Causative Factors in Human Cancer
المؤلف:
Mary Louise Turgeon
المصدر:
Immunology & Serology in Laboratory Medicine
الجزء والصفحة:
5th E, P477-478
2025-11-04
44
Factors that cause most neoplasms are unknown. They can be classified as environmental factors (e.g., chemical and radiation), host factors and disease associations, and viruses.
Environmental Factors
The incidence of cancer has been correlated with certain environmental factors. Table 1 lists environmental factors that have been definitively linked with cancer, including aerosol and industrial pollutants, drugs, and infectious agents. Radiation exposure is also known to be associated with specific types of cancer (e.g., acute leukemia, thyroid cancer, sarcomas, breast cancer). Women concerned about organochlorine substances (e.g., polychlorinated biphenyls [PCBs], dioxins, pesticides [DDT, banned in 1972]) can be reassured that available evidence does not suggest an association between exposure to these chemicals and breast cancer.
Table1. Selected Environmental Factors Associated With Cancer
Most chemical carcinogens are inactive in their native state and must be activated by enzymes in the cytochrome P-450 or other enzyme systems (e.g., bacterial enzymes or enzymes induced by alcohol).
In radiation carcinogenesis, ionizing particles (e.g., alpha and beta particles, gamma rays, x-rays) hydrolyze water into free radicals, which are mutagenic to DNA by activating protooncogenes. Ultraviolet (UV) light, especially UVB, induces the formation of thymidine dimers, which distort the DNA molecule, leading to skin cancers (e.g., basal cell carcinoma, malignant melanoma).
Host Factors and Disease Associations
Various host factors have been linked to a higher than expected incidence of cancer. For example, the presence of certain genetic disorders (e.g., Down syndrome) is associated with an increased incidence of leukemia. The link between certain genetic abnormalities and leukemia is consistent with a germinal or somatic mutation in a stem cell line.
Familial clustering of germ cell tumors, malignant tumors arising in the testis, has been observed, particularly among sib lings. Cryptorchidism and Klinefelter’s syndrome are predisposing factors in the development of germ cell tumors arising from the testis and mediastinum, respectively.
The incidence of cancer is 10,000 times greater than expected in patients with an immunodeficiency syndrome. The increased incidence of lymphomas in congenital, acquired, and drug-induced immunosuppression is consistent with the failure of normal immune mechanisms or antigen overstimulation with a loss of normal feedback control. Table 2 lists other cancer-related conditions.
Table2. Cancer-Related Conditions
Viruses
Viral causes of some cancers are known. Viruses associated with specific cancers are listed in Table 1. Nonpermissive cells that prevent an oncogenic RNA or DNA virus from completing its replication cycle often produce changes in the genome that result in the activation of proto-oncogenes or inactivation of suppressor genes.
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