Double diagnosis: a twin is at higher risk if co-twin has cancer

If a twin – fraternal or identical – has cancer, a co-twin faces an increased risk of developing cancer, according to a new study. In addition, there is a greater risk for certain types of cancer including prostate, melanoma, breast, ovary, and uterus.

The long-term follow-up study examined data for more than 203,000 Nordic twins for more than three decades, examining cancer rates, genetics and environmental risks.

The research team examined cancer rates among Monozygotic (identical twins – born from one egg) and Dizygotic (fraternal twins – born from two separate eggs).

A total of 27,156 cancer cases were diagnosed in the twin study population. Which means overall there was a 32 per cent cancer incidence.

If one twin was diagnosed with cancer the risk of the co-twin getting the same kind of cancer was:

• 38 per cent of identical twins.
• 26 per cent of fraternal twins.

“Twenty of the 23 cancers we looked at, we see an increased risk of cancer, greater than just the average risk if their brother or sister also developed that cancer ,” Lorelei Mucci, of the Harvard T. H. Chan School of Public Health told Global News.

Mucci said the cancer incidence could be due to shared genetic and environmental factors.

“That familial, that shared family effect that we see is either due to genetic factors or shared environment, ” said Mucci.

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The study explored how much the cancer was due to genetic factors or environmental factors, like smoking or living conditions during childhood. What they found was “it really depended on the cancer.”

“For example, in prostate cancer and interestingly for melanoma as well, we see a very strong genetic effect, “ said Mucci.

The difference in risk for those cancers could be attributed to inherited genetic factors, included 58 per cent for skin melanoma and 57 per cent for prostate, according to Mucci.

As opposed to lung cancer diagnosis, where genetics played a small role, and shared environment played a significant role, most likely due to smoking.

The results were published in JAMA.

“This information about hereditary risks of cancers may be helpful in patient education and cancer risk counseling,” the authors write.

Mucci added even though genes and family history play an important role it is important to focus on the factors people can modify to reduce cancer risk and increase prevention like diet, smoking and exercise.