The study, which looked at more than 42,000 children, found that the risk of cancer among young siblings of a cancer patient is modestly higher than normal. However, the researchers say, the added risk evaporates when families with so-called cancer syndromes -- hereditary disorders prone to the disease -- are excluded.
"We were able to look at the family trees that seemed to have an excess" number of cancers, says Dr. John J. Mulvihill, a medical geneticist at the University of Oklahoma and a co-author of the study. "It was clear that, when there was an excess risk in the siblings, it could be interpreted as an underlying genetic syndrome that was rare but that we already knew about," Mulvihill says. Results of the study appear in the current issue of The Lancet.One in 350 children will develop cancer before age 15, Mulvihill says. Each year some 12,400 Americans under age 20 are diagnosed with cancer, and 2,400 die from it, according to the National Institutes of Health.
Mulvihill and his colleagues, led by Dr. Jeanette Falck Winther of the Institute of Cancer Epidemiology in Copenhagen, Denmark, analyzed medical histories of 42,277 brothers and sisters of 25,605 childhood cancer patients from Iceland, Finland, Norway, Sweden and Denmark.
The researchers say they expected to see 284 cancer cases among the siblings, but instead found 353 -- mostly leukemias and brain and spinal cord tumors -- suggesting that siblings were roughly 25 percent more likely to develop the disease than normal. The increased risk was greatest for siblings up to age 9, who were 2.6 times more likely to come down with the disease.
But when Winther's group excluded from their analysis children from 56 families with previously identified cancer syndromes, the added risk disappeared.
"These syndromes that we recognize cause tumors early in life, so that if they don't have a problem in the first five to 10 years, then it won't manifest itself," Mulvihill says. Some syndromes, such as one that causes retinoblastoma tumors of the eye, can raise a sibling's risk of the disease by 500 percent or more.
The researchers say about 40 percent of the cancers found in siblings of cancer patients can be attributed to known genetic causes, while the remaining 60 percent are still unexplained.
While the mystery causes may be part genetic and part environmental, cancer researcher Brad Pollock says making such a definitive split stretches what experts know about the causes of tumors.
"For childhood cancers, you can't really quantify" the contribution of genes, says Pollock, the recently appointed director of the Center for Epidemiology and Biostatistics at the University of Texas Health Science Center at San Antonio.
Ultimately, he says, genetic factors are likely to interact with environmental risks -- such as exposure to a virus in the womb, for example -- in ways that may differ between people, despite their shared genes.
What To Do
Or, for information designed specifically for kids with cancer, and their parents, check out a special Web page provided by the National Cancer Institute.