CANCER PREVENTION AND TREATMENT: PRESENT AND FUTURE prospering at the expense of its neighbors. In the end—as the clone grows, evolves, and spreads—it can destroy the entire cellular society (Movie 20.1).
In this section, we discuss the development of cancer as a microevolutionary process that takes place within the course of a human life-span in a subpopulation of cells in the body. But the process depends on the same principles of mutation and natural selection that have driven the evolution of living organisms on Earth for billions of years.
Cancer cells are defined by two heritable properties: (1) they reproduce in defiance of the normal restraints on cell growth and division, and (2) they invade and colonize territories normally reserved for other cells. It is the combination of these properties that makes cancers particularly dangerous. An abnormal cell that grows (increases in mass) and proliferates (divides) out of control will give rise to a tumor, or neoplasm—literally, a new growth. As long as the neoplastic cells have not yet become invasive, however, the tumor is said to be benign. For most types of such neoplasms, removing or destroying the mass locally usually achieves a complete cure. A tumor is considered a true cancer if it is malignant; that is, when its cells have acquired the ability to invade surrounding tissue. Invasiveness is an essential characteristic of cancer cells. It allows them to break loose, enter blood or lymphatic vessels, and form secondary tumors called metastases at other sites in the body (Figure 20–1). In general, the more widely a cancer spreads, the harder it becomes to eradicate. It is generally metastases that kill the cancer patient.
Cancers are traditionally classified according to the tissue and cell type from which they arise. Carcinomas are cancers arising from epithelial cells, and they are by far the most common cancers in humans. They account for about 80% of cases, perhaps because most of the cell proliferation in adults occurs in epithelia. In addition, epithelial tissues are the most likely to be exposed to the various forms of physical and chemical damage that favor the development of cancer. Sarcomas arise from connective tissue or muscle cells. Cancers that do not fit in either of these two broad categories include the various leukemias and lymphomas, derived from white blood cells and their precursors (hemopoietic cells), as well as cancers derived from cells of the nervous system. Figure 20–2 shows the types of cancers that are common in the United States, together with their incidence and death rates. Each broad category has many subdivisions according to the specific cell type, the location in the body, and the microscopic appearance of the tumor.
In parallel with the set of names for malignant tumors, there is a related set of names for benign tumors: an adenoma, for example, is a benign epithelial tumor with a glandular organization; the corresponding type of malignant tumor is an adenocarcinoma (Figure 20–3). Similarly, a chondroma and a chondrosarcoma are, respectively, benign and malignant tumors of cartilage.
Most cancers have characteristics that reflect their origin. Thus, for example, the cells of a basal-cell carcinoma, derived from a keratinocyte stem cell in the skin, generally continue to synthesize cytokeratin intermediate filaments, whereas the cells of a melanoma, derived from a pigment cell in the skin, will often (but not always) continue to make pigment granules. Cancers originating from different cell types are, in general, very different diseases. Basal-cell carcinomas of the skin, for example, are only locally invasive and rarely metastasize, whereas melanomas can become much more malignant and often form metastases. Basal-cell carcinomas are readily cured by surgery or local irradiation, whereas malignant melanomas, once they have metastasized widely, are usually fatal.