Cancer is not a single disease but rather comes in many forms and degrees of malignancy. There are three main classes of cancer:
• Sarcomas, in which the tumor has arisen in mesenchymal tissue, such as bone, muscle, or connective tissue, or in nervous system tissue;
• Carcinomas, which originate in epithelial tissue, such as the cells lining the intestine, bronchi, or mammary ducts; and
• Hematopoietic and lymphoid malignant neoplasms, such as leukemia and lymphoma, which spread throughout the bone marrow, lymphatic system, and peripheral blood.