Estimating I-131 Thyroid Uptake from Tc-99m scintigraphy: Implication for calculated I-131 Therapy Activities in Benign Thyroid Disease

For benign thyroid disease, pre-therapeutic radioiodine (131I) dosimetry is recommended given that higher quality of care can be achieved by personalised radioiodine patient treatment (Salvatori and Luster 2010). Pre-therapeutic dosimetry evaluates the 131I kinetics of the thyroid, and calculates 131I uptake (131IU) function, which is used to calculate the radioiodine-therapy administered activity. Thyroid technetium-99m (99mTc) scintigraphy also forms part of patient diagnosis and 99mTc uptake (99mTcU) can be estimated from this scan. Studies have demonstrated the use of 99mTcU to estimate the 131IU and, hence, eliminate the need for 131IU assessment (Smith et al. 1990; Gorur et al. 2009; Szumowski et al. 2016); however, the question remains as to whether 99mTcU may accurately reflect 131IU or whether results in an uncertain prescribed 131I therapy activity. Accordingly, 131IU with 99mTcU datasets were compared for a patient cohort examined between 2012 and 2018. The factors affecting 99mTcU accuracy, including demographic data and laboratory thyroid function test, were analysed. The study sample comprised 133 patients (100 women and 33 men) diagnosed with Graves’ disease (55%), multi-nodular goitre (21%) and toxic nodules (25%). Results showed the correlation between 99mTcU and 131IU is significantly more pronounced in men than women (R2 = 0.90 vs. R2 = 0.27, respectively). The study could not estimate 131IU based on 99mTcU. It was shown that other investigated parameters affected the correlation between the two. It demonstrates how the use of 99mTcU alone, in place of 131IU, would lead to significant over and underestimation of 131I therapy activities.