Use of a dosimetry-based RAI protocol for treatment of benign hyperthyroidism optimises response while minimising exposure to ionising radiation.

BACKGROUND: The optimal treatment strategy for radioiodine (RAI) treatment protocols for benign hyperthyroidism remains elusive. Although individualised activities are recommended in European Law, many centres continue to provide fixed activities. Our institution implemented a dosimetry protocol in 2016 following years of fixed dosing which facilitates the calculation of individualised activities based on thyroid volume and radioiodine uptake. METHODS: This was a retrospective study comparing success rates using a dosimetry protocol targeting an absorbed dose of 150 Gy for Graves' disease (GD) and 125 Gy for Toxic Multinodular Goiter (TMNG) with fixed dosing (200MBq for GD and 400MBq for TMNG) among 204 patients with hyperthyroidism. Success was defined as a non-hyperthyroid state at 1 year for both disease states. Results were analysed for disease specific or patient specific modulators of response. RESULTS: This study included 204 patients; 74% (n = 151) received fixed activities and 26% (n = 53) of activities administered were calculated using dosimetry. A dosimetry-based protocol was successful in 80.5% of patients with GD and 100% of patients with TMNG. Differences in success rates and median activity administered between the fixed (204Mbq) and dosimetry (246MBq) cohort were not statistically significant (p = .64) however 44% of patients with GD and 70% of patients with TMNG received lower activities following treatment with dosimetry as opposed to fixed activities. Use of dosimetry resulted in successful treatment and reduced RAI exposure for 36% of patients with GD, 70% of patients with TMNG, and 44% of patients overall. CONCLUSION: This retrospective clinical study demonstrated that treatment with a dosimetry-based protocol for TMNG and GD achieved comparable success rates to fixed protocols while reducing RAI exposure for over a third of patients with GD and most patients with TMNG. This study also highlighted that RAI can successfully treat hyperthyroidism for some patients with activities lower than commonplace in clinical practise. No patient or disease specific modulators of treatment response were established in this study; however, the data supports a future prospective trial which further scrutinises the individual patient factors governing treatment response to RAI.

Blood Micro-sampling: An alternative blood collection method for radioiodine therapy dosimetry.

PURPOSE: Collecting venous blood samples from patients post administration of high therapeutic activities results in radiation exposure to staff collecting the blood. This study investigated the use of finger-tip capillary-blood collection as an alternative to the venous-blood collection method recommended by the European Association of Nuclear Medicine (EANM) dosimetry protocol for quantifying 131I concentration in the critical organ after therapeutic dose of 131I. METHODS: The study included differentiated thyroid cancer patients referred to a thyroid cancer centre at St James's Hospital, Ireland, for therapeutic and diagnostic oral administration of 131I. The 15 patients recruited for this study provided 30 venous and capillary paired-blood samples. The activity concentration of the blood samples was compared between the type of blood (venous vs capillary) and the geometry/volume of the blood (1.0 ml versus 0.03 ml). Other variables were also investigated including administered activities, dose to staff performing the sampling, duration of sampling and time since administration. RESULTS: Blood samples were taken at 2.0-91.9 h post administration using 0.2 ± 0.0 GBq (n = 2) or 4.0 ± 0.1 GBq (n = 28) 131I activities. There was no significant difference found between different blood sampling types (-1.0 ± 4.3 %, p = 0.223), different blood volumes (-3.2 ± 10.0 %, p = 0.070), or between their combination. No significant correlation was found between the percentage differences and investigated parameters. CONCLUSION: A high degree of accuracy was achieved with blood radioactivity quantified using capillary blood collection using the finger-prick method. Further validation of the method would be required prior to implementation, to investigate patient specific factors which may affect accuracy.