Oncology is a branch of medicine that deals with the prevention, diagnosis, and treatment of cancer. A medical professional who practices oncology is an oncologist. Medical oncology: focuses on treatment of cancer with chemotherapy, targeted therapy, immunotherapy, and hormonal therapy. Surgical oncology: focuses on treatment of cancer with surgery. Radiation oncology: focuses on treatment of cancer with radiation. An oncologist is a doctor who treats cancer. Some of the subspecialties include medical, surgical, radiation, pediatric, and gynecologic oncologists. Oncologists who specialize in blood cancers are called hematologist-oncologists.