I had a CT scan of chest to rule out widen mediastinum. What are the chances of getting cancer from that once CT scan.?

Very small. Very small if not zero. Can be negleted with once ct chest. Ct scans may slightly increase the risk of developing cancer with accumulating doses. This risk is usually very small in adult, may be slightly higher in pediatric population and type of the scan. Because of the possible risk, no imaging exam should be done unless there is a clear indication.