defects may include mutations, rearrangements, amplification of cellular oncogenes, inactivation of tumor suppressor genes, and karyotypic instability (54:117). Once progression is complete, the cancer cell and its progeny overgrow by a series of divisions (48:43). By sequentially replicating themselves, the cell populations of many tumors proliferate at exponential rates.
During the later stages of a neoplastic illness, the malignant spread of cancer cells may take place. Virtually all carcinomas have the capacity for metastasis; however, those that are most likely to spread consist of "large tumors that are poorly differentiated histologically" (4:61). Metastasis essentially involves the development of secondary tumors in addition to the primary one. These remote colonies result from an orderly sequence of physiologic mechanisms. The different stages generally occur as follows: (1) detachment of the primary tumor's cells; (2) infiltration of these cells into the lymphatic and vascular systems; (3) cancer cell circulation; (4) invasion of distant organs; and, finally, (5) the development of the secondary growths (4:65). In general, the metastatic process tends to be exceedingly complex (31:40). Sometimes it even occurs as a cascade. The ultimate result of such an outpouring may include neoplastic involvement of one or more, widely varying ectopic sites simultaneously.
Typically, a variety of different factors contribute to the development of any given cancer. Extrinsic influences may include viruses, radiation, and exposure to carcinogenic chemicals (52:91 92). In addition, the disease may also be related to genetic factors (32:121).
Overall, the various neoplastic diseases represent a significant threat to human health; they are currently responsible for inestimable suffering. For example, in the United States, cancer is the second leading cause of death. There are over 7 million Americans living with th...