Women aged 40 to 49 should talk to their doctors about their individual risk for breast cancer when deciding whether to start getting mammograms, according to new U.S. guidelines released on Tuesday.

Annual mammograms are recommended for older women, but conflicting evidence means there is no one-size-fits-all approach for those in their 40s, the American College of Physicians said.

"We designed our screening mammography guideline based on scientific evidence," said Dr. Lynne Kirk, the president of the college.

"It will empower women between the ages of 40 and 49 to become part of the decision-making process and to encourage them to discuss with their physicians the benefits and risks of mammograms."

A mammogram is an X-ray that can detect small tumours in the breast. The screenings have helped to reduce death rates for breast cancer, according to a study published in the Lancet in late 2006, while randomized trials suggest more modest benefits.

The Canadian Cancer Society recommends that women between the ages of 50 and 69 have a screening mammogram and a clinical breast exam every two years. Women between 40 and 49 should discuss their risk of breast cancer with their doctor.

The new U.S. guidelines are based on evidence that breast cancer risk is not the same for all women under 50.

Women who are in their 40s range from child-bearing to postmenopausal, which changes the risks. Family history and early onset of menstruation also increase risk. 

The college's panel of six experts reviewed scientific literature on mammograms from 1966 to 2005. Unlike earlier reviews, the potential harms from mammograms were considered.

Disadvantages included the risk of false positive results, which can lead to unnecessary biopsies, anxiety and cost, as well as exposure to radiation.

No blanket recommendation

An editorial accompanying the guidelines in Tuesday's issue of the Annals of Medicine concluded that "no simple recommendation applies to all women in their 40s."

Instead, doctors should listen carefully to women between 40 and 49, and tell them about "the benefits and limitations of our imperfect tests."

If a woman decides not to have a mammogram, she and her doctor should take another look at the idea every one to two years, Kirk said.

It could take 5,000 to 10,000 mammograms to detect one case of breast cancer for a woman in her 40s. By the time women are in their 50s, the number of mammograms needed to detect a case drops to about 2,500.

The chances of a woman in her 40s getting breast cancer are about one in 67, U.S. figures suggest.

Over a lifetime, the risk for breast cancer is about one in nine, which applies to women in their 80s who haven't died of another cause. One in 27 will die of it, according to the Canadian Cancer Society.

The college's panel came to different conclusions from the American Cancer Society, which in 2006 recommended annual mammograms for women starting at age 40. The U.S. society called the new guidelines "a step backward."