Paraffin-embedded tumor tissues were consecutively cut into 5 µm thick sections and processed for immunohistochemical staining using the endothelial cell marker CD31, as described previously (23 (link)). Briefly, Antigen retrieval was carried out using the EnVisionTM FLEX Target Retrieval Solution (pH =9.0, Dako), followed by incubation with a rabbit polyclonal CD31 antibody (dilution: 1:300, Proteintech, Rosemont, USA) at 4 °C overnight. Sections were then incubated with the secondary antibody (PV-9000, Zhongshan Jinqiao Biotechnology Co., Ltd., Beijing, China) for 1 hour, followed by color development with 3,3’-diaminobenzamine in Tris-HCl (50 mmol/L, pH =7.5) containing 0.005% hydrogen peroxide, and counterstaining with hematoxylin.