A Mayo Clinic study has found that more breast cancer patients are opting for mastectomy instead of breast-conserving lumpectomy, although the reasons are unclear.

Researchers looked at mastectomy trends among 5,414 women who had surgery for early breast cancer at the Rochester, Minn. clinic between 1997 and 2006. Mastectomy declined from 44 percent in 1997 to 30 percent in 2003, but then rose to 43 percent in 2006.

One factor influencing the trend was MRI imaging studies. 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 more than doubled from 11 percent in 2003 to 23 percent in 2006.

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

The researchers could not say whether MRI was related to mastectomy because it detected more disease than mammography, or because it created greater anxiety about the true extent of disease. Indeed, mastectomy rates also went up in women who did not undergo MRI.

The study was presented earlier this month at the annual American Society of Clinical Oncology meeting.