TK1 and hepatocellular carcinoma: Using a tissue microarray, we observed a significantly higher TK-1 expression in HCC tumors (median immunohistochemical score, 33; range, 0–300) than in the surrounding parenchyma (score, 0; range, 0–50), suggesting that 18F-FLT has the potential to differentiate HCC from surrounding cirrhotic liver (P = 0.004) (Supplemental Figs. 2A–2C).