After a doctor completes medical school and is licensed to practice medicine, the physician may pursue a rigorous, 3 year training program in Internal Medicine. After passing an examination and completing the requirements, the doctor is Board Certified in Internal Medicine, and may be called an Internist. This is different from an intern, who is a doctor who has completed medical school and is in the first year of postgraduate training.
Oncologists (cancer specialists) complete two additional years of training after they finish training in internal medicine in diagnosis and treatment of all types of cancer. Once a person is diagnosed with cancer, the oncologist usually assumes management of their care. They are also experts in radiation and chemotherapy treatments. Leukemia and other disorders of the blood and bone marrow are usually managed by a hematologist-oncologist.