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Context Measurements of thyroglobulin (Tg) and Tg antibodies are crucial in the follow-up of treated differentiated thyroid cancer (DTC) patients. Inter-assay differences may significantly impact follow-up. Objective The aim of this multicenter study was to explore the impact of Tg and Tg antibody assay performance on the differential classification of DTC patients, as described in national and international guidelines. Design Four commonly used Tg and Tg antibody assays were technically compared to reflect possible effects on patients with DTC follow-up. Storage stability at different storage temperatures was also investigated for LIAISON® and Kryptor assays, as this is an underexposed topic in current literature. Results B.R.A.H.M.S. assays yield approximately 50% lower Tg values over the whole range compared to the DiaSorin and Roche assays investigated. These differences between assays may result in potential misclassification in up to 7% of patients if fixed cut-offs (e.g. 1 ng/mL) are applied. Poor correlation was also observed between the Tg antibody assays, when the method-specific upper limits of normal are used as cut-offs. Storage of Tg and Tg antibodies was possible for three to four weeks at -20 °C and -80 °C. Calibration of the assays, however, was found to be crucial for stable results over time. Conclusions Technical aspects of Tg and Tg antibody assays, including inter-assay differences, calibration and standardization, and cut-off values, may have a significant clinical impact on the follow-up of DTC patients.

Background: Although differentiated thyroid carcinoma (DTC) is the most frequent endocrine pediatric cancer, it is rare in childhood and adolescence. While tumor persistence and recurrence are not uncommon, mortality remains extremely low. Complications of treatment are however reported in up to 48% of the survivors. Due to the rarity of the disease current treatment guidelines are predominantly based on the results of small observational retrospective studies and extrapolations from results in adult patients. In order to develop more personalized treatment and follow-up strategies (aiming to reduce complication rates) there is an unmet need for uniform international prospective data collection and clinical trials. Methods and analysis: The European pediatric thyroid carcinoma (ped-DTC) registry aims to collect clinical data for all patients with a confirmed diagnosis of DTC<18 years of age who have been diagnosed, assessed, or treated at a participating site. This registry will be a component of the wider European Registries for Rare Endocrine Conditions (EuRRECa) project which has close links to Endo-ERN, the European Reference Network for rare Endocrine Conditions. A multidisciplinary expert working group was formed to develop a minimal dataset comprising information regarding demographic data, diagnosis, treatment and outcome. We constructed an umbrella-type registry, with a detailed basic data set. In the future, this may provide the opportunity for research-teams to integrate clinical research questions. Ethics and dissemination: Written informed consent will be obtained from all participants and/or their parents/guardians. Summaries and descriptive analyses of the registry will be disseminated via conference presentations and peer-reviewed publications.

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