Every physician knows how important family history can be to assessing risk for cancer as well as heart disease, diabetes and a myriad of chronic illnesses. An excellent method for computing the impact of familial risk is to study twins and this is exactly what was done in the Nordic twin study that was just published in the January 5th issue of JAMA.
The researchers estimated familial cancer risk from a study of both identical (termed mono zygotic or from the same egg) and fraternal twins (dizygotic or from two eggs) twins born in Denmark, Finland, Norway and Sweden. The medium follow-up of the twins was 32 years! The study included 357,377 individual twins. Unlike other developed countries (including the US), residents in Nordic countries each have a unique national registration number and are linked to national cancer registries, mortality registers and of course twin registers (if they are twins). Physicians and pathologists are mandated by law to report every newly diagnosed malignant tumor. This means that case reporting is close to 100% complete.
During the 32 years of follow-up 27,156 cancers were reported in 23,980 individuals of the twin pairs. Cancer was diagnosed in both twins among 1383 monozygotic and 1933 dizygotic pairs. What’s amazing is that 38% of the identical twins and 26% of the fraternal twins were diagnosed with the same type of cancer. There was also an excess all-cancer risk in twins whose co-twin was diagnosed with cancer. The estimated cumulative (for a lifetime) cancer risk was 5% in fraternal twins and 14% in identical twins. The types of cancers with the highest estimated incident in the second twin were prostate, breast, lung, melanoma, ovary and uterus cancer.
There was also an increased risk of getting a different type of cancer if one of the twins had cancer. Among the pairs of twins in which both members developed cancer more than two thirds were diagnosed with a different malignancy. It often took years, however for cancer to appear in the second twin. It is thought that there may be an aggregation of genes that makes a co-twin more at risk for cancer susceptibility from environmental causes (such as smoking, obesity, poor nutrition and pollution). Identical twins will have identical genes, fraternal twins share many but not all, as do siblings born at different times. The authors conclude that “there was a significant excess familial risk for cancer overall …this information about hereditary risk may be helpful in patient education and cancer risk counseling.”
Put succinctly, make sure you are aware of your family history and be sure to tell your physician. The history you know may help save your life!