Most people I see in clinic with a new cancer diagnosis come in not knowing what their grandfather died of. Or they have a vague sense — "something with the stomach, in his sixties" — but the details have been smoothed over by time, distance, and the discomfort of asking. By the time the question becomes urgent, the people who would have known are no longer available to ask.
This is one of the easier mistakes to fix. It only requires a Sunday afternoon and three questions.
The science behind it is well established. Roughly five to ten per cent of all cancers are driven by an inherited mutation that meaningfully shifts a person's lifetime risk. For specific cancers — ovarian, pancreatic, certain kinds of breast and colon cancer — the figure is higher. We can identify many of these inherited risks today through targeted genetic counselling and testing. The trouble is that the testing pathway begins with a question almost no one asks: what cancer ran in my family?
Why family history matters for screening
Cancer screening guidelines for the general population are calibrated for average risk. Mammography starts at 40. Colonoscopy at 45. PSA discussion at 50. These ages assume nothing unusual in the family.
If you have a first-degree relative — a mother, father, sibling — who developed cancer at an unusually young age, the math changes. The standard adjustment is to begin screening at the age your relative was diagnosed minus ten years, or at the standard age, whichever is earlier. A woman whose mother had breast cancer at 38 should not be waiting until 40 to start mammography. She should be starting at 28, with MRI alongside mammogram for her age. That's a twelve-year shift, made on a single piece of family information.
For some patterns — especially clusters of breast and ovarian cancer, or early-onset colon cancer — the conversation goes further. Genetic counselling becomes the right next step. Not because everyone needs a test, but because a thirty-minute conversation with someone trained to read family trees often saves years of unnecessary worry, or, occasionally, a life.
Three questions to ask
You don't need a family medical historian. You need three answers. Ideally from a parent, an aunt, or a grandparent — someone who would know.
1. At what age did anyone in the family have cancer of any kind?
The age is the single most important number. Cancer in a relative who lived past seventy-five is largely an age story. Cancer in a relative under fifty is more often a biology story. The earlier the diagnosis, the more weight it carries. A grandfather diagnosed with prostate cancer at 82 is not a screening signal for you. A maternal uncle diagnosed at 52 is.
2. Was there breast or ovarian cancer in two or more relatives, especially before age 50?
This is the BRCA pattern. BRCA1 and BRCA2 mutations significantly raise lifetime risk of breast, ovarian, and (in men) prostate cancer. The pattern of inheritance is autosomal dominant — meaning every child of a carrier has a 50% chance of inheriting it. So clusters tend to show up. If two women on the same side of the family had breast or ovarian cancer before fifty, BRCA testing is the conversation.
Importantly, BRCA can come down through the father's side. Many families miss this because the father may have been a carrier without ever developing cancer, or the carrier-related cancers in his side of the family were dismissed as "just old age." Look at both sides.
3. Was there bowel cancer, particularly under 60?
Early-onset colorectal cancer — under 50 in the relative — is the strongest signal for Lynch syndrome, a hereditary condition that raises lifetime risk of colorectal cancer as well as endometrial, ovarian, gastric, and several others. Lynch syndrome is more common than people realise, and the screening protocol it triggers — colonoscopy every one to two years, plus surveillance for the other connected cancers — is one of the highest-yield interventions in preventive medicine.
If a parent, grandparent, or sibling was diagnosed with bowel cancer before sixty, the conversation about Lynch testing is worth having. It's also worth having if multiple Lynch-related cancers showed up across the same side of the family, even if none individually were before sixty.
Three answers can change the math. Sometimes by a decade.
What "yes" means
If the answer to any of those three questions is yes, the next step is not panic. It is a single conversation with a doctor or a genetic counsellor. Most of the people who walk into that conversation walk out reassured — the family history, examined carefully, doesn't quite fit a hereditary pattern, and standard screening on a slightly accelerated calendar is enough.
A smaller number walk out with a referral for genetic testing. The test itself is a blood draw. The result, when it comes back, is a number that meaningfully changes the shape of someone's screening, and sometimes the shape of their treatment if cancer ever does come.
For BRCA carriers, for example, modern guidelines offer a wide menu: intensified screening with annual breast MRI alongside mammography, the option of risk-reducing salpingo-oophorectomy (removal of ovaries and tubes) once childbearing is complete, and access to PARP-inhibitor maintenance therapy if cancer does ever develop. None of these options are available without the test result. The test result is not available without the question.
What "I don't know" means
Most families have at least one branch where the answer to any of these questions is "I don't know." A grandparent died young, decades ago, before the diagnostic infrastructure existed to know what they died of. The death certificate, if it exists, says "natural causes." The phrase "she had a tumour" is all that survived.
This is fine. The test of a useful family history isn't completeness — it's a search for clusters. If three of the four branches of your family don't have meaningful cancer history at any age, the absence of information from the fourth branch is not a strong screening signal on its own. If two branches are clean and one is full of early-onset breast and ovarian cancer, the unknown branch matters less.
What to do today
If you're reading this on a Sunday and someone in your family is reachable by phone, this is the conversation. It's awkward exactly once. After that, the answers live in your medical record and shape every screening conversation you have for the rest of your life.
Write down what you find out. Cancer name, age at diagnosis, which side of the family. Bring the list to your next doctor's visit. If anything in it suggests a cluster, ask whether genetic counselling is worth a conversation.
Today is a good day to ask.