FDG-PET in thyroid papillary carcinoma: a case report

The most common form of thyroid cancer is papillary thyroid cancer, accounting for 70-90% of well differentiated thyroid malignancies. The incidence of malignant thyroid tumours is approximately 4/100,000 in women and 1.5/100,000 in men. In recent years, there has been an increase in the incidence of papillary carcinoma; whether this is due to dietary changes over time remains unclear. Although papillary tumours tend to metastasise via lymphatics, they have a good prognosis, with a 90% 10-year survival rate. Positron emission tomography (PET) with the glucose analogue fluorodeoxyglucose (FDG-PET) has a special place in the investigation of papillary thyroid malignancies. These tumours are generally poorly differentiated and have impaired Na+/I- pumps, resulting in poor iodine uptake. However, these tumour cells have an increased expression of GLUT-1 and GLUT-3 glucose transporters. FDG is a glucose analogue that does not enter glycolysis, and so papillary tumour cells accumulate FDG, leading to positive findings on PET. Therefore, FDG-PET is especially important in cases where an iodine 131 (131I) scan is negative but serum thyroglobulin levels remain high. In this scenario, FDG-PET sensitivity has been shown to be greater than 90%.