Oncology, at its most basic level, involves the diagnosis and treatment of cancer. The word oncology comes from the Greek word meaning "mass" or "bulk," referring to tumors. A doctor who specializes in oncology is called an oncologist. Oncology involves a huge range of study. Since cancer can occur in so many of the body's systems, many doctors choose to specialize in a particular branch of it, such as bone cancer or blood diseases. Some doctors specialize in chemotherapy treatments, while others focus on radiation therapy.