The family members of people with cancer commonly worry about the possibility of getting cancer themselves. This article discusses the concept of “relative risk”, and how most individuals with a family history of cancer actually have a lower chance than they think.
As an oncologist, a frequent question posed to me by relatives of someone diagnosed with cancer is “Doctor, does this mean that I have a high risk of getting cancer as well?”
In about 5 to 10% of cancers, there is a strong genetic predisposition to develop cancer. There are certain characteristics in these cases, including:
- Multiple members of the family being involved with the same cancer
- Cancer occurring at an unusually young age
- Particular types/variants of cancer
- Presence of other genetic diseases and syndromes
In these 5 to 10%, the risk of family members developing cancer can be very high, and such people may require genetic counseling, and appropriate risk-reducing strategies. But this is not the topic of discussion for this post.
The remaining 90 to 95% of cancers are sporadic – that is, the predisposition to develop cancer is not inherited.1 But in spite of this, family members of a person with cancer can have a higher chance of developing cancer when compared to the general population. This higher chance is expressed in mathematical terms as “relative risk”.
As an example, let us look at breast cancer, which is the most common cancer in women in the world as well as in India.2 A woman whose mother or sister has breast cancer has approximately two times the risk of developing breast cancer herself.3 That is, her “relative risk” is about 2. Other ways of expressing this are – her risk is doubled, or increased by 100%. But what do these numbers mean in ordinary terms?
A relative risk of 2 (or an increase in risk by 100%) means that compared to the general population, a woman whose mother or sister has breast cancer has twice the chance of developing breast cancer. This may sound scary, but read further!
In India, every year approximately 30 women out of 100,000 are diagnosed with breast cancer.2 So, among women whose mother or sister has breast cancer, this risk is doubled – which means the chances of getting breast cancer are 60 out of 100,000 or 0.06% per year.
Expressed in another way, in a group of 100,000 Indian women, about 30 will develop breast cancer every year. In a group of 100,000 Indian women whose mother/sister had breast cancer, about 60 will develop breast cancer every year. In absolute terms, this is a very small number.
A parent, child, or sibling is a first-degree relative. Aunts, uncles, grandparents, nephews, and nieces are second-degree relatives. Cousins are third-degree relatives. Compared to a first-degree relative, the relative risk is far lower for a second-degree relative and even lower for a third-degree relative. We considered breast cancer as an example because it is the most common cancer in women. For other cancers which are less common, the absolute risk is much lower.
To summarize, apart from the rare hereditary cancer syndromes, the vast majority of cancers are sporadic. The risk of cancer due to a family history is small, and can be estimated by your oncologist.
References:
- Claus, E. B., Schildkraut, J. M., Thompson, W. D., & Risch, N. J. (1996). The genetic attributable risk of breast and ovarian cancer. Cancer, 77(11), 2318–2324. https://doi.org/10.1002/(SICI)1097-0142(19960601)77:11<2318::AID-CNCR21>3.0.CO;2-Z
- Mathur, P., Sathishkumar, K., Chaturvedi, M., Das, P., Sudarshan, K. L., Santhappan, S., Nallasamy, V., John, A., Narasimhan, S., Roselind, F. S., & ICMR-NCDIR-NCRP Investigator Group (2020). Cancer Statistics, 2020: Report From National Cancer Registry Programme, India. JCO global oncology, 6, 1063–1075. https://doi.org/10.1200/GO.20.00122
- https://www.breastcancer.org/risk/factors/family_history