxis underwent static thyroid imaging 3 h after 123I-Ioflupane administration. The measured thyroidal uptake of free 123I was then extrapolated to peak uptake time (24 h post-administration). This value was used to calculate cumulated activity in the thyroid and thus thyroid-thyroid absorbed dose D(r thy ←r thy ) using the relevant S-value in the MIRD method. Results Mean D(r thy ←r thy ) was found to be 13.6 mGy with an SD of 8.8 mGy; this would contribute an additional 0.5 mSv to the effective dose. Conclusion ARSAC recommends in its Notes for Guidance prophylactic thyroid blocking if the absorbed dose to the thyroid is >50 mGy; the maximum thyroid dose in this study cohort was 36.3 mGy. With risk from IIH and its associated cardiac complications increasing with age, this study suggests that iodide prophylaxis with 123I-Ioflupane should be reconsidered for elderly patient.