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Introduction: It's difficult to make a scientific, evidence-based approach about the timing of radioiodine remnant ablation (RRA) in patients with differentiated thyroid carcinomas (DTCs). Primary aim of the study was to reveal whether timing of RRA relates to achievement of non- structurally incomplete response (non-SIR) in low/intermediate and high-risk patients. Another aim was to reveal the correlation of timing with non-SIR status in reproductive-age women., Materials and Methods: Records of 279 low, intermediate, and high-risk patients were analysed, retrospectively. Number of days between surgery and RRA is referred to as timing. Low/intermediate-risk patients, high-risk patients, and low/intermediate-risk reproductive-age women were divided into non-SIR and SIR groups, according to 2015 American Thyroid Association guidelines for therapy response. The relationship between timing and therapy response was analysed statistically., Results: We could not find any significant relationship in patients with low/intermediate risk between timing and non-SIR, including women between 18-49 years of age (p >0.1). For high-risk patients, we found a statistically significant relationship between timing and non-SIR response. According to ROC analysis, RRA ≤58 days was found as a cut-off value. The sensitivity, specificity, positive likelihood ratio, and negative likelihood ratio were calculated as 83.3%, 70.0%, 2.78, and 0.24, respectively., Conclusion: RRA must be initiated within 58 days after surgery in patients with high-risk DTCs. Under this approach, risk of SIR and associated mortality risk may be reduced. RRA timing for women in reproductive ages with low/intermediate risk groups may be planned according to their pregnancy/breastfeeding intent. For other low/intermediate risk groups, they can safely proceed according to the capacity of the medical facility and related logistical considerations., (Copyright © 2019 Sociedad Española de Medicina Nuclear e Imagen Molecular. All rights reserved.)

Objective: There is a special group of patients, according to 2015 American Thyroid Association guidelines. This group is defined as "the patients with conflicting observational data for post-surgery radioiodine ablation (COD for PSRIA)". For this special group of patients RIA is applied after a thorough reassessment of histopathological, clinical and biochemical features, including thyroglobulin (Tg). However, there is no consensus on what is the suitable cut-off value for the radioiodine ablation (RIA) decision or for therapy prediction. Moreover, is also unclear which Tg parameters should be used for these purposes. If we can determine useful and practical cut-off values for excellent response (ER) and non-structural incomplete response (non-SIR) response categories, this will facilitate our therapy response prediction before RIA and may allow us to categorize the group of "COD for PSRIA" based on a higher risk of recurrence/relapse or disease specific mortality rates according to serum thyroglobulin (Tg). This categorization may also enable us to plan the follow-up frequency of patients more scientifically. Consequently, it may provide the more efficient use of medical facility and healthcare system resources., Subjects and Methods: Two hundred forty-nine patients (out of 577 examined) with "COD for PSRIA" were included in this study. Firstly, patients with indeterminate, biochemical incomplete and structural incomplete responses were considered as the non-ER group and compared to the ER group. Secondly, patients with excellent, indeterminate, and biochemically incomplete responses were considered as the non-SIR group and compared to the SIR group. The data were evaluated by MedCalc Statistical Software version 18.9., Results: The cut-off value for ER patients was calculated as ≤6.57ng/mL. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were 67.9%, 75.4%, 55.6% and 83.8%, respectively. The cut-off value for non-SIR patients was calculated as ≤12.7ng/mL. Sensitivity, specificity, PPV and NPV were 78.5%, 91.7%, 35.5% and 98.6%, respectively., Conclusion: If a patient has ≤6.57ng/mL pre-ablative Tg, follow-up intervals of patients with "COD for PSRIA" may be extended due to lower recurrence/relapse rates. However, if a patient has >12.7ng/mL pre-ablative Tg, these patients should be followed-up more frequently in order to determine SIR earlier. This approach may enable more efficient use of medical facility and healthcare system resources and a more scientific planning of their follow-up treatment. This approach seems to have the potential to contribute significantly to cost-effectiveness.