Mastectomies were standard treatment until 1990, when studies showed that women whose cancers were small and confined to the breast did just as well if they had less radical surgery followed by radiation. That set a trend of more women choosing breast-conserving treatment.

Now, a study of about 5,500 women at the Mayo Clinic in Rochester, Minn., shows that mastectomies are on the rise. Doctors say there are signs this is happening elsewhere around the country, too.

"We really don't have a lot of data on why," said Dr. Julie Gralow, a cancer specialist at the University of Washington in Seattle. However, she said, it may be that newer tests like MRI scans are finding more cancers, or flagging so many suspicious spots that women want the breast removed just for peace of mind.

The study was released Thursday by the American Society of Clinical Oncology and will be presented at the group's annual meeting later this month.

In the Mayo Clinic study, about 45 percent of breast cancer patients chose mastectomies in 1997. That declined to only 30 percent in 2003, then started to rise. By 2006, 43 percent were opting for the more radical treatment.

The rise coincided with wider use in the clinic of MRI, or magnetic resonance imaging. Studies show it can detect far more breast abnormalities than ordinary mammograms but it also gives a lot of false alarms.

Women who had MRIs were more likely to choose mastectomy than those who did not have the test, although a rise was seen in both groups.

This could be because MRIs often find additional small tumors in the same breast that had been missed by mammograms, making the more aggressive surgery a better choice, Gralow said.

Women also may choose mastectomy to avoid the need for radiation treatment, which is strongly advised when only a lump is removed. Others may have had genetic testing suggesting they still faced cancer risk. And some may be swayed by better reconstruction techniques available after mastectomy.

However, "even the best reconstructed breast does not have the sensation of the natural breast," said Dr. Nancy Davidson, a cancer specialist at Johns Hopkins University in Baltimore and president of the oncology society.

The study was led by Dr. Rajini Katipamula and Dr. Matthew Goetz at the Mayo Clinic.

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