Quantitative Iodine-123 single-photon emission computed tomography/computed tomography for Iodine-131 therapy of an autonomously functioning thyroid nodule

Abstract

Purpose Autonomously functioning thyroid nodules (AFTNs) are treated with iodine-131 (I-131) therapy, which increases the risk of permanent hypothyroidism; however, the risk can be reduced by separately estimating the accumulated activity for the AFTN and extranodular thyroid tissue (ETT). Methods A quantitative I-123 single-photon emission computed tomography (SPECT)/CT (5mCi) was performed in one patient with unilateral AFTN and T3 thyrotoxicosis. The I-123 concentrations measured at 24h were 12.26µCi/mL and 0.11µCi/mL in the AFTN and contralateral ETT, respectively. Thus, the I-131 concentrations and radioactive iodine uptake expected at 24h by 5mCi of I-131 were 38.59µCi/mL and 0.31 for the AFTN and 0.34µCi/mL and 0.007 for the contralateral ETT. The weight was calculated as CT-measured volume multiplied by 1.03. Results In the AFTN patient with thyrotoxicosis, we administered 30mCi of I-131, which would maximize the 24-h I-131 concentration in the AFTN (226.86µCi/g) and maintain a tolerable concentration in the ETT (1.97µCi/g). The percentage of I-131 uptake at 48h post I-131 administration was 62.6%. The patient achieved a euthyroid state at 14weeks and maintained the state until 2years post I-131 administration with an AFTN volume reduction of 61.38%. Conclusion The pre-therapeutic planning of quantitative I-123 SPECT/CT may enable a therapeutic window for I-131 therapy, which directs optimal I-131 activity to effectively treat AFTN while preserving the normal thyroid tissue.

Key points Question: Is it feasible for I-123 SPECT/CT to be used for planning of I-131 therapy in an autonomously functioning thyroid nodule (AFTN)? Pertinent findings: The radioactive iodine uptake of the AFTN and extranodular normal thyroid tissue can be accurately estimated using the I-123 quantitative SPECT/CT. Implications for patient care: Patient-tailored I-131 therapy can be possible using the pre-therapeutic I-123 SPECT/CT-based planning in patients with AFTN.