Breast Cancer Risk with First-Degree Relative
Having one first-degree relative (mother, sister, or daughter) with breast cancer approximately doubles your risk of developing breast cancer, with a cumulative lifetime risk of about 13% compared to 8% in the general population. 1
Quantifying the Risk
The magnitude of risk depends critically on several factors:
- One affected first-degree relative increases breast cancer risk 1.8-fold compared to women with no family history 1
- Cumulative lifetime risk (to age 80) is approximately 13.3% for women with one affected first-degree relative, versus 7.8% in the general population 1
- Risk to age 50 is 3.7% with one affected relative versus 1.7% without family history 1
The absolute numbers matter here: while the relative risk nearly doubles, eight out of nine women who develop breast cancer do not have an affected mother, sister, or daughter 1. Most women with one affected first-degree relative will never develop breast cancer, and most who do will be diagnosed after age 50 1.
Critical Modifying Factors
Age at diagnosis of the affected relative substantially impacts risk:
- If your relative was diagnosed before age 50, your risk increases more significantly than if she was diagnosed at older ages 2, 3
- Risk ratios are greatest at young ages and increase the younger the relative was when diagnosed 1
- A first-degree relative diagnosed before age 45 is a high-risk criterion warranting genetic counseling 3
Number of affected relatives exponentially increases risk:
- One affected relative: 1.8-fold increased risk 1
- Two affected relatives: 2.93-fold increased risk 1
- Three or more affected relatives: 3.9-fold increased risk 1
When to Pursue Genetic Evaluation
You should be referred for genetic counseling if you meet any of these criteria 2, 3:
- Two or more first-degree relatives with breast cancer at any age 3
- One first-degree relative diagnosed before age 45 3
- Any relative with bilateral breast cancer 3
- Male breast cancer in the family 2, 3
- Family history of both breast and ovarian cancer 2, 3
- Ashkenazi Jewish heritage with any affected relative 2, 3
Screening Recommendations
For women with one first-degree relative and no other high-risk features:
- Annual mammography starting at age 40 or 10 years before the youngest affected relative's age at diagnosis, whichever comes first 4
- Lifetime risk typically falls in the 15-20% range (moderate risk), which does not meet the threshold for routine MRI screening 4
- MRI screening is not recommended for women with only one first-degree relative unless lifetime risk exceeds 20% based on formal risk models 2, 4
Risk assessment should use appropriate models:
- The Tyrer-Cuzick model is preferred when family history is the primary risk factor, as it accounts for age at onset and second-degree relatives 2, 4
- The Gail model underestimates risk in women with strong family histories and should not be used when family history is the primary concern 2, 4
Important Caveats
Both maternal and paternal family history matter equally 2, 3. A common pitfall is focusing only on the mother's side while ignoring the father's family history, which can lead to missed hereditary risk 3.
Small family size can mask hereditary risk 3. If you have few female relatives, the absence of breast cancer in the family doesn't necessarily mean low risk 3.
Family history should be reassessed periodically as new cancer diagnoses in relatives change your risk status 3.
The majority of familial breast cancer risk is NOT explained by BRCA1/2 mutations 5, 6. Even when BRCA mutations are excluded, first-degree relatives of early-onset cases remain at 2-5 fold increased risk 7, 6, indicating other genetic and environmental factors contribute substantially to familial clustering.