HGF and neoplasm: Emerging evidence indicates that the tumor microenvironment contributes to radiation resistance [6] by regulating the levels of cytokines and growth factors, including vascular endothelial growth factor (VEGF) [7], epidermal growth factor (EGF) [8], hepatocyte growth factor (HGF) [9], and basic fibroblast growth factor (bFGF) [10], and promoting extracellular matrix degradation by matrix metalloproteinases (MMPs) [11].