If One Twin Has Cancer, Will the Other?

Most healthy twins of a person with cancer never develop cancer themselves. Their risk is raised, though: about 46% over a lifetime for an identical twin and 37% for a fraternal twin, against about 32% for people in general. Genes explain only about a third of why people get cancer, and the rest comes from life, chance and the body’s own random changes. The extra risk is higher for some cancers, such as prostate cancer and breast cancer at young ages. In one unusual situation, childhood leukemia in young identical twins who shared a placenta, the cancer itself can pass from one twin to the other before birth.

The Healthy Twin’s Lifetime Odds

Identical twins come from one fertilized egg and share nearly all their genes. Fraternal twins come from two eggs and share about half, like ordinary siblings. Across a large group of Nordic twins, about 1 in 3 people developed some cancer by age 100.1Harvard Gazette. When one twin gets cancer, other faces higher risk When one twin had cancer, the other’s lifetime risk was 37% for fraternal twins and 46% for identical twins.2PubMed Central. Familial Risk and Heritability of Cancer Among Twins in Nordic Countries That puts an identical twin’s risk about 14 points above average, and a fraternal twin’s about 5 points above.3PubMed Central. Familial Risk and Heritability of Cancer Among Twins in Nordic Countries

Reading 46% the right way matters. It means more than half of identical co-twins never get any cancer. Part of that figure is the ordinary risk everyone carries, and it covers all cancers combined, not only the type your twin has.

The gap between 46% and 37% exists because identical twins share nearly all their DNA, so more of an identical twin’s inherited risk matches their sibling’s. Fraternal twins still sit above average, which fits with ordinary siblings sharing some inherited risk and often growing up in the same home. When both twins did get cancer, it was the same type in 38% of identical pairs and 26% of fraternal pairs.4Harvard Gazette. When one twin gets cancer, other faces higher risk

These lifetime figures run to age 100, so the risk in any single year is much lower. A young co-twin has most of those years still ahead, and someone in their thirties faces a far smaller near-term chance than the lifetime number suggests.

If you aren’t sure whether you’re identical, some same-sex twins were told the wrong thing at birth, often because of how the placenta looked or because nobody tested them. A cheek-swab DNA test, sold by clinics and by some consumer genetic companies, can confirm it. Which odds apply to you depends on that answer, so it is worth settling before you and your doctor decide how closely to watch your risk.

Why Shared Genes Don’t Mean a Shared Fate

Across cancers overall, genes accounted for about 33% of the differences in who got cancer, and shared upbringing added no clear effect.5PubMed Central. Familial Risk and Heritability of Cancer Among Twins in Nordic Countries Habits still count. In identical twin pairs where only one twin smoked, the smoking twin had about 1.7 to 1.9 times the risk of several cancers other than lung cancer.6PubMed Central. Cancer in twin pairs discordant for smoking: The Nordic Twin Study of Cancer

Chance plays a large part too. Cells divide again and again across a lifetime, and each time the DNA is copied there is a small chance of a mistake. Most are caught and fixed or are harmless, but occasionally one lands in a gene that controls cell growth. These errors happen at random, so two people with identical DNA collect different ones in different cells.

Bodies also drift apart over time. Even identical twins build different immune systems, and common infections can change more than half of the measured immune features.7PubMed Central. Variation in the human immune system is largely driven by non-heritable influences Chemical tags on DNA that switch genes on and off differ between twins, and they can shift before cancer appears.8PubMed Central. Integrative DNA methylome analysis of pan-cancer biomarkers in cancer discordant monozygotic twin-pairs Identical twins who kept four healthy habits had slower “biological aging” of their DNA than their less healthy twins.9PubMed Central. Combined healthy lifestyle score and risk of epigenetic aging: a discordant monozygotic twin study

The same goes for how a cancer behaves once it appears. Immune and tissue differences can change its course, so don’t expect your twin’s experience with treatment to predict your own.

Cancers That Run Most Strongly in Twins

Heritability is how much of the difference in who gets a cancer comes from genes. For several cancers, twin studies put it like this:

These numbers describe whole groups of people, not any one person’s odds. A 58% figure does not mean a 58% chance of getting the cancer. The cancers at the top are where an affected twin’s diagnosis says the most about the co-twin’s inherited risk, but cancers lower down still carry a genetic link. Ask your doctor about screening for your twin’s specific cancer type.

Prostate cancer shows the twin effect clearly. In one study of veteran twins, about 27% of identical co-twins of men with prostate cancer also got it, compared with 7% of fraternal co-twins.17The Prostate. Heredity and prostate cancer: A study of World War II veteran twins When both identical twins got it, the second diagnosis came a median of 3.8 years after the first, compared with 6.5 years in fraternal twins.18PubMed Central. The Heritability of Prostate Cancer in the Nordic Twin Study of Cancer

For breast cancer, identical co-twins of women with the disease got it at about 4.4 times the usual rate, and fraternal co-twins at about 1.7 times.19PubMed Central. Heritable breast cancer in twins The extra risk was highest when the first twin was diagnosed before 40, and it was greatest in the following five years.20PubMed Central. Heritable breast cancer in twins Identical co-twins who reach 75 have at least a 44.5% chance of breast cancer.21PubMed Central. Heritable breast cancer in twins Earlier puberty was also linked to earlier diagnosis in genetically prone twins.22PubMed. Puberty and genetic susceptibility to breast cancer in a case-control study in twins

Childhood Leukemia in Young Twins

Twins who develop leukemia together often carry exactly the same leukemia cells.23PubMed. Leukemia in twins: lessons in natural history The cells start in one twin in the womb and cross to the other through blood vessels that join them in a single shared placenta.24PubMed. Leukemia in twins: lessons in natural history In one pair of identical twins, the same leukemia gene change was found at diagnoses about 3.5 and 4.8 years of age, and it traced back to before birth.25PubMed. Fetal origins of the TEL-AML1 fusion gene in identical twins with leukemia In rare cases, other cancers in newborn twins have also formed in one twin and moved to the other.26PubMed Central. Embryonic origin of cancer in newborn twins

This differs from other cancers in twins, where the cancer is not passed along. Here the same cancer cells travel from one twin to the other before birth.

Only identical twins who shared one placenta have blood vessels that join, and this joined circulation is what lets leukemia cells cross. Identical twins with their own placentas, and nearly all fraternal twins, do not share blood this way. Where the placenta was shared, the healthy twin may carry the same early leukemia cells without being sick yet.

Doctors first check whether the twins are identical and whether they shared a placenta.27The Institute of Cancer Research. Identical twin study sheds new light on the origins of childhood leukaemia They can then test the healthy twin’s blood about every three months for early leukemia cells.28The Institute of Cancer Research. Identical twin study sheds new light on the origins of childhood leukaemia The risk is highest in the first year after the first twin’s diagnosis and falls after that, so screening a healthy twin after about age 10 is probably unnecessary.29The Institute of Cancer Research. Identical twin study sheds new light on the origins of childhood leukaemia

Parents can ask the care team a few specific questions. Did our twins share one placenta? Birth records may say. Should the healthy twin have blood checks, and how often? Which symptoms should we watch for, such as tiredness, bruising or fever?

Screening and Genetic Testing for the Healthy Twin

Some cancers come from an inherited gene change that is present in every cell, such as a change in BRCA1.30PubMed. Breast cancer in monozygotic twins Testing the twin who has cancer can show whether the healthy twin should be tested too. Doctors usually test the sick twin first, because a result in the person with the disease is the clearest evidence of an inherited cause. If a harmful change turns up, the healthy twin can be tested for that specific change, which is simpler than searching broadly. In one case, testing an identical twin for the DICER1 gene was a key step in her care.31PubMed. Case report: an identical twin with Sertoli-Leydig cell tumor If nothing is found in the sick twin, that does not rule out every inherited cause, so the healthy twin still needs a sensible screening plan.

Results need expert interpretation, because a gene change can show up only in a tumor or only in blood cells.32PubMed Central. Somatic TP53 variants frequently confound germline testing results Most inherited breast cancer in twins also seems to come from several genes acting together, not from one gene.33PubMed Central. Heritable breast cancer in twins A positive result means a higher risk, not that cancer will develop, and it usually leads to closer screening or preventive options. A negative result in the healthy twin is most reassuring when the sick twin’s specific gene change is known and the healthy twin lacks it.

A genetic counselor can help here. They review your family history, explain what a test result would mean for you, and help you decide whether testing is worth it. Your doctor can refer you.

Standard screening rules may miss twins at risk. In one reported case, a non-smoking identical twin developed lung cancer after her sister, and earlier screening might have caught it sooner.34Clinical Medicine. Lung cancer in non-smoking identical twin sisters: highlighting the need for reassessing screening strategies So ask whether you should start your twin’s cancer screening earlier or have it more often. Keep healthy habits too, especially not smoking.35PubMed Central. Cancer in twin pairs discordant for smoking: The Nordic Twin Study of Cancer

When you see your doctor, say what cancer your twin has and the age at diagnosis, and whether you are identical or fraternal. Ask about mammograms, PSA tests or skin checks, depending on your twin’s cancer.