Research over the past few decades has shown that genes play a key role in the development and behavior of cancers. In short, genes are the instructions that tell our cells how to grow and function to keep us healthy. Therefore, changes within those instructions can cause cells to lose their regulation and grow out of control, leading to a tumor.
The majority of cancers are sporadic, or occur just by chance. Within cancer cells, genetic mutations are found that are different from the healthy, non-cancerous cells in the body. As noted above, these genetic mutations are understood to cause and influence the abnormal growth of the cancer cells. Typically, sporadic cancers occur in people when they are older, and there is not a strong pattern of cancer within the family (although because cancer is common, it is not unusual to have some history of cancer in the family). The causes for sporadic cancers are largely unknown, but may include environmental and lifestyle risk factors (e.g. smoking, UV exposure) as well as the natural and unavoidable process of aging.
In about 5-15% of cancers, the underlying cause is due to an inherited gene mutation that increases the risk to develop certain types of cancer during the lifetime. An inherited mutation (passed from parent to child) is present from conception and is found in every cell of the body. Although by itself the mutation is not cancerous, it increases the chance for other random mutations to occur and accumulate in the cells, leading to the development of cancer.
Here are some things that may indicate that someone has a higher risk for hereditary cancer:
A third category for classifying cancer risk is familial cancer. These descriptions may be used when there appears to be more cancer in a family than we would expect by chance alone, but the cause is not identified. There may be a combination of multiple genetic and/or environmental risk factors that are increasing the risk for cancer in the family, or there could be a genetic cause for the cancer that has not yet been identified. Therefore, until more clear answers can be found, the most important message is that the family should be considered at some level of increased cancer risk, and be managed based on their unique history.