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Clinical Determinants of Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography in Differentiated Thyroid Cancer Patients with Elevated Thyroglobulin and Negative 131Iodine Whole Body Scans after 131Iodine Therapy SHAMIM, Syed Ejaz; NANG, Lee Boon; SHUAIB, Ibrahim Lutfi & MUHAMAD, Nor Asiah Abstract   Background: A cross-sectional prospective study has been conducted on differentiated thyroid cancer (DTC) patients using negative 131Iodine (131I) whole body scans and elevated thyroglobulin (Tg) levels. The main objective of this research was to determine the prevalence of the conversion of differentiated to dedifferentiated thyroid cancer patients during follow up at the Hospital Kuala Lumpur. It has been demonstrated that fluorodeoxyglucose (FDG) uptake is inversely proportional to the iodine concentration and to differentiation of the cells.   Methodology: Thirty-five patients with histologically proven DTC that have undergone total or near total thyroidectomy, and post 131I radioactive iodine ablation therapy, were selected and prospectively analysed. The patients also had to show at least one negative whole body scan and Tg levels of 10 ìg/L and above. The results of the FDG-Positron Emission Tomography/Computed Tomography (PET/CT) were then studied to determine the association and the predictors influencing the outcome by using univariable and multivariable analyses.   Results: Out of the thirty-five patients, 60% of them (twenty-one) showed positive results and 40% (fourteen) showed negative. Age, gender, and type of histopathology (HPE) showed significant associations with the positive results of the FDG-PET/CT. The results also showed no correlations observed between the Tg levels and standardised uptake value (SUV)max in the DTC patients with positive disease findings in the FDG-PET/CT. The predictor for this study was age.   Conclusion: The prevalence of the conversion of differentiated to dedifferentiated thyroid cancer among patients with negative 131I and elevated Tg was 60%, with age as the predictor. DTC patients aged 45 year-old and older were seven times more likely to have positive results of FDG-PET/CT imaging. Keywords thyroid cancer; iodine; whole body scan; thyroglobulin; fluorodeoxyglucose F18; positron-emission tomography

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