Value of the Post ablative Thyroglobulin measurement for assessment of disease-free status in patients with differentiated thyroid cancer Dessoki N1, Nasr I 2. Badawy A3. Ali I4. 1, 3Oncology and Nuclear Medicine Department, Faculty of Medicine, Cairo University. 2Oncology & Nuclear Medicine Department, Faculty of Medicine, Zagazig University. 4Radiodiagnosis Department, Faculty of Medicine, Zagazig University, Egypt. Indian J. Nucl. Med., Jan 2019 (In press) Abstract Aim: To evaluate the value of TSH stimulated sTg measurements by the end of the first year post ablation in DTC patients with initial biochemical incomplete response. Methodology: 100 patients with DTC, underwent near total thyroidectomy and radioactive remnant ablation (RRA) by iodine-131 (I131) with regular follow up every 6 months during the first two years and at 6–12-month intervals thereafter by I131 WBS, Neck ultrasound (US) and sTg measurement in the hypothyroid state TSH > 30mU/L. Patients were divided according to the imaging findings and sTg level into 3 groups: Excellent response (ER): no evidence of disease by imaging and stimulated or suppressed Tg <1 ng/mL, Indeterminate or acceptable response (AR): Non-specific findings on imaging studies and stimulated sTg less than 10 ng/mL and Incomplete response (IR): Patients with incomplete structural and/or incomplete biochemical response (Tg>10 ng/mL). Results: The follow up at 6 months post ablation showed ER in 3 (3%) patients, AR in 29 (29%) patients and IR in 68 (68%) patients. A second follow up at 9-12 month post ablation showed dramatic biochemical response with excellent, indeterminate and incomplete response in 50 (50%), 34 (34%) and 16 (16%) patients respectively and 14 (14%) patient had structural recurrence. This change is highly statistically significant (P value 0.00). In the last follow up (ranges from 3-10 years) 53 (55.8%) patients achieved ER and (44.2%) 42 AR and no patient with IR. The change in patients with IR between the second and the last follow up is also statistically significant (P value 0.001). Conclusion: Stimulated Tg measurement by the end of the first year is more reliable test and considered significant predictor of disease free status for the follow-up of patients with DTC and biochemical incomplete response. Patients with biochemical IR still have the chance to achieve ER or AR by passage of time without additional therapies. Key words: Differentiated cancer thyroid (DTC), I131 whole body scan (I131WBS), TSH stimulated thyroglobulin (sTg), biochemical incomplete response.