Platelets are primarily known for their central role in primary hemostasis. However, they are increasingly recognized for their participation in various non-hemostatic processes, such as cancer progression and clinical expression. Experimental and clinical data indicate that the involvement of platelets in the pathophysiology of cancer goes far beyond the realm of cancer-associated thrombosis. Several experimental studies have shown that platelets can promote the metastatic process by various mechanisms. However, while it has been shown in vitro that direct contact with platelets initiates tumor cells for metastasis, it remains unclear whether such contacts occur in solid tumors. In addition to their ability to promote metastasis, platelets have been shown to stimulate angiogenesis and play a crucial role in lymphangiogenesis. Considering that blood vessels, lymphatics and immune cells are major components of the tumor ecosystem, our hypothesis is that platelets contribute to the development and / or regulation of the tumor microenvironment. This is because platelets stabilize tumor blood vessels by permanently repairing vascular damage caused by immune cells infiltrating tumors. Targeting platelets destabilizes tumor vessels, causing intra-tumor hemorrhage, which allows intra-tumor accumulation of intravenously administered anti-tumor drugs such as paclitaxel and improves their efficacy. Studies have also reported the role of platelets in several pathogenic mechanisms of cancer: thrombocytosis is a paraneoplastic syndrome which suggests a poor prognosis in patients with solid tumors; a negative correlation between the platelet count and the response to chemotherapy has been reported in several types of cancer; histological analyzes of esophageal cancer suggested a possible association between the presence of platelets in the tumor stroma and the level of tumor lymphangiogenesis and lymphovascular invasion; finally, a recent study reported the expression of one of the main targets of immunotherapies, PD-L1, on the platelets of patients suffering from different types of solid cancers. All of these data support our hypothesis that platelets are components and / or regulators of the tumor microenvironment and therefore potential targets for the improvement of anti-tumor therapies. In this context, the objectives of our project are to determine whether platelets are components of the microenvironment of tumors of the central nervous system, and to study the possible correlations between the intratumoral presence of platelets and the evolution of patients with central nervous system tumors