The invention relates to an ultra-low molecular weight heparin with an average molecular weight of 2000 to 3000 Daltons, an anti-FXa activity of 145 to 180 U/mg and an anti-FIIa activity of less than 5 U/mg, in particular semuloparin, for use as a prophylactic antithrombotic agent in cancer patients receiving chemotherapy for locally advanced solid tumors, wherein said use improves the survival of patients with locally advanced cancer.