When Several People in One Family Develop Cancer

Cancer is common. Roughly one in three people will be diagnosed with cancer at some point. In a large family, several diagnoses can happen with no shared cause at all.

That is not a dismissal of what you are seeing. It is the baseline against which patterns are read. Three relatives diagnosed in their seventies, at three different sites, looks very different from two sisters with breast cancer in their thirties.

Only about 5% to 10% of cancers are caused by an inherited variant. The rest come from DNA damage built up over a lifetime. Those can still cluster in families through shared environment, shared behaviors, or chance.

Genetics services look for specific features rather than a simple count.

• Unusually young ages at diagnosis, above all under 50.
• The same cancer in several close relatives. For example, a mother, a daughter and sisters with breast cancer.
• Cancer across several generations. That fits autosomal dominant inheritance.
• All the cancers on one side of the family, rather than scattered across both.
• More than one separate primary cancer in the same person, or cancer in both of a paired organ.
• Rare tumors, such as male breast cancer, medullary thyroid cancer, retinoblastoma, or sarcoma in a child.
• Known combinations, such as breast with ovarian, colorectal with uterine, or pancreatic with melanoma.
• Ancestry associated with founder variants, such as Ashkenazi Jewish heritage.

None of these is decisive alone. An assessment weighs them together.

Sporadic. The cancers are unrelated to each other. This is common at older ages, at different sites, with no clear pattern.

Familial. There is more cancer than expected, but no single strong gene has been found. The cause may be shared environment. It may be shared behaviors, such as smoking. It may be many small-effect variants, or some mix of these. This group still often qualifies for increased screening.

Hereditary. A single inherited variant with a strong effect. It usually follows autosomal dominant inheritance. That means each child of a carrier has a 50% chance of inheriting it.

Most families who worry about their history fall into the first two groups. Many of those still gain from an assessment.

Write it down first. For each affected relative, note which organ the cancer started in, the age at diagnosis, and which side of the family. Cover three generations if you can: grandparents, parents and their siblings, your own siblings and cousins. Note ancestry too.

The primary site matters more than anything else. Cancer that spread to the bones is not bone cancer, and that distinction changes the whole picture.

Take it to a genetic counselor. Referral is standard, and telehealth appointments have widened access a lot. The National Society of Genetic Counselors keeps a public directory. The assessment itself has value whether or not testing follows.

If someone in the family has cancer now, that is the person to test first. A result from an affected relative tells you far more than one from an unaffected relative. If a variant is found in them, everyone else can be tested for that single change.

An assessment can end in several ways, and none of them is wasted.

You may be offered testing. You may be told your history does not meet criteria, with an explanation of why. You may test negative and still be put on earlier or more frequent screening on the strength of the history alone. Care often follows the family history rather than the test.

Or a variant may be found. That gives relatives something specific to be tested for. It gives everyone options the previous generation did not have.

The only consumer BRCA test authorized in the US checks three variants. It misses about 80% of harmful BRCA variants. A reassuring result from a mail-in kit tells you very little about a family history like the one you are describing.

• NCI — Genetics of Cancer overview
• NCI — Genetic Testing for Inherited Cancer Risk
• National Society of Genetic Counselors — Find a Genetic Counselor