Cancer is a relatively common disease. While many people may be diagnosed with cancer or have a family member with cancer, most of these cases are considered “sporadic”. Sporadic cancers are isolated occurrences caused by a combination of factors such as the natural processes of aging, environment, and lifestyle. In families with sporadic cancers, the age at diagnosis of cancer are usually later, there are few family members with cancer, and the types of cancer in the family are varied and unrelated. Family members have the same chance of developing cancer as the average person in the general population.
A small number of cancers are called “hereditary”, meaning that they are caused by a single major genetic risk factor that is passed down from generation to generation. Hereditary cancer accounts for 5-10% of most cancers, such as breast cancer and colon cancer. Some types of cancer are more likely to be hereditary. For example, 20-25% of ovarian cancers are caused by inherited risk factors. In families with hereditary cancer, we often see cancers diagnosed at young ages, multiple people with the same or related cancers, and we may even see some people who have developed more than one separate type of cancer. With hereditary cancer, some family members have a much higher chance of developing certain types of cancer than other people. It is important to find families that have a major inherited risk factor, because testing can identify which family members are at higher risk and which are not. This information can then be used to personalize options for early detection or prevention of cancer.
There are also families where the pattern of cancer seems neither sporadic nor clearly hereditary. These families are sometimes described as having “familial cancer,” or a “medium-“ or “moderate-“ risk of cancer. For people in these families the chances of getting cancer are a little higher than the average person. The cause of cancer in these families is “multifactorial,” meaning the cancer is caused by a combination of minor genetic and environmental risk factors.