Causes of Cancer

What causes cancer?

There is no one single cause for cancer. Scientists believe that it is the interaction of many factors together that produces cancer. The factors involved may be genetic and/or environmental.

Diagnosis, treatment, and prognosis for childhood cancers are different than for adult cancers. The main differences are the survival rate and the cause of the cancer. The overall five-year survival rate for childhood cancer is about 83 percent, while in adult cancers the survival rate is 68 percent. This difference is thought to be because childhood cancer is more responsive to therapy, and a child can tolerate more aggressive therapy. While there have been improvements in survival over the past few decades, some childhood cancers do not respond as well to modern therapy. Further, many survivors of childhood cancer have “late effects” due to their therapy that extend well into adulthood.

Childhood cancers often occur or begin in the stem cells, which are simple cells capable of producing other types of specialized cells that the body needs. A sporadic (occurs by chance) cell change or mutation is usually what causes childhood cancer. In adults, the type of cell that becomes cancerous is usually an epithelial cell. Epithelial cells line the body cavity and cover the body surface. Cancer occurs from environmental exposures to these cells over time. Adult cancers are sometimes referred to as acquired for this reason.

What are the risk factors for cancer?

As mentioned, some cancers, particularly in adults, have been associated with repetitive exposures or risk factors. A risk factor is anything that may increase a person's chance of developing a disease. A risk factor does not necessarily cause the disease, but it may make the body less resistant to it. The following risk factors and mechanisms have been proposed as contributing to cancer:

• Environmental factors. Smoking tobacco, eating a high-fat diet, and working with toxic chemicals are examples of environmental exposures that may be risk factors for some adult cancers. Most children with cancer are too young to have been exposed to these lifestyle factors for any extended time. However, exposure to these factors by parents could also contribute to increased risk of cancer in their children. Exposure to pesticides, fertilizers, and power lines has also been researched for a direct link to childhood cancers. However, whether prenatal or infant exposure to these agents causes cancer is still unknown.
• Genetic factors. Family history, inheritance, and genetics may play an important role in some childhood cancers. It is possible for cancer of varying forms to be present more than once in a family. Additionally, genetic disorders, such as Li-Fraumeni and neurofibromatosis type 1, and Beckwith-Wiedemann syndromes are known to predispose children to certain cancers.
• Exposures to certain viruses. Epstein-Barr virus has been linked to an increased risk of developing certain childhood cancers, such as Hodgkin and non-Hodgkin lymphoma and nasopharyngeal carcinoma. The exact reasons for this link are not fully understood. However, it is suggested that the virus alters a cell in some way causing it to reproduce uncontrollably.
• Medical exposures. In some cases, children who have been exposed to some forms of high-dose chemotherapy or radiation may develop a second malignancy later in life. These strong anticancer agents can alter cells and/or the immune system, which can lead to a second malignancy later in life.

How do genes affect cancer growth?

The discovery of certain types of genes that contribute to cancer has been an extremely important development for cancer research. The majority of cancers are observed to have some type of genetic alteration. Some of these alterations are inherited, while non-inherited alterations can occur at any point during life.

Usually the number of cells in any of our body tissues is tightly controlled so that new cells are made for normal growth and development, as well as to replace dying cells. Ultimately, cancer is a loss of this balance due to genetic alterations that "tip the balance" in favor of excessive cell growth. There are many types of genes that can affect cell growth. Some of these genes are altered (mutated) in certain types of cancers, including the following groups of genes:

• Oncogenes. These genes help normal cells grows. When these genes become mutated or have too many copies in a cell, the cell grows out of control. This uncontrolled cell growth allows abnormal cancer cells to also grow out of control. Scientists are currently unsure what causes these changes in oncogenes that lead to cancer growth.
• Tumor suppressor genes. These genes are able to recognize abnormal growth and reproduction of damaged cells, or cancer cells, and can interrupt their reproduction until the defect is corrected. If the tumor suppressor genes are mutated, they become unable to control normal cell growth and allow cancer cells to begin to grow.
• DNA repair genes. These genes help recognize errors when DNA is copied to make a new cell. If these genes are not working properly, errors in the DNA can be transmitted to new cells, causing them to be damaged. This damaged DNA can then lead to the formation of cancer in some cells.