When it comes to elective surgery in the United States, where patients live and which doctors they see play a big role how they are treated, U.S. researchers said on Thursday.

The study of elective procedures among patients over age 65 covered by Medicare, the government health insurance program for the elderly, found wide regional differences in the way U.S. doctors treat patients, suggesting that patient preferences are often being ignored.

For example, an elderly woman in Victoria, Texas, who has early breast cancer is seven times more likely to have a mastectomy than a woman living in Muncie, Indiana.

An elderly man with early-stage prostate cancer who lives in San Luis Obispo, California, is 12 times more likely to have surgery to remove his prostate than a man in Albany, Georgia.

"These striking variations are the by-product of a doctor-centric medical delivery system," Shannon Brownlee of the Dartmouth Institute for Health Policy and Clinical Practice, who led the study, said in a statement.

The wide variation in care suggests "patients' preferences are not always taken into account when medical decisions are made," she said.

The report is the latest from the Dartmouth Atlas Project, which last year uncovered wide differences in spending by region, a finding that became a touchstone in the debate about the need for healthcare reform in the United States.

In this study, researchers analyzed Medicare data from 2003 to 2007 on the rates of various elective procedures, including mastectomy for breast cancer; coronary artery bypass surgery; back surgery; knee and hip joint replacement; carotid artery surgery; radical prostatectomy for prostate cancer; and prostate cancer screening.

"What we find is physicians differ very strongly in their opinions about the value of these procedures," said Dr. David Goodman, co-leader of the Dartmouth Atlas Project.

"There are regional differences or differences in cultures of care that develop partly related to how physicians are trained or the history of the place."

Dr. Michael Barry, president of the Foundation for Informed Medical Decision Making who helped lead the study, said the findings suggest patients are often not full partners in making decisions about how they want to be treated.

"We found patients were really ill-informed. They weren't asked their opinions as often as they should have been. Doctors were often assuming they knew patient preferences rather than asking," said Barry.