It is estimated that 70 to 90 percent of newly produced tumor cells in humans die spontaneously. Were this not the case, many patients would likely die before entering the health care system, because tumors could double in size in a matter of days. The mechanisms responsible for cell death are poorly understood, but they are probably related to changes in the structure of the cancer cell genome, apoptosis, what can broadly be termed cell nutrition, various immunologic mechanisms, and other factors. For example, defects in chromosome structure can emerge during replication and segregation of chromosomes. Abnormal numbers of chromosomes can also be segregated during mitosis, so many daughter cells undoubtedly lack either the genes or the regulatory mechanisms required for survival.
Tumors have an abnormal vascular system.
Rupture of vessels, coagulation, and hemorrhage are common. Poor oxygenation and lack of nutrients can kill many cells. The role of the human immune system in tumor cell death is obscure, but it is likely that certain classes of lymphocytes also contribute to this process. Furthermore, specific genes regulate the duration of life of many cell types. Distinctive changes in nuclear morphology and degradation of DNA into fragments are the hallmarks of cells undergoing apoptosis.