When Janet Halloran last saw her primary care physician, the doctor asked whether she had undergone her annual mammogram. Yes, she replied, she had.

At 76, Ms. Halloran, a real estate broker in Cambridge, Mass., is past the age that most medical guidelines recommend breast cancer screening for someone with no history of the disease. Even for younger women, the guidelines call for a mammogram every other year, not annually.

So Ms. Halloran could consider stopping mammograms, or at least having them less often. But her doctor has never discussed that prospect. “She says, ‘These are the things you need to do,’” Ms. Halloran said. Besides, she added, it’s an easy test: “Go once a year, hold your breath and you’re done for another year. It’s just routine.”

But for older women, should it be?

“There’s been a lot of uncertainty,” said Dr. Xabier Garcia-Albéniz, an oncologist and epidemiologist at RTI Health Solutions and lead author of a new observational study that tries to answer that question. “This is an area with a complete lack of randomized clinical trials.”