FDG uptake in the pathologically proven papillary thyroid cancer

Metastatic thyroid cancers with I-131 uptake have been known to show no increase of FDG uptake whereas those without I-131 uptake tend to demonstrate increased uptake on PET. In this study, we evaluated the degree of FDG uptake in primary thyroid cancers of papillary histology before surgery. Forty FDG PET studies were performed on the patients who had papillary cancer proven by fine needle aspiration. The degree of FDG uptake was visually categorized as positive or negative (positive if the tumor showed discernible FDG; negative if the tumor didn't) and the peak standard uptake value (peak SUV) of the papillary thyroid cancer (PTC) were compared with the size of PTC. The mean size of 26 PTC with positive FDG uptake was 1.9± 1.4 cm (0.5 ∼ 5 cm). In 13 PTC with negative FDG uptake, the mean size of those was 0.5 ± 0.2 cm (0.2 ∼ 0.9 cm). All PTC larger than 1 cm (2.5 ±1.4 cm, 1 ∼ 5 cm) have positive FDG uptake (peak SUV = 6.4 ± 5.7, 1.7 ∼ 22.7). Among the micropapillary thyroid cancer (microPTC; PTC smaller than 1 cm), 8 microPTC show positive FDG uptake (peak SUV = 2.9 ± 1.3, 1.7 ∼ 5.5), while 13 microPTC show negative finding (peak SUV 1.3 ± 0.2, 1.1∼ 1.7). The size of microPTC with positive FDG uptake is significantly larger than that of microPTC with negative FDG uptake (0.7 ± 0.1cm vs 0.4 ± 0.2 cm, ρ = 0.01). All PTCs larger than 1cm show positive FDG uptake in our study. In other words, thyroid lesions larger than 1cm with negative FDG uptake are unlikely to be PTC. So far, only poorly differentiated thyroid cancers are known to show increased FDG uptake. Our results seem to be contradictory to what is known in the literature. Further study is needed to understand better the significance of increased FDG uptake in PTC in relation to expression of NIS and GLUT