Patients with active cancer and symptomatic pulmonary embolism, proximal deep vein thrombosis, iliac or caval thrombosis will be randomly assigned to receive either dalteparin using the CLOT regimen or to oral rivaroxaban using the conventional dosage given in the Einstein studies. Experimental and control treatments will be given for three months. The main outcome at three month will include all symptomatic and incidentally discovered venous thromboembolic events including pulmonary embolism (either objectively confirmed and death due to pulmonary embolism), lower limb and upper limb deep vein thrombosis, iliac, caval and visceral thrombosis and any worsening of vascular obstruction which will be collected systematically at inclusion and at day 90. The safety end-points will consist of the rate of major bleedings and the composite of major and non-major but clinically significant bleedings at day 90. All outcome events will be blindly adjudicated by a central independent adjudication committee.