When more than one person in a family is afflicted with cancer, some may think that the cause is due to heredity, especially if the cases are repeated in individuals from different generations, but the presence of a family history of infection does not necessarily mean that there is an inherited gene that causes the disease, and the presence of a genetic change that increases the risk of infection does not mean the inevitability of cancer.
Cancer arises due to genetic changes that occur within cells, but these changes are not always inherited. They may appear in a person’s life as a result of environmental factors or natural errors that occur during cell division, while some people inherit genetic changes that make them more susceptible to cancer.
The US National Cancer Institute explains that what can be passed from parents to children is the genetic predisposition that increases the risk of cancer, not the cancer itself.
Hereditary cancer is less common than expected
Dr. Tawfiq bin Omran, Head of the Department of Genetic and Genomic Medicine at Sidra Medicine Hospital in Qatar, says that most cancer cases are caused by non-genetic factors, including aging, smoking, obesity and other risk factors, while cancers associated with inherited genetic changes represent a limited percentage of cases.
The US National Cancer Institute also indicates that about 5% to 10% of all cancers are believed to be caused by genetic changes inherited from a parent.
This is consistent with what was reported by a scientific review published in the journal “Genes” in 2023, indicating that hereditary cancer syndromes represent about 10% of all cancer cases.
Discovering the genetic change associated with these syndromes helps determine the level of infection risk and develop appropriate plans for follow-up and preventive examinations for the patient and his family members.
Bin Omran added that genetic predisposition does not mean inheriting the disease itself, but rather the presence of genetic changes that may increase the probability of infection. Even in hereditary cancer syndromes, infection is not inevitable, and genetic counseling, examinations, and appropriate follow-up can help reduce this risk.
Why does the disease recur within the family?
Cancer may recur among family members without a known inherited genetic change being found. Bin Omran explained that the presence of more than one condition in a family does not necessarily mean the existence of a common genetic cause, as there may be an overlap between genetic, environmental and behavioral factors that family members share.
The US National Cancer Institute explains that the disease may appear to be familial due to family members being exposed to common environmental factors or adopting similar behaviors, such as smoking. Moreover, more than one individual may be affected by a common cancer without the cause being an inherited genetic change. A group of genetic variants may also contribute, each of which adds a small increase to the risk.
Hence, doctors differentiate between cancer resulting from a known inherited genetic change, and recurrence of infection within a family without a specific genetic cause that explains this recurrence. Therefore, the number of infections within a family is important in assessing risk, but it is not sufficient alone to rule that cancer is hereditary.
When does family history become significant?
Bin Omran added that the possibility of a hereditary factor becomes more apparent when the disease appears at an early age, especially before fifty, or infections are repeated in several first-degree relatives, or cases appear in more than one generation, or one person develops more than one independent cancer.
He said that genetic counseling and genetic evaluation are recommended when there is a strong family history of the disease, or when a known inherited genetic change is discovered in a relative, because early evaluation can help determine the need for examination and develop an appropriate plan for follow-up and prevention.
These indicators are consistent with what is reported by the American National Cancer Institute, which considers infection at a younger age than usual, the presence of several affected relatives, the appearance of more than one cancer in the same person, or knowledge of the presence of a genetic change that increases the risk of infection in the family, are among the factors that may require discussion of genetic counseling and testing with specialists. When possible, it is preferable to start screening with a family member who has previously had cancer, because the result is more useful in explaining the risk among the rest of the family members.
Genes do not work alone
Bin Omran stressed that the risk of cancer is formed by a complex interaction between inherited genes, environmental factors, and lifestyle. Genetic predisposition may increase the risk of infection, but the appearance of the disease remains linked to other factors as well.
The importance of these factors is highlighted in a study by researchers from the American Cancer Society, published by CA: A Cancer Journal for Clinicians in 2024, which estimated that about 40% of cancer cases in adults aged 30 years and over in the United States in 2019 were associated with risk factors that can be avoided or reduced. Cigarette smoking was the largest factor, followed by excess weight, in addition to other factors whose impact can be reduced.
This does not mean that changing lifestyle can eliminate genetic predisposition or prevent cancer with certainty, but it shows that genes are not the only factor that affects the risk of infection. Therefore, prevention and a healthy lifestyle remain important, even in people with a family history of cancer.
What does the genetic test say?
Genetic screening is not a test that definitively predicts who will develop cancer, but rather looks for known inherited changes associated with increased risk. Its result can help doctors estimate the risk of infection and determine appropriate follow-up, and may also provide important information for other family members.
But a negative result does not always mean that the risk has returned to the usual level, especially when the family history is clear and the examination does not reveal a known genetic change. The cause of recurrence of the disease within the family may not yet be known, so follow-up remains linked to personal and family history and other risk factors.
Thus, the recurrence of cancer within a family does not carry a prejudgment on the rest of its members or subsequent generations, but it does carry a sign that deserves to be read medically, not with fear. Knowing who was infected, at what age, and degree of kinship may transform family history from a source of concern into a tool that helps the doctor estimate the risk, determine who needs genetic counseling or closer follow-up, and who is sufficient for the usual prevention and follow-up measures.