 Many highly specialized fields require formal training beyond residency.Examples of these include cardiology, endocrinology, oncology after internal medicine; cardiothoracic anesthesiology after anesthesiology; cardiothoracic surgery, pediatric surgery, surgical oncology after general surgery; reproductive endocrinology/infertility, maternal-fetal medicine, gynecologic oncology after obstetrics/gynecology.There are many others for each field of study.