CHICAGO - A deadly gene's path can hide in a family tree when a woman has few aunts and older sisters, making it appear that her breast cancer struck out of nowhere when it really came from Dad.

A new study suggests thousands of young women with breast cancer - an estimated 8,000 a year in the U.S. - aren't offered testing to identify faulty genes and clarify their medical decisions.

Guidelines used by insurance companies to decide coverage for genetic testing should change to reflect the findings, said study co-author Dr. Jeffrey Weitzel of City of Hope Cancer Center in Duarte, Calif. Testing can cost more than $3,000.

"Interestingly, it's about Dad," Weitzel said. Half of genetic breast cancers are inherited from a woman's father, not her mother. But unless Dad has female relatives with breast cancer, the faulty gene may have been passed down silently, without causing cancer. (Men can get genetic breast cancer, too, but it's not common.)

Weitzel said doctors often overlook the genetic risk from the father's side of the family.

The study, appearing in Wednesday's Journal of the American Medical Association, looked at the genetic test results from 306 women diagnosed with breast cancer before age 50.

None of the cancer patients in the study had a family history of breast or ovarian cancer.

Among the women with plenty of female relatives, about 5 percent had BRCA gene mutations. But among those with few sisters and aunts older than 45 (when breast cancer would be likely to appear), almost 14 percent had mutations of the genes BRCA1 or BRCA2. That suggests that these cancer patients were unaware of their genetic mutations because there were so few women in the family to signal a cancer risk.

The researchers defined few female relatives as fewer than two on either the father's or mother's side of the family.

Women who were adopted and don't know their family medical history should be aware of the findings, Weitzel said. Women whose female relatives died young before breast cancer had time to show up also are affected.

When such a woman gets breast cancer before age 50, she should get a genetic test, said Dr. Noah Kauff, a cancer geneticist at Memorial Sloan-Kettering Cancer Center in New York. That would help her decide whether to have the unaffected breast or her ovaries removed to prevent more cancer. Kauff was not involved in the research, but wrote an accompanying editorial.

"The study allows physicians and patients to make an argument to insurance carriers that, although there's not a family history of breast cancer, it's still reasonable to test and it should be a covered benefit," Kauff said.

Genetic testing helps a woman choose her next medical steps. A woman with breast cancer who has a BRCA gene mutation has a four times greater risk of developing cancer in the other breast and a 10 times greater risk of ovarian cancer than does a woman with breast cancer who has no BRCA gene mutation.

Some women with a family history of breast cancer choose to have a BRCA genetic test so they can decide whether to reduce their cancer risk by removing their ovaries and breasts before any cancer appears. Drug therapy and monitoring with annual MRI tests offer alternatives.

Testing the genes of more women would cost more money, but Weitzel said that won't add significantly to health care costs and will prevent cancer in some of the women.

The study also showed that three commonly used predictive models don't accurately estimate the genetic breast cancer risk for women without a family history of cancer. The American Cancer Society recently based its recommendation for annual MRIs on risk assessments from the predictive models.

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