BDNF and neoplasm: Based on current research, we consider that although anatomical factors, cells, nerve growth factors (NGF, BDNF, etc.), chemokines in the tumor microenvironment are involved in the development of PNI, there are other factors that may also contribute to PNI, such as extracellular vesicles (EVs) derived from tumor cells, or other cells in tumor microenvironment and perineural niches (Figure 1).