A 40 years old female patient attends the OG department with swelling of legs for past one day. There is no history of fever. 2 years back she had a metastatic breast adenocarcinoma which was treated with paclitaxel and radiotherapy. Blood pressure 120/76 mmHg, Pulse rate 84/min, temperature 37.6 C, Respiratory rate 16/ min. Saturation was 99% in room air. There was warmth and tenderness over the left leg and left thigh. Auscultation of lungs revealed clear fields. Urine pregnancy test was negative. CT chest was normal. USG of left leg revealed thrombus in the superficial femoral vein. The most appropriate treatment is