A Mayo Clinic study has found that more patients with early breast cancer are opting for mastectomy instead of breast-conserving lumpectomy, although it's unclear why.

Mastectomy at the Rochester, Minn., clinic declined from 44 percent in 1997 to 30 percent in 2003, but then rose to 43 percent in 2006.

One factor: MRI, which detects both cancerous and benign lesions, is now recommended to supplement mammography for some women at high risk of breast cancer. The fraction of women who had MRIs rose from 11 percent in 2003 to 23 percent in 2006.

More than half the women who had an MRI before surgery chose mastectomy, compared with 41 percent who did not have the scan.

But the researchers could not determine the reason.

- Marie McCullough