CHICAGO (Reuters) - Women with high-risk genetic mutations who have their ovaries and fallopian tubes removed lower their risk of cancer in those organs by about 80 percent but can still be afflicted, US researchers said last week.

They analysed pooled data from several studies of women at high risk for the cancers because of faulty copies of the BRCA1 and BRCA2 genes.

They found the surgery cut a woman's risk of breast cancer by about 50 percent. But it only reduced the risk of ovarian or fallopian cancer by 80 percent, because even though the organs had been taken out, small remnants of tissue may have been left behind.

"It only takes a few cells to result in an ovarian cancer," said Timothy Rebbeck of the University of Pennsylvania in Philadelphia, whose study appears in the Journal of the National Cancer Institute.

"It's really interesting. It's telling us that removing your ovaries even before you know there might be cancer there only reduces risk by 80 percent," Rebbeck said in a telephone interview.

"While that is a huge amount of risk reduction, it's still not 100 percent. Earlier studies suggested it was much higher, like 95 percent to 100 percent, but it's really not."

Rebbeck said the findings may suggest that women with these rare and highly risky mutations are waiting too long to have the surgery or that surgeons might need to do more to ensure all cells from the ovary and fallopian tubes are removed.

He and colleagues set out to get a better estimate of how the surgery — known as salpingo-oophorectomy — improves a woman's odds at avoiding a future cancer.

Based on data from 10 published studies, they found the surgery reduced the risk of ovarian and fallopian tube cancers by 79 percent and the risk of breast cancer by 51 percent.

"It allows us to have a better estimate of risk reduction than we've ever had before," he said.

The reduction in breast cancer risk was similar regardless of whether a woman had a BRCA1 or BRCA2 mutation. The study population was too small to separate out the effects on ovarian and fallopian tube cancers by mutation.

Having a BRCA1 or BRCA2 mutation raises the risk of breast cancer by 50 percent to 80 percent and of ovarian cancer by between 16 and 60 percent.

The researchers said the findings raise the question how to lower risk more, either by improving the surgical technique or encouraging women to have surgery sooner.

"We have really strong evidence that this works, but we need to get women to use the surgery and to use it appropriately,' Rebbeck said.