More than 100 types of cancer develop in the various organs in the body. Cancers are described according to where in the body the cancer originated, what type of tissue it originated in, and what type of cell it started in. For example, breast cancer describes any cancer that originated in the breast.
If the cancer spreads to a new organ, such as the lungs, the tumor is called metastatic breast cancer, not lung cancer.
Epithelial tissue forms a protective layer of cells that covers organ surfaces and lines body cavities. Shown here is a layer of simple squamous (scaly) epithelium under magnification. Cancers that arise in epithelial tissues, called carcinomas, account for approximately 90 percent of all human cancers. Each organ in the body is composed of different types of tissue, and most cancers arise in one of three main types—epithelial, connective, or blood-forming tissue. Carcinomas are cancers that occur in epithelial tissues—the skin and inner membrane surfaces of the body, such as those of the lungs, stomach, intestines, and blood vessels. Carcinomas account for approximately 90 percent of human cancers. Sarcomas originate in connective tissues—such as muscle, bone, cartilage, and fat—that support and connect other parts of the body. Much rarer than carcinomas, sarcomas account for less than 2 percent of all cancers. Leukemias develop in blood cells, and lymphomas originate in the lymphatic system. Combined, these cancers of the blood-forming tissues account for about 8 percent of all human cancers.
Connective tissues include bone, cartilage, fat, ligaments, and tendons. These tissues support and connect parts of the body. The structure varies depending on the purpose of the tissue. The diagonal red band in this image shows elastin fiber, which allows connective tissue to spring back into shape following deformation. Cancers called sarcomas orginate in connective tissues. Rare, sarcomas constitute only about 2 percent of all human cancers. Sarcomas are elusive in their early stages. They may arise deep within connective tissues, making them more likely to spread to distant parts of the body before they are detected.
Cancers are further identified according to the type of cell affected. For example, squamous cells are flat, scalelike cells found in epithelial tissue. Cancers that originate in these cells are called squamous cell carcinomas. Adenomatous cells are glandular or ductal cells, and carcinomas that originate in these cells are called adenocarcinomas. Sarcomas that develop in fat cells are called liposarcomas, and those that develop in bone cells are called osteosarcomas.