The fraction that is spontaneous is unknown and cannot be calculated until all known external causes have been accounted for. This is not a feasible proposition. Alcohol, occupational and medical carcinogens, and certain patterns of reproductive behavior also contribute to the cancer burden. Cancers that cannot be attributed to these causes, and for which no other causes can be found, could be considered spontaneous and to arise from endogenous processes. Epidemiological evidence suggests that spontaneous and induced cancers share the same mechanism.
Cancer is a genetic disorder of somatic cells.
An accumulation of mutant genes that control the cell cycle, maintain genomic stability, and mediate apoptosis is central to carcinogenesis. Spontaneous mutation may cause spontaneous cancer. Mechanisms underlying many of these cellular defenses are under genetic control. Thus, germ line mutations or polymorphisms of genes that govern them may also contribute to spontaneous cancer. The author thanks the Cancer Research Campaign and the Medical Research Council of the United Kingdom for financial support.