 PHACS was a randomized multi-center clinical trial. Eligible patients were enrolled and underwent baseline screening ultrasonography of the lower extremities to rule out pre-existing DVT and a chest CT scan to rule out PE. If negative, patients were then randomized to receive either dalteparin 5000 units subcutaneously daily or observation for a study period of 12 weeks. The first day of dalteparin prophylaxis coincided with the first day of initiation of a new systemic chemotherapy regimen. The patients were seen every 4 weeks 1 week at the time of regularly scheduled chemotherapy cycle visits for serial ultrasonography of lower extremities during study period i.e. at 4, 8 and 12 weeks. A chest CT scan was performed at 12 weeks. Compliance was measured by asking patients about missed doses at these 4-weekly visits as well as by asking patients to fill an injection diary. 