Oncologists are physicians with advanced training, experience and education that provide them with the skills necessary to examine, diagnose and treat cancerous tumors. Some of the many diseases they treat are bone tumors, breast cancer, brain tumors, Hodgkin's disease, malignant melanomas, lymphomas and stomach cancer. Hematologists diagnose and treat disorders of the blood, such as anemia, clotting, white blood cell problems (including leukemia) and other bone marrow disorders.