Venous Thrombus Embolism is an increasingly frequent complication of cancer and anti-cancer therapies. It is associated with increased mortality and other significant adverse consequences. Risk factors for VTE in the cancer populations with the use of thromboprophylaxis. However, prophylaxis studies in the ambulatory cancer population have been largely unsuccessful primarily because of low event rates in patients chosen on the basis of one or two risk factors. The use of a validated risk model that incorporates multiple factors to identify high-risk patients represents a novel way to identify an appropriate population for prophylaxis. This study will evaluate the safety and efficacy of prophylaxis in a high-risk subgroup of cancer patients identified by a validated risk model. Correlative studies will evaluate the benefit of tissue factor as a predictive biomarker of VTE.