Not all women who are carriers of the BRCA 1 and BRCA 2 genetic mutations show the same risk for developing breast cancer, according to a study published in the January 9/16 issue of the American Medical Association.

Many women fear that once they are found as carriers of the genes they also can develop breast cancer. Health officials inside the American Society of Breast Surgeons still believe that a woman has about an 80 percent lifetime risk of developing breast cancer if she carries one of the mutations.

The new study comes to deny the previous ones, finding a wide variation in risk among families carrying mutations, indicating that the mutations are also influenced by other genetic and possibly environmental factors.

“Risk among carriers varies considerably, and, if you accept that that's the case, there must be a reason why it varies, and the most plausible explanation is that there are other genetic factors affecting breast cancer risk that are being passed on through these families,” said study author Colin Begg, chairman of the department of epidemiology and biostatistics at Memorial Sloan-Kettering Cancer Center in New York City.

In conclusion, a woman who knows she has a BRCA1 or BRCA2 mutation still cannot know precisely if she will develop breast cancer during her lifetime, the researchers said.

“Because if some carrier families have higher risks than other carrier families presumably there are other genes being passed through these families that elevate or lower the risks,” Begg said.

Begg and his team based their results on the study of 2,000 women in the Women’s Environmental Cancer and Radiation Epidemiology Study who were diagnosed with breast cancer and the families of 181 patients who had BRCA genetic mutations. All patients had been diagnosed early, before the age of 55.

The researchers discovered that 5 percent of the women with cancer in one breast had a BRCA1 or BRCA2 mutation and 15 percent of those with cancer in both breasts did. However, only 25 percent of all the patients had a close relative with breast cancer and 58 percent of those with BRCA1 or BRCA2 mutations had a mother, sister or other close relative with the disease.

Age also mattered in the study. Younger breast cancer patients were more likely to have a mother, daughter or sister with a history of breast cancer. Besides the two genes, there were other genes, which seem to play an important role in developing breast cancer. The researchers also blamed environmental and lifestyle factors to increase women’s chance of developing breast cancer.

All these results led researchers to conclude that a close relative of a BRCA carrier with breast cancer has a 40 percent risk of developing the disease herself by age 70. However, this is not information that applies to all cases, the researchers finding a considerable variation in risk from one family to another.

“There is a perception out there that all that matters is if you’re a carrier or not, and if you’re a carrier that you’re virtually certain to get breast cancer. What I think is important is that when a woman makes decisions, she should make them fully informed of what her risks are really likely to be,” said Begg.

About 180,000 U.S. women were diagnosed with breast cancer last year, the most common tumor in women and 41,000 died from it, according to the American Cancer Society.