Even fraternal twins may share cancer risk 

A woman under age 45 with a twin sister who has breast cancer faces roughly eight times the average risk of getting the disease, a new study shows. Moreover, a man whose male twin has testicular cancer confronts nearly 38 times the normal risk for that rare disease.

 Researchers at the London School of Hygiene and Tropical Medicine reached these unsettling conclusions in an analysis of data gleaned from Britain's nationwide birth and cancer registries. After locating twins who were cancer patients in England and Wales between 1971 and 1989, they determined how often both siblings of a pair developed the same cancer.

 In comparing same-sex twins, both identical and fraternal, researchers found that of 301 women between the ages of 20 and 44 whose twins had breast cancer, 22 had the disease. Of 113 men whose twins had testicular cancer, 3 had received the same diagnosis. Both incidences greatly exceed the cancer risk faced by the general population. Among both groups of twins, the onset of cancer at a young age in one twin increased the chance that the other twin would develop the disease.

"This does suggest something prenatal," says epidemiologist Anthony J. Swerdlow, a coauthor of the report in the Dec. 13 Lancet. Both fraternal and indentical twins share the intrauterine environment, although fraternal twins diverge genetically and identical twins do not.

 Swerdlow and his colleagues also found that fraternal twins generally have higher breast and testicular cancer risks than identical twins. While about two-thirds of all twins are fraternal, the data reveal that among the men with testicular cancer whose twin status was known, 85 percent were fraternal. Of the twins with breast cancer, 77 percent were fraternal. 

 The findings hint that estrogen plays a role in cancer risk. Earlier studies relating breast cancer to high birthweight suggested that exposure to high concentrations of estrogen in the womb may increase the risk of cancer later in life (SN: 2/15/97, p. 108). Women pregnant with twins have high concentrations of estrogen, and some data suggest that women carrying fraternal twins have even higher amounts, says Neil E. Caporaso, an epidemiologist and oncologist at the National Cancer Institute in Bethesda, Md. The new study provides only indirect support for the estrogen theory, Swerdlow says, since "there might be other factors."

 In any case, the increased risk of cancer stemming from being a twin seems to decline with age for both breast and testicular cancer, Swerdlow says.

 The study "is a milestone," says Dimitrios Trichopoulos, an epidemiologist at Harvard University School of Public Health in Boston. "It's the best study on this subject that's ever been done."